1. What is WB0?

WB0 is the wet bulb zero pressure level. This parameter is important since evaporation in the troposphere will change the temperature structure within the troposphere. Evaporation is a cooling process and the amount of cooling depends on the quantity of evaporation that occurs. Full evaporation into air that previously had a large dewpoint depression will result in significant cooling. This cooling, if occurring through a deep layer of the lower and middle troposphere will lower the FRZ (freezing) level closer to the surface.

2. How is the WB0 located?

To find the WB0, the computer program first finds the wet bulb temperature for every pressure level. This is done by artificially saturating the troposphere (evaporating moisture into the troposphere until the relative humidity is 100% throughout the tropospheric column). Once the troposphere is saturated, the new FRZ level is the WB0 level. The WB0 is always closer to the surface than the initial FRZ level, unless the troposphere was already saturated in which case they would be equal. The sounding below shows a wet bulb zero level of 625 mb. The FRZ level is 571 mb. Thus, if complete evaporation occurs the freezing level will drop from 571 mb to 625 mb.

3. Operational significance of WB0:

Winter weather: If the FRZ level is aloft while the WB0 level is at the surface, that is an indication that precipitation may first begin as rain, then change over to winter precipitation after evaporational cooling occurs. WB0 is a better indication of what the FRZ level will be once precipitation moves into the area and saturates the lower troposphere from aloft.

Hail: Dry air in the middle levels of the atmosphere will enhance the hail potential since the entrainment of dry air into the thunderstorm air will produce evaporation and a lowering of the FRZ level. Dry middle level air also enhances convective wind gusts from storms since the intense evaporational cooling supports strong downdrafts. Evaporation cools the air making it negatively buoyant, thus accelerating the air toward the earth's surface since it has a higher density.