How to Find Thermals
Thermals are upflows of air. They occur as the air nearest to the ground heats up and becomes lighter than the surrounding air, causing it to rise until it meets air of equal density. These upflows are used by gliders to remain aloft for hours without the need for an onboard power source. To find these phenomena, you need to read the environment and understand what types of terrain and weather are best suited for the development of thermals.
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Things You'll Need
- Topographic charts or an understanding of the geography of the local area
- Aviation weather forecasts (METARs and TAFs)
Instructions
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How to find thermals
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Look at the historical data for your area. Learn how quickly the area warms. This will give you an idea of what to expect as a typical day progresses.
Thermals are typically stronger during the summertime, due to the increased amount of solar radiation received. Areas that usually have early afternoon thermals are dark, level fields, or asphalt. Bodies of water absorb heat much more slowly than land, but also release that heat much more slowly, creating the potential for thermal activity in the evening when the surrounding land has already cooled.
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Look at the aviation weather forecasts, such as the Terminal Aerodrome Forecast (TAF). These reports come out every day and cover a 24-hour period. TAFs can be found at the Aviation Digital Data Service, or ADDS, website.
The website also provides information on convective activity, which is caused by thermal activity, as well as other useful information for pilots. The information available can be somewhat confusing, so go to a local airport and ask a pilot for help in understanding what the TAF says.
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As the day progresses, watch the clouds. With an air front moving through the area, convective activity will show up as cumulus clouds. These clouds build and fall in three stages--updraft, mature and downdraft.
The updraft stage is the first stage and is what begins to build the clouds. At this point, the thermal is at its strongest and can be effectively used for gliding. However, during the mature and downdraft stages, the flow of air is reversed. This can lead to a dangerous situation in an unpowered aircraft. Severe downdrafts can have speeds exceeding 57 miles per hour, or 5,000 feet per minute. To be safe, don't attempt to use cumulus clouds as a source of thermals until you have gained a significant amount of experience in judging the formation of storm clouds.
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Watch for birds, especially birds of prey. Eagles, hawks and falcons all stay aloft looking down on the world for their next meal. Birds are masters at using thermals to their advantage, remaining airborne for long periods of time without exerting additional energy. Watch where these birds congregate and see what types of terrain they stay over. This will give a good indication of thermal locations.
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Tips & Warnings
To help you in learning how to read clouds and weather to find thermals, talk with glider pilots. An experienced glider pilot can remain airborne for hours using nothing but thermals.
Do not attempt to fly in unstable air, such as squall lines, as the winds they generate can rapidly shift direction and intensity, causing disaster if you are at a critical phase of flight when these changes occur.
