A side view of a cold front (A, top) and how it is represented on a weather map (B, bottom). As a cold front moves into an area, the heavier (more dense) cool air pushes under the lighter (less dense) warm air, causing it to rise up into the troposphere.
Lifted warm air ahead of the front produces cumulus or cumulonimbus clouds and thunderstorms, like in the image on the left (A). There is a sudden drop in temperature, and also heavy rain, sometimes with hail, thunder, and lightning.
After a cold front moves through your area, you may notice that the temperature is cooler, the rain has stopped, and the cumulus clouds are replaced by stratus and stratocumulus clouds or clear skies. The triangles are like arrowheads pointing in the direction that the front is moving.
Notice on the map that temperatures at the ground level change from warm to cold as you cross the front line. A side view of a warm front (A, top) and how it is represented on a weather map (B, bottom).
Warm fronts often bring stormy weather as the warm air mass at the surface rises above the cool air mass, making clouds and storms. Warm fronts move more slowly than cold fronts because it is more difficult for the warm air to push the cold, dense air across the Earth's surface.
Warm fronts often form on the east side of low-pressure systems where warmer air from the south is pushed north. As the front passes over an area, the clouds become lower, and rain is likely.
Winds blowing parallel to the front instead of perpendicular can help it stay in place. On a weather map, a stationary front is shown as alternating red semicircles and blue triangles like in the image at the left.
Wind changes direction as the front passes and the temperature either warms or cools. On a weather map, shown to the left, an occluded front looks like a purple line with alternating triangles and semicircles pointing in the direction that the front is moving.
These air masses are designated P for “polar” (cold), T for tropical (warm), M for maritime (wet) and C for continental (dry). Prior to that, it was thought that if today happened to be colder, it was simply yesterday’s air with some heat that was lost to space.
But Bjerknes realized that there were masses of colder and warmer air that swept across the globe and bumped into each other and in the process produced zones of unsettled weather near and along their respective boundaries. Cold fronts are marked on weather maps with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel, and are placed at the leading edge of the cooler air mass.
With the cold front, warm air is rapidly forced upward (like the shavings) in advance of the actual front (the “cutter”), creating towering cumulus clouds, some hard showers and quite possibly a few gusty thunderstorms followed by a push of cooler and drier air in its wake. Winds out ahead of a cold front tend to blow from the south and southwest, and then shift after the frontal passage (called a “fro pa” by meteorologists) into the northwest.
But on occasion, a cool air mass might build up over eastern Canada and sink southward through northern New York and New England. Such a scenario is called a “back door” cold front and usually occurs during the spring and summer months.
The sensible weather associated with a warm front can stretch for as much as a thousand miles out ahead of it and as much as 36 to 48 hours prior to its actual arrival. Increasing moisture ahead of the warm front first arrives in the high levels of the atmosphere in the form of thin, wispy cirrus (ice crystal) clouds.
You might even catch sight of a halo around the sun or moon; sailors would interpret this as a sign that weather was likely to turn unsettled within the next 18 to 24 hours. And indeed, with the passage of time, these thin clouds gradually lower and thicken and eventually a steady light rain or drizzle will start to fall.
Near the actual frontal boundary, precipitation tends to become steadier and heavier and there could also be areas of fog as well. And especially during the winter months, the cold air that a warm front is attempting to displace is often heavy and dense and is stubborn to dislodge.
As a result, the shape of some warm fronts end up contorted, seemingly looping around the colder air mass. Stationary fronts are depicted by alternating red half-circles and blue spikes (pips) pointing in opposite directions, indicating no significant movement.
The resultant weather is usually low cloud cover and long duration precipitation, and not much in the way of wind. When this happens the warm air is forced up away from the ground, and their associated low pressure system is said to be occluded.
Occluded frontal passages are usually marked gusty winds and bouts of heavy rain, perhaps even thunderstorms. They are marked on the weather map by a purple line with alternating half-circles and triangles pointing in their direction of travel.
Finally, a trough (pronounced “trophy”) is an elongated region of relatively low atmospheric pressure, often associated with fronts that can occur either at the Earth's surface or at higher altitudes. The passage of an upper level trough might make its presence felt by producing a build-up of clouds, followed by a quick shot of precipitation, then clearing skies.
In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air. When two air masses meet together, the boundary between the two is called a weather front.
The greater the temperature difference between the two air masses, the stronger the winds will be. In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air.
A front may become stationary if an air mass is stopped by a barrier, such as a mountain range. A stationary front may bring days of rain, drizzle, and fog.
Spring and summer: The air is unstable so thunderstorms or tornadoes may form. Spring: If the temperature gradient is high, strong winds blow.
Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching. As the front approaches, alto cumulus and alto stratus clouds appear and the sky turns gray.
As the warm air mass approaches, temperatures rise and snow turns to sleet and freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog.
Coriolis Effect curves the boundary where the two fronts meet towards the pole. This is called a warm occlusion. The weather at an occluded front is especially fierce right at the occlusion.
Remember, a weather front is basically the boundary between two air masses of different densities. This means that weather is typically sunny within air masses, but their temperatures could vary with the season and humidity could vary based on the source region of the air mass.
Often times, they are part of a larger rotating system called a mid-latitude cyclone. This type of cyclone will be discussed later in this chapter, but as an introduction it is a low pressure system that is usually mixing warmer air from the south (in the Northern Hemisphere) and colder air from the north.
When air masses move from the areas they form in, to other areas, they can collide and form weather fronts in the places they meet. The zone may be 20 miles across or it may be 100 miles across, but from one side of a front to the other, one clearly would sense that the properties of an air mass had changed significantly (e.g., contrasts in temperature and dew point, wind direction, cloud cover, and ongoing weather).
The frontal zone represents the leading edge of a wedge of cold/cool air. If the wedge is retreating and warmer air is moving into an area previously occupied by cool air, the front is termed a warm front.
To locate a front on a surface map, look for the following: sharp temperature changes over relatively short distances, changes in the moisture content of the air (dew point), shifts in wind direction, low pressure troughs and pressure changes, and clouds and precipitation patterns. Cold fronts tend to move the farthest while maintaining their intensity.
In winter, cold fronts move into Oklahoma mainly from the Canadian prairies but sometimes from the Arctic Circle or the eastern Pacific. Cold fronts usually bring cooler weather, clearing skies, and a sharp change in wind direction.
Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (strati form clouds and light to moderate continuous rain). Behind the warm front, skies are relatively clear (but change gradually).
Clouds and precipitation are quite prevalent to the north of the warm front. This results from the fact that low-level southerly winds in the “warm sector” of the cyclone rise up and over the cooler, more dense air at the surface located north of the warm front.
The lifting leads to saturation, cloud formation, and, ultimately, to some form of precipitation. In Oklahoma, warm fronts are rare in the winter and non-existent in the summer.
Occluded fronts are indicative of mature storm systems (i.e., those about to dissipate). OK-FIRST Project, Oklahoma Climatological Survey, 100 East Boyd Street, Suite 1210, Norman, OK 73019.