Glider pilots use various types of lift to make progress from one part of the country to another. Thermal lift is generated by the rising parcels of air, created by the sun's heat on the ground. The trick is to know if a particular day is going to produce good thermals. This is my summary of how to predict thermals strength from the available weather forecasts (with strong acknowledgement to Weatherjack and others).
When the sun heats the ground, this will also cause the parcel of air immediately above the ground to be warmed up. Since this parcel of air is now warmer than the surrounding air, it is less dense, and therefore more bouyant than its surroundings. It will therefore rise, and as it rises, the pressure it experiences will be reduce. The rising parcel of air is what the gliding community call a thermal and is what provides the energy to allow gliders circling in the thermal to climb. As the parcel of air rises, the reduction in pressure in the atmosphere with height will cause the parcel to expand and cool. If the parcel continues to find itself surrounded by cooler air than itself, it will continue to rise. If it does not, then it will stop. Thus, for good gliding thermals, we look for
Suprisingly, the temperature profile with height in particular air mass can be very different according to the past history of that air mass.
The rising pockets of warm air that we hang glider pilots call thermals are known as a parcels of air to meteorologists. These parcels of air being warmer and therefore lighter than the surrounding air rise as long as they are warmer than the air around them. While rising, a thermal expands and cools at about 5.5 *F for every thousand feet that it goes up. The rate of temperature change as we change altitude is called the lapse rate and this 5.5 *F or 3 *C rate of cooling is called the dry adiabatic lapse rate. Again, as long as the parcel of air or thermal is warmer than the air around it it will rise but sooner or later it will cool to the temperature of the surrounding air and will stop rising. If the air above us is not cool enough compared to the surface temperature then there will not be any thermals.
Air contains a percentage of moisture in it and if a thermal cools to a certain temperature based on that amount of moisture it will condense and form a cumulus cloud. This temperature is called the dew point. We see this happen on and near the surface in the form of dew on the grass and fog. As condensation takes place heat is released and the thermal's upward velocity will increase because it cools slower at the moist adiabatic lapse rate instead of 5.5 *F per thousand feet.
The Upper Level Sounding
If we are to predict what the day's thermals are to be like then we need to know what the temperature of the air above us is. All over the world balloons are sent up from specific weather stations at the same time to measure atmospheric conditions. This is done at 0 Zulu and 12 Zulu for every station. Radio transmitters aboard the balloons then send information about temperature, humidity, and wind velocity back from various altitudes. Because instruments that measure altitude operate on barometric pressure and the rate of pressure change is not always the same, these altitudes are referred to as pressure levels. Inches of mercury are the units we are used to seeing for barometric pressure but upper level soundings use millibars. The information that is sent back from the balloon is only for certain heights or levels and is determined by two criteria. These are called Mandatory Levels and Significant Levels Mandatory Levels are those used for every sounding and are listed below with approximate altitudes.
|sea level||0 ft|
|1000 mb||300 ft|
|850 mb||5000 ft|
|700 mb||10000 ft|
|500 mb||18000 ft|
|300 mb||30000 ft|
|200 mb||40000 ft|
|100 mb||53000 ft|
Significant levels are points where there are significant changes in temperature or relative humidity. If the straight-line segment between two plotted points is more than 1 *C in the troposphere then it is considered a significant level and the data is plotted in addition to the mandatory level data. Taking and plotting upper level soundings has been done for many years but now computers are used to analyze the data to create atmospheric profiles which are the heart of weather forecasting. A variety of methods can be used for graphing these soundings but the most common in the United States is the skew-T and that is what we will concentrate on in this article.
|dry adiabatic lapse rate||When a parcel of dry air is lifted, it expands and therefore cools at approximately 5.5 degrees F (3 degrees C) for every one thousand feet that it ascends. In other words it is said to cool adiabatically. The dry adiabat represents this cooling.|
|moist adiabatic lapse rate||When a parcel of rising air cools to the same temperature as the dew point, condensation occurs forming a cloud and heat is released giving the thermal new life. The thermal will then cool slower at the moist adiabatic lapse rate and will ascend faster. The amount of water vapor present in the parcel determines the rate of cooling that is usually between 2*F and 5*F per 1000 ft. Slower cooling increases the upward rate of the thermal and is what causes towering cumulus and cloud suck.|
|standard lapse rate||The standard or average lapse rate determined by many years of recording weather is 3.5 *F for every 1000 feet of altitude.|
|temperature inversion||If the temperature increases with altitude we are said to have a temperature inversion.|
|saturation lines||While the temperature of our parcel cools at 5.5 *F, the dew point decreases at about 1 *F per 1000 ft. This means that the dew point and the temperature approach each other at 4.5 *F. Hence, if we subtract the surface dew point from the surface temperature and divide the resultant by 4.5 the answer times 1000 will be the altitude agl. of cloud base.|
Forecasting Thermals With A Skew-T
To make things easier we will use only a blown up section of the skew-T illustrated above to predict the height the thermals will rise to and the height of cumulus cloud base if any.
Step #1 Using the predicted high temperature (23 *C) for the day, plot a line from the surface parallel with a dry adiabat until it intersects the actual temperature sounding. This is illustrated on the graph below with a blue line. The point where the blue line intersects the sounding (the red line) will be the height that a dry thermal will rise to during the time of peak heating. In this case approximately 725 mb. As will be seen in the next step of our example this altitude will be well above cloud base. This area from the surface to the maximum height of a dry thermal is called the adiabatic layer.
Step #2 The next step of plotting the saturation line from the surface dew point is a bit trickier because the dew point changes as the air warms up and it is difficult to predict what its value will be at the time of peak heating. An easy way to find the predicted dew point is from forecasts such as the NGM-MOS or MM5 models found on the internet at. Using this value (11 *C) we plot a brown line from the surface, this time parallel to the saturation lines until we intersect the blue line that we plotted from the surface temperature. This (810 mb) then is the predicted cloud base at the best time of the day when peak heating occurs. This can be done for several temperatures-dew point spreads to forecast the height of the thermals and cloud base at different times of the day.
Step #3 We can use the following chart to convert the level in millibars that we got from the skew-T to height in feet. The graph is an approximation but it is accurate enough for our purposes. Using the height of the adiabatic layer that we found in our example we find that 725 mb is equivalent to 9,000 ft. msl. Doing the same thing for the value of 810 mb for cloud base we find that it is roughly 6,000 ft. msl. Subtract the msl altitude of the launch at the site we are going to fly from 6,000 and we have the maximum altitude gain we can expect. Remember though that this is a forecast and just as we complain about the weathermen, we will have our own blown forecasts.
Upper level and aviation temperatures are given in *C while many other forecasts are in *F. This means that we will have to convert back and forth to make use of them. Conversion programs are available on the internet and some weather sites even provide one. The following formulas can be used for conversions.
Celsius to Fahrenheit: F = (9/5 C)+32
Fahrenheit to Celsius: C = 5/9 (F-32)
The thermal index is a value that allows us to compare the temperature of a thermal as it rises with the temperature of the air around it. To find the thermal index for a given altitude we find the temperature on the sounding plot (red line) for a specific altitude and then subtract the temperature on the blue line for the same altitude. The result is the thermal index. For an example if we find the temperature at 900 mb on the sounding (11 *C) and then subtract the temperature where the blue line crosses the 900 mb level (17 *C). 11-17= -6 This tells us that the thermal index for 900 mb or 3200 ft. msl is a negative six. A negative number is favorable while a positive number indicates stability. A few more examples are given in the chart below. Sailplane pilots claim that soaring can be done to an altitude where the thermal index is a -3 *F (1.66 *C).
|Height||Thermal Index *C||Thermal Index
Estimating climb rates is a bit subjective but a general rule of thumb is that the higher the thermals go, the better the climb rates. In other words the height of the adiabatic layer plays an important part in the strength of the thermals. Another indicator for climb rate is the thermal index. Higher negative numbers indicate better climb rates while lower negative numbers tell us to expect poor climb rates.
What Data To Use
Because of the times that the soundings are taken and the delay before we can acquire them it is not always feasible to wait for the 12 Zulu one. Zulu time is 5 hours ahead of Eastern Standard Time or 4 hours ahead of Eastern Daylight Time and it is not unusual to wait for one to two hours after the balloon is sent up to get the data.. Fortunately there are other options available to us. One option is to use the the 0 Zulu sounding from the night before but it is sometimes hard to determine which one to use and how far up wind it should be. The other option is to use the forecasted soundings available at several places on the internet.
A BASIC INTRODUCTION Basically, a tephigram shows the temperature of a vertical profile of the atmosphere. Because a rising thermal acts as an enclosed parcel of air, it cools at a different temperature to the surrounding air. Depending on how the temperature of the surrounding air falls with height, the thermal will either be able to continue to rise, or not. If it can, at some point it will condense and form cloud, if it continues to rise, a shower and rising further a cumulonimbus. By looking at the tephigram we can determine how likely this is to happen. If the temperature profile falls quickly with height (leans to the left), the chances are that a rising thermal will continue to rise, if the temperature falls less quickly or even rises with height the thermal will not rise as much and cloud may not form.
These diagrams are theoretical, simplified Skew-T's
|Millibar (hpa) pressure levels are on the left in blue
Surface pressure is normally around 1000 mbs
Each millibar corresponds to about 30 feet
(in lowest part of the atmosphere)
850 mbs is about 5,000 feet an important level
700 mbs is about 10,000 feet
500 mbs is about 18,000 feet
300 mbs is about 30,000 feet cirrus level
|Wind arrows (and numbers) are down the right hand side
short dash is 5 knots-----long dash is 10-----triangle 50 knots
The direction is shown by the arrows
difference between true and magnetic can be ignored
---In this example:
At 500 mbs (about 18,000 feet) triangle and short dash means about 55 knots (numbers give 57) from about 320°
At 30,000 feet (300 mbs) the wind is approx 340°/75
Not all charts use quite the same axes nor the same colour schemes and units
|The red arrows point to numbered red diagonal lines.
These are temperatures. eg, 0, 10, 20 etc
The red line that wiggles its way up the page is the environmental lapse rate line (ELR)
ie a plot of temperature with height.
It can be seen that the environmental line meets the bottom horizontal 1000 mbs line roughly midway between the two red diagonal lines marked 10 and 20
This is the surface temperature of around 15°C
(before any heating raises it later)
|Now follow the ELR up to the 850 mb level.
Here the temperature is about 3°C
At the 700 mbs level, it also happens to be 3°C in this theoretical example
At 300 mbs (30,000 feet) the temperature is -38°C
You will recall that the red line is the ELR
The green line represents the dewpoint.
Thus at 10,000 feet, the dewpoint is -22°C.
At the baseline, it is 8°C, ie surface dewpoint 8°C
The dewpoint is the temperature at which the air can hold no more water vapour, ie it is saturated.
Surface dewpoint is of major importance
On the real sounding on the next page, the dewpoint line is not of course green, but is a the solid line on the left.The two arrows point to brownish/red lines .
At the surface dewpoint (green line intercepting base), the mass of water the air can hold at that temp is indicated.
As that air rises in a thermal, it expands, cools (3°/1000) until it becomes saturated and cloud forms.
But - and this is important - the total water the air is holding has not changed from that it held at the surface
|Sea Breeze rule of thumb
Deep penetration inland only likely if total depth of convection, including cumulus, is between 3,000 and 10,000 feet
Less or greater depth of convection will probably mean any sea breezes confined to very near the coast
So with light winds and appropriate depth of convection on 15th August, sea breezes were forecast to move well inland
Not shown on this Skew-T, but using information from other sources, cooler air between 850 and 800 mbs was expected to move in from the west during the day raising the depth of convection.
That was taken into account when making the forecast for 15th August
|Straight lines (arrowed in black) run diagonally up at 45° from the bottom right to the top left.
These are the Dry Adiabatic Lapse Rate (DALR) lines
Adiabatic means no external heat added nor taken away
from the air mass
A rising parcel of air cools (because it expands) at the DALR until such time as it becomes saturated.
DALR can be taken as being about 3°C per 1,000 feet.
So a thermal which leaves the surface with a temperature of 20°C will have cooled to 14°C by the time it has reached 2,000 feet
|When the thermal rises far enough and cools sufficiently for condensation to occur, cloud forms. Condensation takes place if the air continues to rise, and latent heat is given out by the condensation process. Thus the temperature in the cloudy thermal falls off rather more slowly than it does in a dry thermal.
At low levels, this can be taken to be roughly 1½°C per 1,000 feet
These Saturated Adiabatic Lapse Rates lines (SALR) are curved and indicated by the green arrows
Air behaves either as being dry or saturated. It is not a gradual process of change between one state and the other
|Now we should know what all the various lines mean. So how do we use them? Let's recap|
|The red ELR line starts from about the 1000 mb level.
Now using those red temperature lines that go diagonally upwards to the right, it can be seen that the ELR reaches the surface at about 15°C
Just to the right of this point, there is one of the black DALR lines (they go diagonally up to the left) which intercepts the 1000 mb surface at about 17°C
So if the surface temperature reaches 17°C, then a bubble of air (being warmer than the environment) will rise as a thermal and follow that DALR line (temperature decreasing by 3°C every 1,000 feet as it does so)
Eventually, that thermal reaches the red ELR line at about 850 mbs (5,000 feet) and if it were somehow able to rise above that point, would in fact be cooler than the environment. This cannot happen, (ignoring orographic effects) so where this particular DALR line from a surface temperature of 17°C reaches the environmental line, the air stops rising, and this marks the top of the thermal.
|It can be seen by further study that about 15°C would have been needed before any convection can start (the trigger temperature).
We now consider the absurd case of the surface temperature rising to 30°C A line drawn (or visualized) from 30°C parallel to a DALR will reach the environment line at about 730 mbs (9,000 feet) - rather a good day!
In this illustration, formation of cumulus has deliberately been ignored as a simplification.
|This was a midnight sounding. It looks dreadful, doesn't it?
Lets blow up the bottom bit.
With some difficulty, it can be seen that the dry and the dewpoint lines are very close or even coincide near the surface.
The dewpoint probably won't change much during the day.
Now imagine that the bottom of the dry (environmental line) reaches the surface at 20° later in the day
The later midday sounding shows that we were correct with the earlier interpretations.
(It should be clear that once cumulus has formed, it will rapidly grow to great heights)
You will need to work this out for yourselves.
The experts will note the super-adiabatic very near the surface.
|Blue or not blue? This was a fairly predictable blue day. Let's see why.|
|The surface dewpoint is around 3°C
Follow the moisture content (purple line) up to the right
It would not intercept the ELR until about 700 mbs
Unless totally impossibly high surface temperatures of around 42° are reached, a DALR (from a realistic temperature) meets the ELR well to the right of the moisture content line; no cumulus can form
If the dewpoint were a little higher, or the air at 900 to 800 mbs a little cooler, then cumulus would develop.
In forecasting, there has to be inspired (and experienced) interpretation in situations like this
|Many days are touch and go whether or not blue. Forecast soundings can be used to see the likelihood of a blue day, but all these factors make predicting with certainty very difficult in some situation.
Many days are easy. There will definitely be cumulus or it will definitely be blue. But so many in Britain are borderline