1. What is FRZ?
FRZ (freezing level) is the pressure level in the troposphere where the temperature is freezing (32 F, 0 C).
2. How is the FRZ located?
FRZ is located as the intersection between the 0 degree C isotherm and the temperature sounding. In the sounding below the FRZ level is 571 mb. You can verify this by following the 0 C isotherm from the bottom of the diagram up to the 571 mb pressure level. At this pressure level the sounding temperature will be intersected. To get our bearings, at 571 mb, the dewpoint is -10 C, temperature is 0 C and theoretical temperature an air parcel raised from PBL would be is 4 C.
3. Operational significance of FRZ:
Winter weather: A FRZ level at the ground indicates precipitation type will most likely be snow or freezing drizzle. The identifier BG (Below Ground) indicates that the surface temperature is below freezing. FRZ can be used to determine at what elevation in mountain areas precipitation type will change from rain to snow. FRZ can be used to get an idea of how much evaporative cooling is needed to lower the FRZ level to the surface. Since evaporation and melting are cooling processes, the FRZ level will lower toward the surface when precipitation falls into unsaturated air.
Hail: In a severe thunderstorm environment, a low FRZ level indicates hailstones will have more time to grow in the updraft and will have less time to melt as it falls to the surface. A FRZ level with a pressure level of 650 mb or closer to the surface in a severe weather situation generally will support large hailstones.
Icing: Used to determine elevations aircraft will experience icing within clouds and during precipitation.
FRZ level can change rapidly after initial sounding time. Atmospheric lift or sinking, thermal advection, convection, and evaporative cooling can rapidly change the freezing level. Pay attention to forecast soundings to see how the freezing level is expected to change.