Frontal analysis
Today, we will continue our discussion of frontal systems. We will primarily be focusing on identifying them from a weather analysis map and satellite photographs. We will also discuss the weather and cloud formations one would expect to see as a particular type of frontal system passed over his or her location.
Frontal analysis
Frontal boundaries exhibit specific characteristics. Meteorologists look for those characteristics when diagnosing the presence or development of a frontal system.
Since a front is the leading edge of an air mass, with the front's passage, there is a marked temperature increase or decrease depending on whether the front was a warm front or cold front. Therefore, on a station analysis map, fronts can be identified by examining temperature trends.
Consider the weather analysis below where a warm front exists:
Notice the marked temperature increase on the back side of the warm front. Also, notice the wind direction shift. With the passage of a warm front, the temperature will increase and the winds will shift from a predominantly easterly direction to a predominantly southerly direction. You should be able to envision why there would be some low-level convergence at a warm front.
Now consider the cold front below:
Observe the sharp temperature change that occurs following the passage of the cold front. Also observe the wind direction shift that takes place. Following the passage of a cold front, winds usually shift from a predominantly southerly direction to a predominantly northwesterly direction.
Let's now take a look at what one may expect as different frontal systems approach with the aid of the table below:
| Weather element | In advance of the front | During frontal passage | Following frontal passage |
|---|---|---|---|
| Cloud type | Cirrus thickening into cirrostratus then altostratus | Stratus, nimbostratus | Cumulus congestus, cumulonimbus (if unstable), altocumulus |
| Weather | Increasingly cloudy | Steady light to moderate precipitation persisting for several hours | Broken skies, showers and some thunderstorms possible in unstable conditions |
| Temperature | Cool | Cool | Much warmer |
| Wind speed/direction | Light/Northeast-Southeast | Light/South-Southeast | Stronger/South-Southwest |
| Humidity | Dry | High | High |
| Barometer | Falling | Falling | Falling-steady |
| Weather element | In advance of front | During frontal passage | Following frontal passage |
|---|---|---|---|
| Cloud type | Cumulus congestus, cumulonimbus (if unstable), altocumulus | Cumulonimbus | Clear skies, stratocumulus if moisture is present in middle atmosphere, cumulonimbus in unstable conditions |
| Weather | Partly cloudy, some showers or thunderstorms in unstable conditions | Squally, intense precipitation of shorter duration. Strong to severe thunderstorms sometimes occur | Clearing skies |
| Temperature | Warm | Falling | Cold |
| Wind speed/direction | Breezy/south-southwest | Strong/west | Strong/west-north |
| Barometer | Falling | Falling-steady | Rising rapidly |
Refer once again to the maps above. Notice the light gray lines which are isobars. Remember that isobars are lines connecting points of equal barometric pressure.
Notice the "kinks" in the isobars that occur. This occurs because a front is a site of low-level convergence. This is another technique used by meteorologists to diagnose the presence of a frontal boundary.
Because there are characteristic cloud formations associated with frontal systems, fronts often leave characteristic signatures on satellite images.
Precipitation distribution near frontal boundaries
Because fronts are sites of convergence and because mechanisms that generate vertical motion are in place near frontal boundaries, they are often associated with abundant cloud development and precipitation. However, the nature of the precipitation is different when considering a warm front versus a cold front.
With a warm front, precipitation usually begins to fall several hundred miles in advance of the actual surface front. It usually is of light to moderate intensity and is usually steady, meaning it persists for long periods of time.
After the front passes, the skies usually break and the precipitation usually stops, or becomes much more scattered and isolated in nature.
With a cold front however, the cloud development tends to be much more vertical in nature. Precipitation typically will occur much closer to the frontal boundary. The nature of the precipitation will often be much more "squally" as well and it won't last as long. Because of the violent nature in which air is forced upward with a cold front, intense thunderstorms often develop along a cold front.
Refer to the two cross-sections below:
