Warm Front
A warm front is the boundary between a warm air mass and the colder air it is replacing, where the warm air rides up and over the denser cold air at a gentle angle. Warm fronts produce widespread, extended periods of cloud and precipitation, and are associated with some of the most hazardous icing conditions in Canadian aviation.
Warm Front Structure
The warm front slope is very gradual — typically 1:100 to 1:200 (1 ft rise per 100–200 ft horizontal). This means the zone of cloud and precipitation can extend 500–1,000 km ahead of the surface warm front position. The weather associated with a warm front develops slowly and can persist for 24–48 hours.
Warm Front Weather Sequence (Ahead of Front)
- First sign: high-level cirrus cloud at 24–48 hours before frontal passage
- Cirrostratus: halo around sun/moon (12–24 hours before)
- Altostratus: widespread grey overcast; sun dimly visible
- Nimbostratus: thick, dark cloud; continuous moderate rain or snow begins
- Fog, freezing rain, and ice pellets in the cold air ahead of the warm front
- At frontal passage: wind veers, temperature rises, pressure stops falling
Aviation Hazards
- Freezing rain (FZRA): The most dangerous hazard — warm air above, sub-freezing surface air below; widespread and prolonged
- Ice pellets (PL): Indicator of freezing rain above
- Embedded turbulence: Convective cells can be embedded in the stratiform cloud mass, invisible until entered
- Icing: Continuous moderate to severe icing in the warm frontal cloud
- Low ceilings and visibility: Can persist for extended periods
On the TC Written Exam
Warm fronts are extensively tested in meteorology. Know: gentle slope; widespread stratus and prolonged precipitation; freezing rain hazard; the sequence of cloud types; gradual weather change over many hours.
Related Terms
See also: Cold Front, Trowal, Freezing Rain, Icing.