Using the SkewT-logP diagram
Copyright Robert Hart
For glider pilots, the weather is everything. Unless the weather cooperates by producing lift that a glider can use, any glider flight is going to be a fairly short glide back down to the ground.
There are several sorts of lift used by gliders, but the most commonly used here in Australia is that of thermals - rising columns/bubbles of warm air. A bubble of air that is warmer than the surrounding air is less dense than the surrounding air and so weighs less than the same volume of that cooler, surrounding air. As it weighs less, the surrounding air pushes it up - and the bubble rises. The bubble will continue to rise until it is the same temperature as the surrounding air - when it weighs the same and so it is no longer pushed up. Things get a bit more complicates because the issue of air pressure must be taken into account.
Air pressure decreases with altitude and our warm bubble of air must always be at the local pressure (as it is not confined). This means it expands in volume as it rises and this spreads out the heat energy over the larger volume and the temperature of the air falls. This rate of cooling (known as the dry adiabatic lapse rate or DALR) is 3o Celsius per 1,000ft.
So, provided the surrounding air temperature is falling by 3o Celsius per 1,000 ft, our bubble will continue to rise for ever! Well not really, as this model really only applies in the bottom part of the atmosphere (known as the troposphere). What normally happens is that a bubble of warmer air will rise to some height at which it has cooled to the temperature of the surrounding air mass - and at that point it stops.
Glider pilots want to know what that height will be as this will be the maximum height to which they can climb - and they may not be able to climb that high as cloud may well form in the rising bubble due the the moisture it contains condensing.
The air around us contains a fair amount of moisture, existing as water vapour. The amount of water that can exist as vapour in air is limited by the air temperature - the warmer the air, the more water it can hold as vapour.
In earlier paragraphs, the dry adiabatic lapse rate (DALR) was mentioned. For air containing water vapour, as it rises through the atmosphere, it will cool at the DALR (even though it contains water vapour) until it reaches what is known as the dew point - the temperature at which the air can no longer hold all the water as vapour. As the air cannot hold all the water as vapour, some of it condenses into tiny droplets of liquid water and we see this as cloud.
Once cloud has started to form, as the air contimues to rise it now cools at the saturated adiabatic lapse rate (SALR), which is less than the DALR due to the warming effect of the energy released by the condensation of vapour into water droplets. (It is this effect that causes the lift to increase inside the cloud and this in turn sucks up the air faster just below cloud base, causing the increase in lift often found below but close to cloudbase.)
So, in order to work out if the weather will give us good thermals to a reasonable height, we need to know some details about the atmosphere - in particular how the temperature and dew point vary with altitude. This is measured by taking a sounding - sending up a weather balloon that, as it climbs, measures this data and transmits it back to the ground. Unfortunately, most places we want to glide are quite a long way from the weather stations that actually take soundings, but with the wonders of modern technology, we can obtain a forecast atmospheric sounding for any latitude and longitude we like. The US National Oceanic and Atmospheric Administration (NOAA) provides a web site that does this here.
When you get to the NOAA page, you will need to enter the lat/long of the place for which you want a sounding in decimal degrees. I fly mainly from Darling Downs Soaring Club (DDSC), which is located at -27.351 151.512 (ie S27.351o, E151.512o). Once you have entered the location, click Continue. This brings up a new page with a great number of options.
What we want is a sounding. In the Soundings drop down box, select GFS Model (0-84h, 3hrly, Global).
This brings up a new page and you will need to select the time for which you want the sounding - in Universal Coordinated Time (UTC - what in the good old days was known as Greenwich Mean Time and what is somewhat confusingly also known as Z or Zulu time as you can see from the line at the bottom of the plot).
As Darling Downs Soaring Club is 10 hours ahead of UTC, choosing today's date at 0300 UTC will give us 1300hrs (or 1pm) local time (EST), which will give us a pretty good idea of what is going to happen. To get a plot for tomorrow, select tomorrow's date and choose 0300 UTC and so on.
Choose no animation, full sounding, graphic and text, the SkewT Log-P and the plot resolution required (I use 1200 pixels as this gives me a plot that is large enough to be usable on screen or printed), enter the displayed access code (which is there to stop automated systems asking for plots from overloading the NOAA computers) and press Get Profile.
You will then get a plot looking something like the following.
There is a great deal of information on this chart! In pink, along the bottom are the lat/long, date (Note: this is in US mm/dd/yyyy format) and time Z (Zulu or UTC)
Looking at the plot, the blue numbers on the left hand side are atmospheric pressure (in mBars or hectoPascals). Pressure decreases with altitude, and this table gives the translation from mBars into feet using the NASA atmospheric model from the surface to 20,000ft.
The squiggly red line is the measured temperature (called the environmental lapse rate or ELR) and the squiggly green line is the measured dew point line. The temperature axis is skewed (which explains that part of the name of the plot) so that lines of equal temperature (red lines with red numbers on the right of the plot) run from bottom left to top right. The black lines (you can see they are actually slightly curved) running from bottom right to top left are the dry adiabatic lapse rate (DALR) lines.
The short red lines running from the bottom of the plot part way up to the right (with small red numbers above them) are mixing lines. The black, slightly curved lines running almost vertical on the right of the plot but curving away to the left on the left of the plot are the saturated adiabatic lapse rate (SALR) lines. The temperatures for these lines are in black (on this diagram at about the 200mb pressure altitude). These two families of lines are important for cloud predictions.
So, to find out if the weather is going to be any good for gliding, all we need to add is the predicted surface temperature, which for this particular day was (according to the Bureau of Meteorology) predicted to rise to 36oC at Dalby.
We now have to correct for the altitude of the site, which is 1,200ft (and why the plot does not go all the way to 0ft pressure altitude). A 36oC temperature at this height places us on the 39.6o dry adiabatic line (36+1.2*3). Alternatively, pick the 36oC point at ground level as the starting point for the DALR.
Now, along the bottom of the plot, we slide up the dry adiabatic line - that is, from the 36oC point at the surface (1,200ft), imagine a line parallel to the adjacent DALR lines and slide up it (it's easier to do this if you print out the plot). This intersects the squiggly red line (ELR) at just under 700mb (say 9,500 amsl).
Checking the Area 40 forecast (at Airservices Australia and follow the area briefing link), we find that they were forecasting
OCNL MOD THERMALS BELOW 9000 INLAND FROM 00 TILL 06
(ie from 10am to 4pm EST as times on briefings are UTC) on this day, which suggests that the prediction from the SkewT-LogP plot agrees pretty well with the Bureau of Meterology! It is worth noting though that the area forecasts do not often include thermal predictions, but do refer to "turbulence" of various strengths below a certain height fairly frequently.
Whatever the temperature, the strong low level inversion (rising temperature with increasing height) meant the thermal activity would not start early, as early warmed air will only rise to the inversion level until it is warmer than the air at the inversion level.
Many thanks to George Lee for his guidance on this section
In order to decide if there is going to be any cloud and if so at what height, we need to look at the surface dew point - which is approximately 13oC. (Note: theory requires an early morning dewpoint value must be discounted by 2 to 3 degrees to allow for the convection process). As there is 06:00(est) data on line for Dalby and Oakey (close to DDSC), these dew points can be fed into the considerations as a cross check with the NOAA surface dew point prediction.
From this dew point value, now draw a line parallel to the mixing lines (the short red lines sloping up from left to right) through the (wiggly red) ELR line.
The maximum cloud base height is shown by the point where this line cuts the DALR line from the maximum temperature predicted for the day.
Now draw a line parallel to the DALR lines back down to the surface from where the mixing line cuts the ELR. The point where this intersects the surface line gives the temperature required for the first cloud to form (and the height of the mixing line ELR line intersection gives the initial cloud base).
To get the maximum cloud top height, draw a line parallel to the (curved) SALR lines from the max cloudbase point up until it intersects with the ELR. That intersection gives the maximum predicted cloud top.
If the forecast talks of thunder cloud (Cumulo Nimbus), for these to become a reality, there must be a significant gap between the SALR line we have just drawn and the ELR.
There is one other point worth noting: if the dew point line approaches close to the ELR line, then there could well be problems with additional cloud. When the dew point line approaches close to the ELR, it indicates that there is high humidity and small perturbations in temperature or humidity can result in significant additional cloud, even well below the expected cloud base. This gives rise to the typically raggedy based clouds associated with widespread rain.
When this occurs at mid to high levels, there could well be problems with convection being suppressed to some extent by the formation of mid to high level cloud.
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