EXCLUSIVE TO THE VALLEY PATRIOT
By: Al Kaprielian – June, 2015
On a surface weather map we commonly see what we call “fronts”.
A front is caused by differences in air pressure in the atmosphere. Fronts separate two different air masses.
An air mass is defined as a region of air where the air around it has similar temperatures and dew points.
Dew point refers to how much moisture is in the air expressed in degrees. When the temperature and dew point are both the same 45/45 the air is totally saturated with moisture.
Warm air is lighter and rises and holds more moisture.
Cooler air is heavier and sinks and holds less moisture.
TYPES OF WEATHER FRONTS
There are different types of fronts. There are cold fronts, warm fronts, occluded fronts and stationary fronts.
A cold front brings cooler & less humid air into the region. A warm front brings warmer, more humid air into the region. An occluded front occurs when the cold front catches up and overtakes the warm front. A stationary front means a front has stalled (stopped moving) in the atmosphere. This is usually the result of the winds in the upper atmosphere being parallel to the front. Cold fronts usually move from west to east but not always.
A backdoor cold front can occur most often in the spring and it usually moves from northeast to southwest. A backdoor cold front can be the result of differences between the temperature of the land versus the ocean.
Remember, the ocean warms more slowly than land in the spring because of evaporation and mixing.
The ocean is a fluid, which causes it to constantly move. Due to the fact that the ocean constantly moves it takes more heat from the sun to warm the ocean one degree versus the land.
Evaporation occurs every day (365 days) over the ocean. Evaporation is a cooling process in that it cools the air over the ocean.
LAND vs OCEAN AIR MASSES
Since the ocean is colder than the land in the spring months, this causes the air to be heavier and more dense over the ocean. Meanwhile, the air is warmer and lighter over the land areas. This causes an imbalance of air pressure and hence, a front forms.
The colder air over the ocean has a higher air pressure in that it is denser. The warmer air over the land is lighter and therefore has lower pressure.
With the approach of a backdoor cold front, there can be some gusty winds. Winds are also caused by differences in air pressure. The wind direction ahead of the backdoor cold front is usually south/southwest. Behind the front, winds shift to northeast/east and temperatures can drop sharply.
Fronts cause clouds and precipitation because there is a clash between warmer air and cooler air. This clashing of warmer & cooler air causes an imbalance for the atmosphere. Warm air & cool air are opposites. The atmosphere wants to be balanced and when fronts approach, it is no longer balanced. This imbalance causes clouds and precipitation to form.
Clouds form when the air rises. As this air rises it cools due to expansion (the spreading out of air). When the air cools, this causes it to cool to the dewpoint.
When this occurs, the air is totally saturated and as a result, condensation occurs. This condensation causes moisture to form in the way of clouds. When the clouds grow and get large enough, they cannot hold any more water droplets so the clouds burst and then rain falls, or in the winter months it can be snow/sleet.
In the winter, the precipitation can be snow. Fronts usually bring unsettled weather with clouds and precipitation because there is a clash between two different air masses.
When fronts stall (stop moving) this can cause rain and unsettled weather to continue for a few to sometimes several days. An example is May 31 into June of this year, when a cold front stalled just south of New England.
In conclusion, fronts usually bring unsettled weather and in the spring and summer thunderstorms, which can become severe and produce tornadoes can occur along and ahead of a cold front. Fronts can play an important role in our weather and can bring an end to sunshine and a dry stretch of weather. Next time you see fronts on a surface weather map, I feel this article will give you a much better understanding of how they can impact our weather.