Cross-Country Gliding Weather

Cross Country Gliding Weather

The aim of this article is to suggest some simple guides to good soaring weather. For this purpose a good soaring day is defined as one on which it is possible to make a closed circuit flight of 300km or more or more over the centre of the North Island or Hawkes Bay. The statistics in this article have been taken from an article in Sailplane and Gliding and have been adapted to New Zealand conditions as there is no information such as this gathered in New Zealand. The monthly totals on which good cross-country days are possible are as follows:

October	12
November	15
December	16
January	18
February	16

A few good days also occur in September and March, but they are rare and usually wave conditions are much more prevalent.

Weather Conditions for Closed Circuit Flights

There should be an even distribution of thermals marked by cumulus cloud. There should be no large areas of showers or regions of extensive layered clouds, nor should there be a complete absence of cumulus.
The cloud base over level country should rise to at least 4000ft above general ground level by early afternoon.
The winds between the surface and 5000ft should not exceed 20 knots. A flat calm does not produce ideal conditions either and speeds between 5 and 15 knots seem best.
Visibility should not be less than about 10 miles and on most good days the visibility should exceed 20 miles. Flights using wave lift require different conditions and have not been considered in this article.

Wind

The wind speed and direction is often the dominating factor in cross-country gliding in which thermals are used as the source of lift. Wind speeds exceeding 20 knots appear to prevent successful closed circuit flying. The wind speed measured by one or more of the stations on good cross-country days showed the following distribution of wind speeds at 5000ft.

Speed:	Percentage of days:
0-4kts	17.50%
5-9kts	24.40%
10-14kts	25.80%
15-19kts	12.70%
20-24kts	20.60%

The increase of good days with wind speeds in the 20-24kts range may be due to the use of cloud streets for out and return flights. However, on such days, triangles are difficult to complete.

Wind Direction

On a great many days the pressure gradient was so weak that no definite direction could be assigned for the wind over the area of cross-country flying. These days were listed under the “variable” class.

Direction:	% of days:
variable	15%
180-240 degree	30%
030-060 degree	7%
All others	11%

There was a gap between the 180 and 060 degrees due to the fact that cold moist air comes from that direction and the 240-030 due to the warm moist air from there.

Track of Air previously

Good thermals are more likely to occur when the air reaching the country is relatively cold and also when this air has not previously been heated over Australia. Air which has been heated over that Continent usually arrives with a well defined inversion just above the surface. The heat necessary to break down the inversion takes several hours to accumulate and soarable weather is delayed until early afternoon.

Previous Track:	% of days:
Winds from WSW to SSE	55%
All Others	19%
Stagnant	26%

i.e. Over the country the previous day.

From the “other directions” group there were no examples of good soaring days when the air had come from the North-East, North or North-West. The critical feature of winds which had come from North-East or East was how long the air had been in the northern latitudes. Most good soaring days in such air had occurred when the NE winds had come from the South and were curving back after a brief stay over Northern latitudes. If the trajectory of the air passed north of latitude 30 degrees the extra moisture picked up prevented good soaring weather developing.

Deductions from the Isobaric Pattern

The patterns of isobars on a weather map give no direct information about the depth of stability and amount of convective cloud, but there are some valid inferences one can make just by looking at a forecast weather map with its fronts and isobars. The general rules are as follows:

Where there is an area of low pressure there is likely to be an area of slowly ascending air aloft leading to extensive cloud cover. If the depression is moving, the area of cloud generally extends forward in the direction of motion.
Where the surface chart shows an area of high pressure, there will probably been large scale slow descent of air aloft. This descent dries and warms the air and makes it more stable.

This effect is not confined to the centres of pressure systems. Troughs and ridges in the pattern are also associated with rising or descending air aloft. A trough tends to have increased cloud cover near it, the cloud cover being extended ahead of it when the trough is moving. In contrast, a ridge is usually an area where cloud cover decreases and the air becomes more stable. For good soaring only small amounts of cloud cover are desirable, so ridges and regions near high pressure centres usually give the best conditions.

A simple test for good soaring weather is- will the route be nearer to a high pressure system or a ridge than to a low pressure centre or trough line?

Note: 86% of good soaring days occurred when the area was nearer to a ridge or anticyclone.

6% of good days occurred half-way between high and low pressure systems.
8% were actually nearer to a low than to a high pressure centre, but on these days the low was moving away.

Curvature of Isobars

It is not always easy to determine from a small weather map whether the area is nearer a trough or a ridge or anti-cyclone. A simpler test of good soaring weather is to look at the way the isobars curve. Isobars which (travelling down-wind) turn to the left are said to have “anti-cyclonic curvature”. Curving to the right is called “cyclonic curvature”. Good soaring conditions are more likely to occur if the isobars have ant-cyclonic curvature.

78% of good soaring days had anti-cyclonic curvature of the isobars.
15% occurred when the isobars were straight.
7% of the days were good despite cyclonically curved isobars.

Summary of an Ideal Surface Chart

A number of outstanding soaring days had the following features in common:

During the period 12-24 hours previous, a cold front had passed across the country.
The air following the cold front had come from a region further south. The main trajectory lay in a broad sector from West through South to South-South-East.
The depression associated with the cold front moved well clear of New Zealand and a ridge or anti-cyclone moved close to or over the country.
The isobars showed anti-cyclonic curvature over the area of the cross-country flights. The spacing between isobars was wide enough to prevent the winds reaching 25 knots at flying levels.

Surface Pressure

The surface pressure is usually not a critical factor in good soaring weather but it was included because of its association with successful flights. The figures are:

Pressure range % good soaring days
1001 – 1005   7%
1006 – 1010 12%
1011 – 1015    25%
1016 – 1020                                           41%
1021 – 1025 13%
1026 – 1030 3%

91% of occasions had a surface pressure of 1009mb or higher

Additional Features

Sunshine:

Sunshine provides nearly all the energy for thermals which develop overland. Most good soaring days had sunshine figures in excess of nine hours. And a minimum of seven hours recorded sunshine at an inland station within the cross-country area seems essential. In latitude 40 degrees south, the sun is above the horizon for more than twelve hours between October 21 and February 26. This is the greater part of the cross-country soaring season. It seems that unless at least 50% of the possible sunshine is actually recorded at ground level, the day is unlikely to be ideal for long g flights. Strong thermals can of course be found under an almost overcast sky but the distribution of thermals is apt to be irregular. The wide gaps between thermals make it difficult to achieve a fast speed.

Temperatures:

The most important layer of the atmosphere is that between the surface and 5000ft. The temperatures at the top and bottom of this layer are a useful guide to soaring conditions. The temperature at ground level is mentioned in most press and radio/TV forecasts. The 5000ft temperature is only available by telephoning a Met Office. Taking surface temperatures first, the following figures seem to be the lowest acceptable values for maximum temperatures:

September           10 C
October    16
November    18
December 23
January                26
February    25
March                   21

There is no similar upper limit, but very hot days are not necessarily the best soaring days. Hot days generally occur when air has come from Australia. This air often contains a large inversion at dawn. This takes time to break down even if the sun shines continuously. Consequently, thermals do not develop until early afternoon and are likely to die out well before sunset.

Temperatures Aloft:

On good soaring days the temperature at 5000ft is at least 11 degrees Celsius colder than the maximum surface temperature. On days with strong thermals the difference can be about 16 degrees C. If the temperature continues to fall the same amount in the next 5000ft, (i.e. between 5000ft and 10,000ft) the air is likely to prove too unstable for good soaring. A drop in temperature of 10 C between 5000ft and 10,000ft may indicate that there will be heavy cumulus development by the afternoon with the subsequent risk of showers cutting off parts of the route.

On an ideal soaring day, the temperature falls at least 11 degrees C to 5000ft but only 5 degrees C in the next 5000ft.

Changes of Temperature with Time:

On a good soaring day the 5000ft temperature remains constant or decreases slowly during the day. Days when the temperature aloft decreases with time, are usually days when thermals last until late in the day. Eighty per cent of good soaring days had constant or slowly decreasing temperatures at 5000ft.

In contrast, rising temperatures at 5000ft usually result in weakening thermals which end early in the day. The maximum rise in the 5000ft temperature on a good day was 4 degrees C, but this was an exception.

Humidity:

Humidity is important because as a general rule the more humid the air is, the lower the cloud base will be and the greater the cloud amount. Humidity is often reported as the difference between the actual air temperature and dew point. (The dew point is the temperature below which the moisture in the air condenses out as droplets of water, resulting in dew, fog or cloud depending on the circumstances.)

Airfield weather reports often give the temperatures and dew point. There is a useful but not exact relationship in the difference between air temperature and dew point and the base of convective cloud. This rule is only valid while the air is rising.

Multiply the difference between dry bulb and dew point by 400 to get the cloud base in feet. For example, with a reported temperature of 22 C and a dew point of 12 C, the base of cumulus (if any) will probably be about 4000ft.

Humidity and Spreading out of Cumulus:

Cross-country pilots are often faced with the problem of cumulus which, within a few hours of development, spreads out to form an almost continuous layer of stratocumulus.

This occurs when:-

(a) A well marked inversion or very stable layer acts as a lid to convective currents and the cumulus clouds all cease rising at a uniform height.

(b) When the air beneath is moist for a depth of at least 1500ft.
There is no infallible way of detecting this from the surface chart. One must have access to a representative upper air sounding. As a general rule, if the sounding shows there that there is a very stable layer beneath which the dew point is within 5 C of the air temperature, then there is a risk of cumulus spreading out to form a layer. The risk is greatest when the inversion begins at levels between about 4000 and 7000ft.

If the inversion starts below 4000ft there is a good chance of thermals stirring up the air enough to bring down the very dry air usually found above such inversions. This only occurs well inland however. Coastal areas are particularly prone to persistent stratocumulus with onshore winds.

Inversions are a common feature of good soaring days:

The difference between a good day and a poor one often just few degrees between the dew point and the air temperature. The drier air results in well broken shallow cumulus instead of a continuous sheet of stratocumulus.

A Check List for Good Soaring Weather:

1. After the passage of a cold front.
2. Near or ahead of a ridge or centre of high pressure.
3. When the winds up to 5000ft are:-
(a) Less than 25 knots
(b) Or from a direction WSW through South.
(c) Or wind direction variable and less than 15 knots.
4. When the air has come from a more southerly latitude the day before.
5. When the isobars are curved anti-cyclonically.
6. When the area is nearer to the adjacent high or ridge than to a low or a
trough.
7. Provided the afternoon temperatures reach the values mentioned earlier.
8. When the mid-afternoon dew point is at least 10 C below the air
    temperature.
9. Surface pressure above 1001mb, preferably in the range 1011 to
    1021mb.
10. The 5000ft temperature at least 11 C below the surface temperature
    by mid-afternoon and not expected to rise.
11. At least 50% of the possible sunshine reaches the ground.

Bad Signs:

1. Nearby or approaching toughs or depressions.
2. Winds from a SE though East to NNW direction, especially if the wind is
    expected to increase and is preceded by the formation of high cloud
    such as cirrus.
3. Winds of Beaufort force six or more in nearby areas. (Mentioned as
    “strong to gale”.
4. Fog or poor visibility such as smoke haze near the coast if winds are
    onshore. This suggests a low inversion with very limited thermals inland.

Making the Most of Weather Forecasts:
The bulletins broadcast by Radio and TV is required to be brief since the majority of listeners and viewers do not wish to be confused by details. Glider pilots need to be alert to the significant points.

Summary of the Situation:

This is the most useful part since it sets the stage for the forecast and often tells the listener or viewer if the weather has any promise. The brief glimpse of the TV weather chart is worth a lot of words. Use it for items one and two on the check lists.

Assuming that there is no adverse feature (such as a depression or trough moving towards the country), here are some pointers to good weather which can be picked out of a forecast.

1. Tonight’s weather
Decreasing winds and clearing skies is often a good sign especially if the pressure is rising and a front or trough is moving away. Are the night’s temperatures expected to be unusually low? This often means the dew point is low and a low dew point can mean a high cloud base next day. A night frost in the spring and early summer can precede a day of strong thermals. But the opposite can apply in late summer and autumn.

2. Tomorrow’s weather
The cheering phrases here are “sunny periods” or “sunny spells”. In contrast though, “sunny intervals” is not so hopeful. This phrase implies much more cloud and to a glider pilot, it may mean overdevelopment of convective cloud.

Showers may not spoil the day. If the term “isolated” or “scattered” is added it can still be a good day. However if you hear the adjectives… “Widespread, frequent, heavy or prolonged” used… you should prepare for disappointment.

If the winds are predicted to be light or moderate on the ground, then the upper winds will probably be light enough for closed circuit flying. Terms such as “fresh to strong” mean that the surface wind is expected to exceed 16 knots and if so, the upper winds may be too strong for triangular flights. The term “strong” is used for the range 22 – 27 knots and in such cases there is almost no hope of completing a triangle.

Conclusion:

The check list for good soaring conditions was intended to show the various weather features which go to make a day an average club pilot has a good prospect of completing a 300km triangle in a glider such as a Skylark 3 or a Ka-6.

There are exceptions to almost every weather rule quoted and the check list has many limitations. It is generally true that the more items for which the criteria are satisfied, then the better the prospect of good conditions. However, the list does not cover the occasions when a narrow bands of very good weather develop in an otherwise mediocre pattern of weather.

WEATHER FOR WAVE SOARING:

Conditions for Wave Development:

Synoptic situation for lee waves: The airstream conditions required for wave development can be satisfied by a variety of synoptic conditions. The most favourable conditions for wave development are:-

(a) Behind cold fronts on the east coast.
(b) With an upper air trough in the Tasman Sea producing a westerly flow across the mountains and a surface cold front or occluded front approaches from the south-west.
(c) Ahead of a warm front, when wind and temperature conditions for limited periods are suitable for wave flow.
(d) In the outer regions of high pressure areas where winds are 15 knots or greater, if subsidence provides the shallow stable layer required.

Required Winds Aloft:

Stability and Moisture Requirements:

There should be marked stability (isothermal) layers or inversions) at levels where the air is disturbed by the mountains. In the diagram note the more stable layer just above the mountain top and the nearly isothermal layer above that. This very stable need not extend to the surface. However layers both below and above the very stable layer should exhibit only slight stability.

The amplitude of the lee wave is greatest in the stable layer. The stable layer produces larger wave amplitudes (see diagram) when it is a shallow layer of great stability than when it is only moderately stable and relatively deep. The strength of the lift depends on the wave amplitude, wave length and wind-speed. Strong lift is favoured by large amplitudes, short wave lengths and strong winds.

Presence of a Jet Stream:

The presence of a jet stream with high wind speeds and strong vertical wind shear has been determined to be an important factor in the formation of mountain waves. In a study of mountain waves, 80% of the pilot reports of wave were within 200 miles of a jet stream.

Topographic Effects:

A long mountain ridge is more effective than a short ridge or isolated mountain peak in producing lee waves; also a ridge which presents a concave face to the wind will produce a greater wavelength than if the face is convex.

When the spacing between successive mountain ridges down-wind is about the same as the wave-length of the lee wave, the amplitude is increased, but if the distance between the ridges is much different than the wave length, the amplitude is diminished and the wave may be damped out entirely.

The shape of the lee slope and its height above the lee plain has more effect on the waves than the windward slope. As the height above the plain, or the lee slope increases, so do the waves.

Use of Satellite Photographs:

Considerable work has been done in the application of satellite cloud pictures to wave soaring. For about nine years, weather satellites using visual sensors have photographed wave patterns to the lee of mountain ranges in various parts of the world. Information on wave patterns obtained by satellites can be requested by the pilot when he receives a briefing from the Weather Office. Every Weather Office does not have the actual photographs, but a satellite photograph narrative is received at all weather stations by teletype. This narrative describes wave cloud patterns as well as other clouds being observed.

Wave clouds visible from the ground and photographed by the weather satellites do not always indicate soarable waves. The waves may be too weak with too little vertical motion for soarable lift. The wind and stability conditions help us determine how strong waves will be.
The Weather Briefing:

What is available from the weather briefer? Knowing what you will need will assist in briefing. The information listed below can be obtained by telephone from the Weather Office.

(a) Winds aloft forecast up to 29000ft for a number of locations.

(b) The latest observed lapse rate including levels of inversions, if a nearby
sounding is available.

(c) Wave clouds observed by satellite as indicated in the satellite narrative
message.

(d) Strength of surface and low level winds and the intensity of turbulence.
Often there vare good waves but it is too windy and turbulent to fly.

(e) Cloud conditions and visibility over the area of your flight.

Insert:

WEATHER FOR RIDGE SOARING:

Ridge soaring is the oldest form of soaring. Pilots used the currents moving up over ridges in 1920, some six years before thermal soaring was discovered. Although ridge soaring is not as popular as thermal soaring, it is done in some areas and can be an easy way to fulfil the five hour duration requirement for the FAI silver badge.

Recently there has been a renewal of interest in ridge soaring. In 1968, an out and return world record of 476 miles was made along the ridges of the Appalachian mountains in America. In 1973, a new out & return world record of 782 miles was established along the same mountains. These records have demonstrated that ridge lift used in conjunction with wave and thermal lift along the ridges can produce long flights.

Conditions of Wind and Stability for Ridge Lift:

The direction and speed of the wind are the main factors in ridge soaring. A wind that flows at right angles to the line of the ridge is best, but if the direction is within 40 degrees of perpendicular, there may still be sufficient airflow up the ridge for soaring. The average wind speed from the base up to the top must be at least 15 knots. If the gradient wind (wind about 1500 to 2000ft above the ground is 20 knots from the right direction, the ridge will probably be working.

The height to which a glider pilot can soar on the ridge does not increase in simple proportion to the wind speed. Strong winds tend to raise the turbulence without raising the soaring level.

In addition to the wind speed and direction, the air’s stability is an important factor. Stability usually provides the best conditions for slope (ridge) soaring. With a moderate wind, a sailplane may be able to soar up to three times the height of the ridge. A particularly favourable temperature profile is one where a neutrally stable layer of air at low levels is capped by a sharp temperature inversion. When these conditions are met, sailplanes may climb several thousand feet above the escarpment. Such conditions occur more commonly in the evening hours than during the heat of the day.

During the early morning hours, a marked nocturnal inversion over the valley may extend up to the level of the ridge. Then, despite a 20 knot gradient wind, the air beneath the inversion remains calm. There will be no flow of air up the ridge until insolation (incoming radiant solar energy) breaks down the inversion.

During the day as surface heating proceeds and the air becomes less stable, the upper limits of soaring may extend to the top of the unstable layer. The lift over the ridge will be stronger and more turbulent as thermals, developing in the air flowing up the slope, reinforce the existing ridge lift. The degree of turbulence depends on the wind speed, the lapse rate and the roughness of the terrain. Consequently, it is difficult to predict the strength of the lift.

Inclination and Profile of the Ridge:

The effect of the steepness of the hill or slope is difficult to assess. The best inclination for easy soaring is between 20 and 45 degrees. Within certain limits, the steeper the slope, the greater the vertical component of the airflow, but this relationship breaks down when the slope is very steep. As the slope steepens beyond 45 degrees, the area of lift becomes more restricted and turbulence greater. If the angle exceeds 60 degrees the slope may become difficult to soar.

A gradual change in the angle of the slope is best for soaring since the air can follow the profile smoothly (laminar flow) without breaking away in turbulent eddies.

Where the slope is steep, a rotor may form at the foot of the cliff and remain there. These rotors have been termed “bolster eddies”. They alter the effective shape of the cliff, making it appear less steep to the oncoming airflow.

Weather Briefing:

When the pilot interested in ridge soaring calls his/her nearest Weather Office or Flight Service Station for a weather briefing, the following information can be obtained:-

(1) Air mass moisture conditions… is the airmass over the area dry enough so that the ridge will be free of and remain free of cloud?

(2) Wind speed and direction… since observational wind data will not normally be available for such a small area and for such low altitudes; conditions for up slope wind will have to be estimated. The surface wind in the valley must be averaged with the forecast wind at the first available level above the ground. Ask for the latest winds-aloft forecast for the first two levels above sea level. If the upper wind observations are taken nearby, this will aid significantly because you can get more detail of the lower winds. If your slope or ridge is near the sea, a forecast sea-breeze is usually a guarantee of a steady 15 knot breeze of stable air.

(3) The likelihood of thermals or wave developing… a local study by pilots relating local winds to the general weather pattern can help greatly in arriving at conditions to be expected.

…………………………………………………………………………

Further reading: Meteorology for Glider Pilots by Tom Bradbury.
This article prepared by Peter Lyons for the Hawkes Bay Gliding Club and also presented at the 1991 Matamata Cross-Country Course.

CROSS COUNTRY GLIDING WEATHER

October 12, November 15, December 16, January 18, February 16. A few good days also occur in September and March, but they are3 rare and usually wave conditions are much more prevalent.

WEATHER CONDITIONS FOR CLOSED CIRCUIT FLIGHTS
(a) There should be an even distribution of thermals marked by cumulus cloud. There should be no large areas of showers or regions of extensive layered clouds, nor should there be a complete absence of cumulus.
(b) The cloud base over level country should rise to at least 4000ft above general ground level by early afternoon.
(c) The winds between the surface and 5000ft should not exceed 20 knots. A flat calm does not produce ideal conditions either and speeds between 5 and 15 knots seem best.
(d) Visibility should not be less than about 10 miles and on most good days the visibility should exceed 20 miles. Flights using wave lift require different conditions and have not been considered in this article.
WIND
The wind speed and direction is often the dominating factor in cross-country gliding in which thermals are used as the source of lift. Wind speeds exceeding 20 knots appear to prevent successful closed circuit flying. The wind speed measured by one or more of the stations on good cross-country days showed the following distribution of wind speeds at 5000ft.
Speed: 0- 4kts Percentage of days 17.5%
5- 9kts 24.4%
10- 14kts 25.8%
15- 19kts 12.7%
20- 24kts 20.6%
The increase of good days with wind speeds in the 20-24kts range may be due to the use of cloud streets for out and return flights. However, on such days, triangles are difficult to complete.
Wind Direction
On a great many days the pressure gradient was so weak that no definite direction could be assigned for the wind over the area of cross-country flying. These days were listed under the “variable” class.
Direction variable % of days 15%
180-240 degree 30%
030-060 degree 7%
All others 11%
There was a gap between the 180 and 060 degrees due to the fact that cold moist air comes from that direction and the 240-030 due to the warm moist air from there.
Track of Air previously.
Good thermals are more likely to occur when the air reaching the country is relatively cold and also when this air has not previously been heated over Australia. Air which has been heated over that Continent usually arrives with a well defined inversion just above the surface. The heat necessary to break down the inversion takes several hours to accumulate and soarable weather is delayed until early afternoon.
Previous Track % of days
Winds from WSW to SSE 55%
All Others 19%
Stagnant 26% i.e. Over the country the previous day.
From the “other directions” group there were no examples of good soaring days when the air had come from the North-East, North or North-West. The critical feature of winds which had come from North-East or East was how long the air had been in the northern latitudes. Most good soaring days in such air had occurred when the NE winds had come from the South and were curving back after a brief stay over Northern latitudes. If the trajectory of the air passed north of latitude 30 degrees the extra moisture picked up prevented good soaring weather developing.
Deductions from the Isobaric Pattern.
The patterns of isobars on a weather map give no direct information about the depth of stability and amount of convective cloud, but there are some valid inferences one can make just by looking at a forecast weather map with its fronts and isobars. The general rules are as follows:
(a) Where there is an area of low pressure there is likely to be an area of slowly ascending air aloft leading to extensive cloud cover. If the depression is moving, the area of cloud generally extends forward in the direction of motion.
(b) Where the surface chart shows an area of high pressure, there will probably been large scale slow descent of air aloft. This descent dries and warms the air and makes it more stable.
This effect is not confined to the centres of pressure systems. Troughs and ridges in the pattern are also associated with rising or descending air aloft. A trough tends to have increased cloud cover near it, the cloud cover being extended ahead of it when the trough is moving. In contrast, a ridge is usually an area where cloud cover decreases and the air becomes more stable. For good soaring only small amounts of cloud cover are desirable, so ridges and regions near high pressure centres usually give the best conditions.
A simple test for good soaring weather is- will the route be nearer to a high pressure system or a ridge than to a low pressure centre or trough line?
Note: 86% of good soaring days occurred when the area was nearer to a ridge or anticyclone.
6% of good days occurred half-way between high and low pressure systems.
8% were actually nearer to a low than to a high pressure centre, but on these days the low
was moving away.
Curvature of Isobars.
It is not always easy to determine from a small weather map whether the area is nearer a trough or a ridge or anti-cyclone. A simpler test of good soaring weather is to look at the way the isobars curve. Isobars which (travelling down-wind) turn to the left are said to have “anti-cyclonic curvature”. Curving to the right is called “cyclonic curvature”. Good soaring conditions are more likely to occur if the isobars have ant-cyclonic curvature.
78% of good soaring days had anti-cyclonic curvature of the isobars.
15% occurred when the isobars were straight.
7% of the days were good despite cyclonically curved isobars.
Summary of an Ideal Surface Chart.
A number of outstanding soaring days had the following features in common:
(a) During the period 12-24 hours previous, a cold front had passed across the country.
(b) The air following the cold front had come from a region further south. The main trajectory lay in a broad sector from West through South to South-South-East.
(c) The depression associated with the cold front moved well clear of New Zealand and a ridge or anti-cyclone moved close to or over the country.
(d) The isobars showed anti-cyclonic curvature over the area of the cross-country flights. The spacing between isobars was wide enough to prevent the winds reaching 25 knots at flying levels.
Surface Pressure
The surface pressure is usually not a critical factor in good soaring weather but it was included because of its association nwith successful flights. The figures are:-
Pressure range % good soaring days
1001 – 1005 7%
1006 – 1010 12%
1011 – 1015 25%
1016 – 1020 41%
1021 – 1025 13%
1026 – 1030 3%
91% of occasions had a surface pressure of 1009mb or higher.
Additional Features
Sunshine: Sunshine provides nearly all the energy for thermals which develop overland. Most good soaring days had sunshine figures in excess of nine hours. And a minimum of seven hours recorded sunshine at an inland station within the cross—country area seems essential. In latitude 40 degrees south, the sun is above the horizon for more than twelve hours between October 21 and February 26. This is the greater part of the cross-country soaring season. It seems that unless at least 50% of the possible sunshine is actually recorded at ground level, the day is unlikely to be ideal for long g flights. Strong thermals can of course be found under an almost overcast sky but the distribution of thermals is apt to be irregular. The wide gaps between thermals make it difficult to achieve a fast speed.
Temperatures
The most important layer of the atmosphere is that between the surface and 5000ft. The temperatures at the top and bottom of this layer are a useful guide to soaring conditions. The temperature at ground level is mentioned in most press and radio/TV forecasts. The 5000ft temperature is only available by telephoning a Met Office. Taking surface temperatures first, the following figures seem to be the lowest acceptable values for maximum temperatures:
September 10 C
October 16
November 18
December 23
January 26
February 25
March 21
There is no similar upper limit, but very hot days are not necessarily the best soaring days. Hot days generally occur when air has come from Australia. This air often contains a large inversion at dawn. This takes time to break down even if the sun shines continuously. Consequently, thermals do not develop until early afternoon and are likely to die out well before sunset.
Temperatures Aloft
On good soaring days the temperature at 5000ft is at least 11 degrees colder than the maximum surface temperature. On days with strong thermals the difference can be about 16 degrees C. If the temperature continues to fall the same amount in the next 5000ft, (i.e. between 5000ft and 10,000ft) the air is likely to prove too unstable for good soaring. A drop in temperature of 10 C between 5000ft and 10,000ft may indicate that there will be heavy cumulus development by the afternoon with the subsequent risk of showers cutting off parts of the route.
On an ideal soaring day, the temperature falls at least 11 degrees C to 5000ft but only 5 degrees C in the next 5000ft.
Changes of Temperature with Time
On a good soaring day the 5000ft temperature remains constant or decreases slowly during the day. Days when the temperature aloft decreases with time, are usually days when thermals last until late in the day. Eighty per cent of good soaring days had constant or slowly decreasing temperatures at 5000ft.
In contrast, rising temperatures at 5000ft usually result in weakening thermals which end early in the day. The maximum rise in the 5000ft temperature on a good day was 4 degrees C, but this was an exception.
Humidity
Humidity is important because as a general rule the more humid the air is, the lower the cloud base will be and the greater the cloud amount. Humidity is often reported as the difference between the actual ait temperature and dew point. (The dew point is the temperature below which the moisture in the air condenses out as droplets of water, resulting in dew, fog or cloud depending on the circumstances.)
Airfield weather reports often give the temperatures and dew point. There is a useful but not exact relationship in the difference between air temperature and dew point and the base of convective cloud. This rule is only valid while the air is rising.
Multiply the difference between dry bulb and dew point by 400 to get the cloud base in feet. For example, with a reported temperature of 22 C and a dew point of 12 C, the base of cumulus (if any) will probably be about 4000ft.
Humidity and Spreading out of Cumulus
Cross-country pilots are often faced with the problem of cumulus which, within a few hours of development, spreads out to form an almost continuous layer of stratocumulus.
This occurs when:-
(a) A well marked inversion or very stable layer acts as a lid to convective currents and the cumulus clouds all cease rising at a uniform height.
(b) When the air beneath is moist for a depth of at least 1500ft.
There is no infallible way of detecting this from the surface chart. One must have access to a representative upper air sounding. As a general rule, if the sounding shows there that there is a very stable layer beneath which the dew point is within 5 C of the air temperature, then there is a risk of cumulus spreading out to form a layer. The risk is greatest when the inversion begins at levels between about 4000 and 7000ft.
If the inversion starts below 4000ft there is a good chance of thermals stirring up the air enough to bring down the very dry air usually found above such inversions. This only occurs well inland however. Coastal areas are particularly prone to persistent stratocumulus with onshore winds. Inversions are a common feature of good soaring days.
The difference between a good day and a poor one often just few degrees between the dew point and the air temperature. The drier air results in well broken shallow cumulus instead of a continuous sheet of stratocumulus.
A Check List for Good Soaring Weather
1. After the passage of a cold front.
2. Near or ahead of a ridge or centre of high pressure.
3. When the winds up to 5000ft are:-
(a) Less than 25 knots
(b) Or from a direction WSW through South.
(c) Or wind direction variable and less than 15 knots.
4. When the air has come from a more southerly latitude the day before.
5. When the isobars are curved anti-cyclonically.
6. When the area is nearer to the adjacent high or ridge than to a low or a trough.
7. Provided the afternoon temperatures reach the values mentioned earlier.
8. When the mid-afternoon dew point is at least 10 C below the air temperature.
9. Surface pressure above 1001mb, preferably in the range 1011 to 1021mb.
10. The 5000ft temperature at least 11 C below the surface temperature by mid-afternoon and not expected to rise.
11. At least 50% of the possible sunshine reaches the ground.

Bad Signs
1. Nearby or approaching toughs or depressions.
2. Winds from a SE though East to NNW direction, especially if the wind is expected to increase and is preceded by the formation of high cloud such as cirrus.
3. Winds of Beaufort force six or more in nearby areas. (Mentioned as “strong to gale”.)
4. Fog or poor visibility such as smoke haze near the coast if winds are onshore. (This suggests a low inversion with very limited thermals inland).
Making the Most of Weather Forecasts
The bulletins broadcast by Radio and TV is required to be brief since the majority of listeners and viewers do not wish to be confused by details. Glider pilots need to be alert to the significant points.
Summary of the Situation:
This is the most useful part since it sets the stage for the forecast and often tells the listener or viewer if the weather has any promise. The brief glimpse of the TV weather chart is worth a lot of words. Use it for items one and two on the check lists.
Assuming that there is no adverse feature (such as a depression or trough moving towards the country), here are some pointers to good weather which can be picked out of a forecast.
1. Tonight’s weather
Decreasing winds and clearing skies is often a good sign especially if the pressure is rising and a front or trough is moving away. Are the night’s temperatures expected to be unusually low? This often means the dew point is low and a low dew point can mean a high cloud base next day. A night frost in the spring and early summer can precede a day of strong thermals. But the opposite can apply in late summer and autumn.
2. Tomorrow’s weather
The cheering phrases here are “sunny periods” or “sunny spells”. In contrast though, “sunny intervals” is not so hopeful. This phrase implies much more cloud and to a glider pilot, it may mean overdevelopment of convective cloud.
Showers may not spoil the day. If the term “isolated” or “scattered” is added it can still be a good day. However if you hear the adjectives… “Widespread, frequent, heavy or prolonged” used… you should prepare for disappointment.
If the winds are predicted to be light or moderate on the ground, then the upper winds will probably be light enough for closed circuit flying. Terms such as “fresh to strong” mean that the surface wind is expected to exceed 16 knots and if so, the upper winds may be too strong for triangular flights. The term “strong” is used for the range 22 – 27 knots and in such cases there is almost no hope of completing a triangle.
Conclusion
The check list for good soaring conditions was intended to show the various weather features which go to make a day an average club pilot has a good prospect of completing a 300km triangle in a glider such as a Skylark 3 or a Ka-6.
There are exceptions to almost every weather rule quoted and the check list has many limitations. It is generally true that the more items for which the criteria are satisfied, then the better the prospect of good conditions. However, the list does not cover the occasions when a narrow bands of very good weather develop in an otherwise mediocre pattern of weather.

WEATHER FOR WAVE SOARING

Mountain wave soaring has become second nature to sailplane pilots who live near mountains on the east coast of New Zealand. To minimise unproductive trips to the aerodrome let’s review the conditions necessary for wave development and the information available from the Weather Office and Flight Service Stations.. (FSS).
Conditions for Wave Development
Synoptic situation for lee waves: The airstream conditions required for wave development can be satisfied by a variety of synoptic conditions. The most favourable conditions for wave development are:-
(a) Behind cold fronts on the east coast.
(b) With an upper air trough in the Tasman Sea producing a westerly flow across the mountains and a surface cold front or occluded front approaches from the south-west.
(c) Ahead of a warm front, when wind and temperature conditions for limited periods are suitable for wave flow.
(d) In the outer regions of high pressure areas where winds are 15 knots or greater, if subsidence provides the shallow stable layer required.
Required Winds Aloft

The wind speed at levels near the mountain top should exceed a minimum which varies from 15 to 25 knots, depending on the height of the ridge generating the waves. In the Ruahines about 15 – 20 knots is sufficient and in the Southern Alps, about 25 knots. The wind speed should increase with altitude (or at least remain constant) up to the tropopause. (See diagram.)
The airflow should be within 30 degrees of the direction perpendicular to the ridge line and the wind direction should not change with height.
The wave length of lee waves varies within the mean wind speed in the layers contributing to the wave flow. The stronger this speed, the greater the wave length.
Stability and Moisture Requirements
There should be marked stability (isothermal) layers or inversions) at levels where the air is disturbed by the mountains. In the diagram note the more stable layer just above the mountain top and the nearly isothermal layer above that. This very stable need not extend to the surface. However layers both below and above the very stable layer should exhibit only slight stability.
The amplitude of the lee wave is greatest in the stable layer. The stable layer produces larger wave amplitudes (see diagram) when it is a shallow layer of great stability than when it is only moderately stable and relatively deep. The strength of the lift depends on the wave amplitude, wave length and wind-speed. Strong lift is favoured by large amplitudes, short wave lengths and strong winds.
Presence of a Jet Stream
The presence of a jet stream with high wind speeds and strong vertical wind shear has been determined to be an important factor in the formation of mountain waves. In a study of mountain waves, 80% of the pilot reports of wave were within 200 miles of a jet stream.
Topographic Effects
A long mountain ridge is more effective than a short ridge or isolated mountain peak in producing lee waves; also a ridge which presents a concave face to the wind will produce a greater wavelength than if the face is convex.
When the spacing between successive mountain ridges down-wind is about the same as the wave-length of the lee wave, the amplitude is increased, but if the distance between the ridges is much different than the wave length, the amplitude is diminished and the wave may be damped out entirely.
The shape of the lee slope and its height above the lee plain has more effect on the waves than the windward slope. As the height above the plain, or the lee slope increases, so do the waves.
Use of Satellite Photographs
Considerable work has been done in the application of satellite cloud pictures to wave soaring. For about nine years, weather satellites using visual sensors have photographed wave patterns to the lee of mountain ranges in various parts of the world. Information on wave patterns obtained by satellites can be requested by the pilot when he receives a briefing from the Weather Office. Every Weather Office does not have the actual photographs, but a satellite photograph narrative is received at all weather stations by teletype. This narrative describes wave cloud patterns as well as other clouds being observed.
Wave clouds visible from the ground and photographed by the weather satellites do not always indicate soarable waves. The waves may be too weak with too little vertical motion for soarable lift. The wind and stability conditions help us determine how strong waves will be.
The Weather Briefing
What is available from the weather briefer? Knowing what you will need will assist in briefing. The information listed below can be obtained by telephone from the Weather Office.
(a) Winds aloft forecast up to 29000ft for a number of locations.
(b) The latest observed lapse rate including levels of inversions, if a nearby sounding is available.
(c) Wave clouds observed by satellite as indicated in the satellite narrative message.
(d) Strength of surface and low level winds and the intensity of turbulence. Often there are good waves but it is too windy and turbulent to fly.
(e) Cloud conditions and visibility over the area of your flight.

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WEATHER FOR RIDGE SOARING
Ridge soaring is the oldest form of soaring. Pilots used the currents moving up over ridges in 1920, some six years before thermal soaring was discovered. Although ridge soaring is not as popular as thermal soaring, it is done in some areas and can be an easy way to fulfil the five hour duration requirement for the FAI silver badge.
Recently there has been a renewal of interest in ridge soaring. In 1968, an out and return world record of 476 miles was made along the ridges of the Appalachian mountains in America. In 1973, a new out & return world record of 782 miles was established along the same mountains. These records have demonstrated that ridge lift used in conjunction with wave and thermal lift along the ridges can produce long flights.

Ridge lift can be used as a stepping stone in the search for thermal and wave lift. A pilot can fly ridge lift until the thermals begin, then go from a thermal directly into wave to make much higher altitude gains than if he/she had been towed directly into wave. Long flights along mountain ranges or ridges with limited options are not for beginners.
Conditions of Wind and Stability for Ridge Lift
The direction and speed of the wind are the main factors in ridge soaring. A wind that flows at right angles to the line of the ridge is best, but if the direction is within 40 degrees of perpendicular, there may still be sufficient airflow up the ridge for soaring. The average wind speed from the base up to the top must be at least 15 knots. If the gradient wind (wind about 1500 to 2000ft above the ground is 20 knots from the right direction, the ridge will probably be working.
The height to which a glider pilot can soar on the ridge does not increase in simple proportion to the wind speed. Strong winds tend to raise the turbulence without raising the soaring level.
In addition to the wind speed and direction, the air’s stability is an important factor. Stability usually provides the best conditions for slope (ridge) soaring. With a moderate wind, a sailplane may be able to soar up to three times the height of the ridge. A particularly favourable temperature profile is one where a neutrally stable layer of air at low levels is capped by a sharp temperature inversion. When these conditions are met, sailplanes may climb several thousand feet above the escarpment. Such conditions occur more commonly in the evening hours than during the heat of the day.
During the early morning hours, a marked nocturnal inversion over the valley may extend up to the level of the ridge. Then, despite a 20 knot gradient wind, the air beneath the inversion remains calm. There will be no flow of air up the ridge until insolation (incoming radiant solar energy) breaks down the inversion.
During the day as surface heating proceeds and the air becomes less stable, the upper limits of soaring may extend to the top of the unstable layer. The lift over the ridge will be stronger and more turbulent as thermals, developing in the air flowing up the slope, reinforce the existing ridge lift. The degree of turbulence depends on the wind speed, the lapse rate and the roughness of the terrain. Consequently, it is difficult to predict the strength of the lift.
Inclination and Profile of the Ridge
The effect of the steepness of the hill or slope is difficult to assess. The best inclination for easy soaring is between 20 and 45 degrees. Within certain limits, the steeper the slope, the greater the vertical component of the airflow, but this relationship breaks down when the slope is very steep. As the slope steepens beyond 45 degrees, the area of lift becomes more restricted and turbulence greater. If the angle exceeds 60 degrees the slope may become difficult to soar.
A gradual change in the angle of the slope is best for soaring since the air can follow the profile smoothly (laminar flow) without breaking away in turbulent eddies.
Where the slope is steep, a rotor may form at the foot of the cliff and remain there. These rotors have been termed “bolster eddies”. They alter the effective shape of the cliff, making it appear less steep to the oncoming airflow.
Weather Briefing
When the pilot interested in ridge soaring calls his/her nearest Weather Office or Flight Service Station for a weather briefing, the following information can be obtained:-
(1) Air mass moisture conditions… is the airmass