The first clouds are thin, wispy cirrus, followed by sheets of cirrus and cirrostratus, and altostratus. The Sun is obscured as the altostratus thickens, rain begins to fall from nimbostratus and the cloud base is lowering. These phenomena describe the approach of a..
Refer to figures.
WARM FRONT. Can be defined as warm air replacing cold air.
The warm, less dense air is overtaking/displacing the cold air => warm air will ride up and over the denser cold air while advancing. The air behind the warm front is warmer and moister than the air ahead of it (when the front passes, the air becomes warmer and more humid).
- The slope of the warm air is approximately 1:100 to 1:150.
- Warm front is slower than a cold front. The front moves at right angles to itself at a speed equal to 2/3 of the geostrophic interval measured along the front.
- The warm air is mostly stable, the clouds that form are mostly stratiform. As the warm front approaches, we typically see this sequence of clouds: Cl, CS, AS, NS.
Cirrostratus CS: at FL200-250 gradually replace the Cl.
Altostratus AS: at FL150-200 as the front gets closer.
Nimbostratus NS: between 6000 - 8000 ft, approx. 150-200 NM away (in some cases up to 300 NM) until the front.
Stratus ST or Stratocumulus SC: can be encountered below the NS
Frontal FOG can sometimes be encountered 20-50 NM ahead of the frontal ground surface.
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