Calculating Trigger Temperature

If you want to fly long distances you need to get going early. The best way to make that happen is to be ready when the thermals start to pop; but how do you know when that will be? Today's post will address how to determine Trigger temperature. My next post will cover how to estimate when trigger temperature will be reached. Like my previous post, the information for this post comes from Dennis Pagen's book, "Understanding the Sky" which is required reading for anyone serious about soaring XC.

The first step to this process is to find the skew-T for the location you plan on launching from. The easiest way to get a skew-T is thought XCSkies point forecast tool or use the tools provided by Dr Jack.To find the trigger temperature you will need to determine the height of the ground inversion layer and the lapse rate of the air layer above it. In the figure below, taken from Understanding the Sky, a notional skew-T chart is shown with only the relevant information displayed.

The ground inversion layer, usually about 1000'-2000' thick, is caused by the relatively cold temperature of the ground at night cooling the air layer closest to the ground. This is the same mechanism that forms the superadiabatic layer near the ground when the surface heats up and warms the lower layer of air. It isn't until the ground inversion layer is warmed to match the lapse rate of the air above it that thermals will escape the ground inversion layer; this warming process is shown in the figure above as the dashed lines. To find trigger temperature, draw a line starting where the temperature profile begins to decrease and follow the dry adiabatic lapse rate (DALR) lines on the skew-T to ground level. The temperature where the DALR line intersects ground level is trigger temperature, represented in the figure as the far right dashed line.

We now know that for trigger temperature to be reached the early morning solar heating must eliminate the cold ground inversion layer, this means that on days following cold clear nights where the ground inversion is thick trigger temperature will occur later in the day.

Hopefully this helps take the mystery out of calculating trigger temperature. Until next time..

Keep soaring,
Michael