What data is shown?
The header includes the date/time of the observations, the station used, the forecast high temperature used in the t.i. calculations, and a cloud base estimate. The cloud base estimate is based entirely on the dewpoint depression (temp minus d.p.), and thus represents the height that clouds will form IF they form at all. The first table is raw data for the lowest 10 levels reported. VirtT is the virtual temperature, and is explained below. The next table shows the TI values, wind data and trigger temperature at 500 foot intervals. The trigger temperature is the ground temperature for which the dry adiabat intercepts the temperature graph, i.e the temperature which will produce a TI value of zero at that altitude. The last table is raw data: P = pressure in millibars, H = height in feet, Tv = virtual temperature in degrees Celcius, T = temperature in degrees Celcius, DP = dewpoint in degrees Celcius, wind direction in degrees, wind speed in knots.
What is a thermal index?
The thermal index at a given altitude is the difference between the actual air temperature and the temperature that a parcel of air would have if it started at the surface and rose adiabatically (as it does in a thermal) to that altitude. Negative values mean that the air parcel is at a higher temperature than the surrounding air, and therefore the air will continue to rise. The altitude for which the t.i. reaches zero (where plot lines cross) can be used as an approximation for the maximum height of thermals for the day. The maximum altitude a sailplane may reach may be lower. Some people use the threshold t.i. value of -3 to estimate the highest they will be able to fly. Due to continuous mixing in the atmosphere, the actual difference between a rising air parcel and the surrounding air is usually not more than .1 C, however. Therefore the t.i. value is not very useful in determining thermal strength. The maximum height of convection is more important.
Where does the data come from?
Texas A&M University obtains weather data from the National Weather Service via a service provided by Alden Electronics. Twice a day, at 0Z (8:00PM C.S.T.) and 12Z (6:00AM C.S.T.), at about 150 locations in North America, weather balloons are sent up to gather data on temperature, pressure, dewpoint, wind speed and wind direction. The data is usually available on the computer within an hour after the observation. The 12Z sounding is particularly useful in the United States for forecasting soaring conditions. This link has chosen the reporting locations nearest to the flying site for its upper atmosphere data and forecast high. These two locations are not necessarily the same since there are more forecast high reporting locations than upper air data reporting locations.
When does the data arrive?
The 12Z data usually starts arriving about 1245Z, but it may be until 1330Z before some station data arrives. For unknown reasons, sometimes certain station data won't arrive at all.
What good is the 0Z sounding data?
The 0Z sounding data can tell you much about the soaring conditions that occurred that day. The convection during the day mixes the atmosphere, and so you will see the actual lapse rate very close to the dry adiabatic lapse rate from the surface up to some altitude. That altitude is either cloudbase or the maximum thermal height. This can be very useful for evaluating days you didn't fly, especially blue days.
The Cloudbase Estimate
The cloudbase estimate is based on taking the average dewpoint in the lowest 500 meters (1600 feet) of the atmosphere as the surface dewpoint. The cloud base (in thousands of feet) is then estimated as the dewpoint depression (temperature minus dewpoint) in F divided by 4.4.
Virtual Temperature
The presence of water vapor in the air makes air less dense than dry air at the same temperature and pressure, the difference depending on the mixing ratio, or water to air ratio (by weight). The virtual temperature is the temperature a dry parcel of air would have at the same pressure and density. Since the buoyancy of an air parcel depends entirely on the density, it makes sense to calculate the thermal index based on the virtual temperature rather than the actual temperature. The difference, however, is usually small, being less than 1 degree in dry conditions, and only 4 degrees in extreme humidity (dewpoint >70 Fahrenheit).