METEOROLOGIST JEFF HABY
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
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.