Air Masses And Fronts

By KD1LD

Almost all weather is caused by the movement of massive bodies of air. commonly referred to as air masses. An air mass is a large body of air having relatively the same horizontal moisture content and temperature distribution. It can be cold or warm and it can be moist or dry depending on it's source region. The two major source regions of air masses that affect us and our country are the polar and the tropical areas. Often the question arises of why air masses move. Lets consider the earth as a smooth sphere with heat occurring at the equator and cooling occurring at the poles. The warm air at the equator and the cold air at the poles would try to move so that a equilibrium will be reached between them. The cold moves southward and the warm air moves northward resulting in a regular pattern of waves in the middle latitudes, but the earths is not a smooth sphere. The water areas, land areas and mountain areas causes this wave pattern to become irregular. Lateral mixing continues as colder air moves south and warm air moves northward producing the movement of air masses within the atmosphere. Some colder air masses originate in the northern waters of the Pacific and the Atlantic. In both cases they are cold and moist because of there origin over water. At these higher latitudes. On the other hand, polar air moving down from Canada would be cold and dry. Dry because of it's origin over land.

Tropical air masses originate in the golf of Mexico and the Atlantic ocean at the same latitudes and flow into the United States. Under normal conditions the south pacific source region is of little importance as far as air masses are concerned in the United States. Since the circulation around the pacific high draws the air away from our continent. The greatest influence on air mass weather is the terrain over which the air mass is passing. Lets consider a polar air mass moving southward during the day. When the air moves over the warmer surface it becomes unstable. It is unstable because vertical currents will develop as a result of the cold air being heated and lifted from below. Now lets make an example. Place a container of cold water over a flame and allow it to heat. As the heating process continues you can see the vertical currents develop and become very lively. This is much the same that happens in a air mass when it is heated from below. The vertical currents that do develop cause the air to be rough and turbulent. Now lets talk about a warm air mass moving over land. Again vertical currents develop as the air is heated from below but because of the moisture contained in the air mass the vertical currents cause cumulus clouds to develop as the air rises and cools adiabaticly. In the initial stages the up draft and down drafts associated with these vertical currants cause the cumulus cloud to develop in scattered or broken layers with open areas in between. The base of the clouds would be several thousand feet above the surface. Some clouds continue to build some times to sixty thousand feet, cover a large area and produce showers and thunder storms. The vertical currents prevent fog from forming. The types of precipitation that will occur in an unstable air mass, are rain showers or snow showers. Now lets talk about an air mass that originates in the gulf in the winter season. The land area being cooler than the air mass will produce cooling from below. No vertical currents will develop and the air mass will be stable. Low stratus clouds and fog will generally form. Remember cooling from below promotes stability. The clouds that form will be decks and sheets that have smooth flat tops and will cover great areas. The base of these clouds will generally be low. Smoke, haze, dust and fog are found in this condition. The types of precipitation associated with stable air are light rain, snow, or drizzle that may be continuous or intermittent. Now lets look at an air mass from the pacific as it moves over the mountains along the west coast. The rapid lifting of the air by the mountains causes the air to become unstable. Vertical currents develop causing turbulence. Cumulus clouds may spring up and cause thunder storms. When the air mass descends beyond the mountain range to the desert areas the cumulus clouds seem to dissipate because of the relatively humidity decreases. The air becomes stable. This decrease in relative humidity is caused by adiabatic heating as the air descends.