Poor, possibly advection fog in winter — Page 313, Lesson 303

I want to walk you through the weather conditions you’ll find in the warm sector of a polar front depression, specifically in winter, and then what happens at and behind the cold front. We’ll also look at the upper winds associated with the whole system. Let’s start with the warm sector. In winter, the warm sector often brings poor visibility, possibly advection fog. Advection fog forms when warm, moist air moves over a colder surface, and that’s exactly the kind of air mass you get in the warm sector. So if you’re flying in winter and you hear you’re in the warm sector, expect low visibility and be alert for fog. Now, let’s move to the cold front itself. At the cold front, the surface wind direction and speed show a sharp veer — that means the wind changes direction clockwise, and you can expect gusts and squalls. The temperature falls suddenly, and the dew point also falls suddenly. The pressure starts to rise. The cloud cover is 6/8 to 8/8 — that’s six to eight oktas, so mostly overcast to fully overcast. The cloud base is low but rising. The cloud types are cumulus, cumulonimbus, and sometimes nimbostratus. Precipitation is heavy rain or snow showers, and thunder and hail are possible. Visibility is good, except in the precipitation itself. Behind the cold front, conditions change. The surface wind is steady or shows a slight veer to the northwest. Temperature is steady and low. Dew point is steady and low. Pressure rises slowly. Cloud cover is 6/8, with the base lifting, and the cloud types are cumulus and cumulonimbus. Precipitation comes as showers, heavy at times, and hail and thunderstorms are possible. Visibility is very good, except in the showers. Now, let’s talk about the upper winds in a polar front depression. At the height of the jet stream, which is about flight level 300, the wind is directly related to the temperature gradient. Because of that, the jet streams generally run parallel to the fronts. Here’s how the upper winds behave in different parts of the depression: Ahead of a warm front, the upper wind is from the northwest. A rapid movement of cirrus from the northwest is a good indication that a jet stream is above. That jet stream will be near the tropopause, parallel to and about 400 nautical miles ahead of the surface position of the front, and it’s in the warm air. Above the warm sector, there is little change from the geostrophic wind near the surface as regards direction, but the speed will be greater. So the wind direction stays roughly the same as what you see at the surface, but it blows faster. Behind the cold front, the upper wind is from the southwest. The jet stream will be near the tropopause, parallel to and about 200 nautical miles behind the surface position of the cold front, again in the warm air. So to summarise: in the warm sector in winter, expect poor visibility and possible advection fog. At the cold front, you get a sharp wind veer, sudden temperature and dew point falls, pressure rising, low cloud, heavy precipitation, and good visibility except in the rain or snow. Behind the cold front, conditions improve: steady low temperature and dew point, slowly rising pressure, lifting cloud base, showery precipitation, and very good visibility except in the showers. And aloft, the jet stream runs parallel to the fronts, with specific wind directions ahead of the warm front, above the warm sector, and behind the cold front.