An air mass is a body of air that is synoptic is size that possesses similar temperature and moisture characteristics about a horizontal plane in the lower troposphere. An air mass takes on the characteristics of the earth's surface it develops over. An air mass will be cold or warm and will also be dry or moist. Of course, there are varying degrees of temperature and moisture properties mainly due to season, how long the air mass builds over the source region and the part of the source region the air mass develops. The four basic air mass types are:
Continental Polar (cP): cold and dry
Maritime Polar (mP): cool and moist
Continental Tropical (cT): warm and dry
Maritime Tropical (mT): warm and moist
Once an air mass leaves a source region it will begin to modify. The cP air modifies by moving into lower latitudes where the sun angle is higher, by precipitation falling into the air mass and also modifies by moving over water surfaces. The mP air modifies by moving into lower latitudes and moving over land surfaces. The cT air modifies by moving into higher latitudes where the sun angle is lower, by precipitation falling into the air mass and also modifies by moving over water surfaces. The mT air modifies by moving into higher latitudes and over land surfaces.
The main focus of this Haby Hint is to analyze in an operational meteorology sense how a stagnant air mass modifies that is no longer in its source region. By stagnant, it in meant that the air within the air mass is no longer advecting. The air mass has settled over a region and the winds within the air mass are light.
The following is a list of processes that modify a stagnant air mass in the lower troposphere:
1. sunlight warming the surface and that heat being conducted upwards into the air mass (most efficient when sun angles high and days long)
2. warm soil temperatures warming an air mass (most efficient when there is a large temperature difference between soil and air)
3. cold soil temperatures cooling an air mass (most efficient when there is a large temperature difference between soil and air)
4. evaporative cooling from precipitation falling into an air mass (most efficient when the air mass initially has a low RH)
5. radiational cooling of the surface which in turn cools the air mass (most efficient on clear and long nights)
6. increase of cloud cover (keeps air mass cooler during the day but warmer at night)
7. Evaporation from earth's surface into the air mass above (most efficient when air mass is shallow with an initial low RH along with saturated soil)
An example of different processes that modify an air mass is given below:
A cP air mass settles over the south-central U.S. This cP air mass is shallow. Differential advection occurs which brings moisture above the cP air. Rain has fallen into the cP air mass and cooled it (through evaporational cooling) and moistened it to a RH of 100%. The temperature in Dallas is 34 degrees F at 5 p.m and had been cooling throughout the afternoon due to evaporational cooling. The skies are cloudy and rain continues to fall. The sun is setting and 14 hours of darkness awaits. The winds are light from the north. The forecast calls for severe icing to occur overnight. However, the icing never occurs and the temperature hovers just above freezing all night. Why? There are 5 main reasons why the ice storm never materialized:
1. Thick low clouds reduced the overnight cooling potential,
2. No significant CAA occurred,
3. The rain falling into the shallow cP air is above freezing. The warmth of the rain helps prevent the cP air mass from cooling overnight,
4. The RH was 100%, thus no further evaporational cooling could occur,
5. The ground soil temperature is above freezing, thus the earth's surface adds heat to the shallow modified cP air mass.