An air mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles.
The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
Classification: 4 general air mass classifications categorized according to the source region.
1. polar latitudes P - typically located poleward of 60 degrees north and south
2. tropical latitudes T - typically located within about 25 degrees of the equator
3. continental c - located over large land masses--dry
4. marine m - located over the oceans----moist
Cold Front-transition zone from warm air to cold air
A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.
Warm Front-transition zone from cold air to warm air
A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and morehumid than it was before.
Stationary Fronts
A cold front is the boundary between cool and warm air when the cool air is replacing the warm air. A warm front is the boundary when the warm air is winning the battle. When the pushing is a standoff, the boundary is known as a stationary front. Stationary fronts often bring several days of cloudy, wet weather that can last a week or more.
Since neither the warm air nor the cold air are advancing, the stationary front weather map symbols combine both the cold front and the warm front symbols.
Maps show stationary fronts with alternating triangles pointing away from the cold air and half circles pointing away from the warm air. Color maps alternate the cold front blue and warm front red.
Occluded Fronts
Generally, cold fronts move faster than warm fronts. Sometimes in a storm system the cold front will "catch up" to the warm front. An occluded front forms as the cold air behind the cold front meets the cold air ahead of the warm front. Which ever air mass is the coldest undercuts the other. The boundary between the two cold air masses is called an occluded front. Occluded fronts are represented on weather maps by a solid purple line with alternating triangles and semi-circles, pointing in the direction of its movement.