Plotting Air Temperature, Dew Point Temperature, and Wet Bulb Temperature

Air temperature is typically plotted on Skew-T charts with a solid red line, while dew point temperature is plotted with a solid green line. These two lines represent the environmental air and dew point temperature with height in the atmosphere. Oftentimes, between the solid green line and solid red line, observed soundings will plot a thin blue line representing wet bulb temperature. Wet bulb temperature represents the air temperature the environment would have if water vapor were continually added to the air through evaporation until saturation, in other words the air can no longer “hold” more moisture. The wet bulb temperature is always between the dew point temperature and the air temperature because adding water vapor increases the amount of moisture, and thus the dew point temperature, while the process of evaporation is also cooling the air temperature. From this perspective, the dew point temperature is increasing and the air temperature is decreasing, so the wet bulb temperature will be where these two meet in the middle.

There are many important applications for wet bulb temperature, with the most important being precipitation-type forecasting. If snow is falling into a relatively dry layer with air temperatures above freezing, the snow can sublimate (go from solid to gas) moisture into the air, effectively cooling that layer until it reaches saturation. This can cause the temperature of that layer to fall below freezing and promote snow to form in a layer which would have otherwise been too warm for snow.

Air, dew point, and wet bulb temperature are plotted with height on a Skew-T with atmospheric pressure representing the height variable. As height or altitude increases, air pressure always decreases, which makes air pressure a reliable variable representing height above the surface.