The lapse rate is used to quantify the stability of the atmosphere in relation to thermal activity. The air is warmed by heat radiating off the ground. Air becomes less dense, and therefore cooler, as you go up in altitude. This means the air is normally warmer at the surface and cools with a gain in altitude, the rate it does this at is known as the Standard Lapse Rate (SLR) and averages 3.6°F/1000 ft. This is an average and a change in conditions, such as a cold front moving in, will increase the standard lapse rate and the rate of cooling. As a parcel of warmed air (thermal) rises through the atmosphere, it cools at 5.5°F/1000 ft, known as the Dry Adiabatic Lapse Rate (DALR. Less than 50%humidity). As long as our parcel of air is warmer and less dense than the surrounding air, it will continue to rise. This happens when the surrounding air is cooling at a rate faster than 5.5°F/1000 ft.
There are many ways to calculate the “stability” of the day. The methods often vary by region and some are more descriptive for certain areas.
Thermal Index
One popular method is the Thermal Index, in which you compare the lapse rate of the atmosphere with the Dry Adiabatic Lapse Rate and create an instability scale. The information you will need for a given day is:
1. An upper level temperature at a given altitude. It is often easiest to use the freezing level altitude; temperatures are given in Celsius.
2. The altitude and expected temperature where you will be flying (surface).For the temperature, we prefer to use what is known as ‘puddle’ temperature. This is an estimated temperature of the air mass near the surface which will be- come the rising parcels. One way to estimate this is to get the mean temperature between the actual surface of the ground and the ambient air temperature, since heat collects on surface and radiates up, diffusing along the way.
Example:
The freezing level, or 32°F is at 12,000ft
The average surface altitude is 3000ft, with a ground temp of 110°F and an ambient of 90°F,
giving us a puddle temp of ~100°F.
There is a difference of 9000ft between our 12,000ft temp and our 3000ft temp. That
difference is multiplied by the DALR:
12000ft – 3000ft = 9000ft * 5.5°F/1000ft = 49.5°
So there should be a temperature increase of 49.5°F from 12000 to 3000 ft to be stable. 32°F + 49.5°F = 82°F (rounded)
So 82° is the stable temperature for 3000ft. However, we expect the temperatures to reach 100° at the hottest part of the day.
82°F - 100°F = -18, this is the Thermal Index (T.I.) number.
Translating this number into usable information is difficult; there are so many factors and variances between locations. The best thing is to get the T.I. number along with all the other weather information, and if you go flying you can compare the number to the conditions you see on launch and experience in the air. To give you a very generalized scale, 0 to -10 is mild, -10 to -20 is moderate, and -20 to -30 is high, and over this can be extreme. This information is an extremely valuable tool for modeling the stability of the day and deciding if, or at what times of the day, the conditions will be appropriate for your level of experience.
Simplified Calculation
In many regions, it is simpler to get the temperature at some altitude above you, and the predicted surface temperature. A difference of 10°F or more for every 3,000 ft should be a soar-able and relatively smooth day. 12.5°F or more should be very thermic and
15°F or more will be extremely strong.