For thermal soaring conditions, a pilot is primarily concerned with the expected surface heating available, and the stability of the air mass. ## Surface Heating For [[Thermal Formation]] and thermal soaring conditions, a pilot is concerned with the value and trend for surface temperature throughout the day, as well as causal factors for surface heating. ### Surface Temperature Even non-pilots care about surface temperature, so there are many resources available for the forecast surface temperature throughout the day. These resources include: - [[GFA: Temperature - Aviation Weather Center](https://aviationweather.gov/gfa/#temps)]() - [[NOAA's National Weather Service - Graphical Forecast](https://graphical.weather.gov/)]() - ForeFlight Daily Forecast - SkySight ### Causal Factors Surface temperature forecasts generally account for the factors that cause large-scale variations in temperature, but thermal soaring conditions are contingent upon localized [[Heat Imbalances]], so some additional products can be used to address the causal factors associated with local surface heating. - [[GFA: Clouds - Aviation Weather Center](https://aviationweather.gov/gfa/#clouds)]() - [[GFA: Precipitation - Aviation Weather Center](https://aviationweather.gov/gfa/#pcpn)]() ## Skew-T Log-P Analysis Using [[Skew-T Log-P Diagrams]], a pilot can observe the stability of the air mass, and determine the strength and maximum altitude of thermals given a forecast for temperature throughout the day. ### Locating an Appropriate Skew-T Log-P Diagram If the flight is being conducted near a location with [[Accessing Skew-T Log-P Diagrams#Observations]], use the morning sounding. If this is not available, use the most relevant [[Accessing Skew-T Log-P Diagrams#Forecast Models]] data available. ### Basic Analysis Procedure Begin with the forecast surface temperature for the proposed time and location for the flight. Plot the temperature and airport surface pressure as a point on the Skew-T Log-P diagram. From this point, draw a line that parallels the dry adiabats until it intersects the environmental temperature line. This line represents a **dry** thermal rising through the surrounding air at that time and place. ### Thermal Index [[Skew-T Log-P Derived Parameters#Thermal Index (TI)]] is simply the expected temperature differential between a thermal and the environmental air, at a given level. It can be calculated for a given level by comparing the temperature of the environmental temperature line and the thermal line derived from a given surface temperature. Generally, -2 is considered the highest value for useable thermals in otherwise good conditions ### Trigger Temperature Trigger temperature is the minimum surface temperature required for thermals to form and rise above the low altitude inversion (if present). How high the thermals must go is not agreed upon; some [[Soaring Forecasts]] calculate trigger temperature from a thermal rising to 3000ft AGL, based on a morning sounding. ### Cloud Bases To estimate cumuliform cloud bases, begin with the surface dewpoint, or better, an averaged dewpoint for the first 50mb. From this point, draw a line that parallels the mixing ratio lines until it intersects the environmental temperature line. This indicates where saturation and condensation will occur, and is known as the **convective condensation level**, or CCL.