Windgram Skew-T Secrets – Lifting Condensation Level and Thermal Trigger Times
The standard forecast tells you what is happening on the ground. The Windgram (or Skew-T) tells you what is happening in the sky.
For the wingfoiler, the vertical profile of the atmosphere determines if a thermal wind will trigger or if a "cap" will kill the breeze.
The Thermal Trigger Temperature
On a Skew-T chart, look for the surface temperature (Red Line). Then look at the air temperature aloft.
For a thermal wind to start, the ground must heat up enough so that a parcel of air becomes warmer than the air above it. This allows it to rise.
- The Trigger: The chart often marks a "Trigger Temp." If the forecast says the day will reach 24°C, and the Trigger is 22°C, the thermals will pop.
- The Cap: Sometimes, there is a layer of warm air sitting at 1000 meters (an inversion). This acts like a lid. If the thermals cannot punch through this cap, the sea breeze will be weak and flat.
The Lifting Condensation Level (LCL)
The LCL is the height where the rising air cools to its dewpoint and forms a cloud.
- High LCL: Clouds form high up. This leaves plenty of room for strong thermals to mix. This predicts a deep, strong wingfoil wind.
- Low LCL: Clouds form low (fog/stratus). This blocks the sun. Without sun, the thermal wind engine dies.
You want a "High LCL" day. This corresponds to a wide temperature/dewpoint spread on the meteogram.
The Wind Gradient Aloft
The Windgram shows wind barbs stacked vertically up to 10,000 feet.
- Aligned: If the wind direction at the surface is the same as the wind at 2,000 feet, they couple. The wind aloft mixes down. This creates a super-boosted 15–25 knots session.
- Shear: If the surface wind is West (thermal) but the wind at 2,000 feet is East (gradient), they fight. This creates turbulence and a "cap." The wind speed will be gusty and unreliable.
Summary
The Skew-T looks complex, but you only need three checks:
- Trigger: Will the ground get hot enough?
- LCL: Are the clouds high enough to let the sun work?
- Gradient: Is the upper wind helping or hurting the surface wingfoil wind?