AT A WARM FRONT:
If the warm air is dry, then expect no clouds or some lower, middle or high stratiform cloud with no precipitation.
If the warm air is moist and stable, then expect layer clouds in the following order as the front approaches your station:
- Cirrostratus (halo is a classical indicator of an approaching warm front)
and continuous precipitation (stratus fractus and frontal fog forming in the precipitation).
If the warm air is moist and unstable, then expect the same clouds as the humid and stable air mass with the addition of cumulonimbus embedded in the layer cloud. Precipitation will be continuous with especially heavy precipitation beneath CB's due to additional showers.
AT A COLD FRONT:
If the warm air is dry and stable, expect very little cloud but some SC/AC possible, no precipitation
If the warm is moist and stable and...
- if advancing slowly, then expect wide spread layer clouds, and intermittent or continuous precipitation.
- if advancing rapidly, then expect a narrower band of layer clouds, and intermittent or continuous precipitation.
If the warm is moist and unstable, expect heap (cumuliform) clouds and showery precipitation.
When approaching an active warm front from the cold air side, precipitation begins in the region of the altostratus cloud deck between 2,500 and 3,000 metres (aprx 8200 et 10000 feet) above the ground. Flying toward the front at low levels from the cold air side, a pilot may encounter snow, snow and ice pellets mixed, ice pellets and freezing rain mixed, and then freezing rain, in succession. At higher levels, but still in the cold air, no ice pellets would be encountered as you would encounter snow and then freezing rain.
Two regions in particular that may present icing problems for aircraft are situated in the cloud of the overrunning warm air, and in the precipitation that falls from the warm air into the cold air.
Icing in the precipitation falling into the cold air
- SN - Snow does not present an icing problem unless it is wet and sticks to the plane and forms ice.
- FZRA - Freezing rain causes a rapid build-up of ice and can cause severe icing.
- PL - Ice pellets have no effect on icing, but note that, since they are frozen raindrops, they always indicate freezing rain aloft.
2nd EditionAndrew Friedland, Rick Relyea 7th EditionMary Cunningham, William Cunningham 3rd EditionAndrew Friedland, Rick Relyea