On weather maps, the boundary between warmer and cooler air is
called a front. If the cold air
advances, it is a cold front. If the
cold air retreats, it is a warm front.
If the boundary doesn't move very much, it is a stationary
front. If the cold front catches up to and merges with the warm
front, the result is called an occluded front.
Another type of front is a dry line. These are explained in more detail below.
Fronts often have clouds, precipitation, strong winds, and turbulence - - all could be flight hazards, as explained below.
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— Flying between cloud layers near a warm front.
Fronts often rotate around a low-pressure center (a "low") similar to spokes around a wheel hub (see Fig. 12.1 below). Also below is a list of the symbols for different fronts. In that frontal list are some definitions: frontogenesis = birth of a new front; frontolysis = death of an old front; squall line = line of convection / thunderstorms along or ahead of a front.
Temperature is not the only thing that changes across the front.
Also changing are wind speed and direction, clouds, humidity, etc. The
diagrams below (from Stull's book "Practical Meteorology") show some of these changes for cold fronts, and for warm
fronts. Isotherms show temperatures. Arrow lengths indicates wind
strengths. These diagrams show how weather varies in the horizontal.
Cold front at the Earth's surface. Actual
temperatures depend on the season.
Warm front at the Earth's surface. Actual temperatures depend on the season:
Cold front (a vertical slice through the atmosphere):
.
Warm front (a vertical slice through the atmosphere):
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— Figures courtesy of US FAA, AC 91-74B (2015).
![]() —Thunderstorms along a cold front. |
![]() —Stratus clouds ahead of a warm front. |
Occluded fronts come in two flavors: cold occlusions and warm occlusions. They both occur when a cold front catches up to a warm front. Where very cold air catches up to, and under-rides, cooler air, then the result is a cold front occlusion. Where cool air catches up to, and over-rides, colder air, then the result is a warm front occlusion.
Regardless of the type of occlusion, there are two characteristics that are important to pilots:
![]() — Thunderstorm (right center) between two stratus layers. |
![]() —Thunderstorm (right center) between two stratus layers. But this thunderstorm is harder to see. |
The boundary between dry and humid air of virtually the same temperature is called a dry line. Don't let the word "dry" deceive you - - it can still be dangerous. Drier air is denser than the moister air of the same temperature. The less-dense moist air will rise over the more-dense dry air, which can trigger thunderstorms along the dry line. Although a dryline cannot be called a cold front or a warm front (because the temperature is often nearly the same on both sides of the front), it behaves very similar to a cold front in its ability to trigger thunderstorms. See more info above and below about cold front hazards.
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— Courtesy of NOAA Aviation Weather Center. In this example, a low (L) in Ontario has warm, cold, and occluded fronts attached. A stationary front crosses the east coast of the USA and another is in Newfoundland-Labrador. A dryline extends through western Texas from a low (L) on the Texas - New Mexico border.
Key words: front, cold front, warm front, stationary front, dry line, frontolysis, frontogenesis, squall line, cumuliform clouds, stratiform clouds, occluded fronts, cold occlusions, warm occlusions
Extra info for experts; not needed for this course.
- FAA AC 00-6A Aviation Weather, Chapter 8 on fronts.
- NOAA Aviation Weather Center (AWC) weather maps and forecasts
- NavCanada/ECCC Surface Weather Maps
- TC AIM. Scroll down this web page to find and download the pdf file for the "MET - Meteorology" chapter. Open the pdf and go to Section 10.0 on Surface Weather Maps.
Image credits. All the photos were taken by Roland Stull, and the drawings were made by Roland Stull, and all are copyright by him and used with his permission. The exception are the color frontal cross sections, courtesy of the US FAA.