Introduction
There are two basic air masses-Tropical (T) and Polar (P) which differ in temperatures. The two are further divided into Maritime (M) and Continental (C), their classifications are based on humidity. Polar maritime is situated over the warmer high latitude oceans of North Atlantic, North Pacific, and the southern hemisphere. The mP air is moister and less cold than the cP air. North America has been experiencing these kinds of air masses in the winter season. The west coast is usually protected from any invasion by cascade barriers and westerly circulations. Polar continental (cP) is usually confined in high latitudinal stretches of Canada and Siberia. (Barry and Perry, 1998).
The dry tropical air
The Tropical Continental (CT) air mass is hot and dry where hot dry winds invade Southern California and Northern Mexico in form of scorching sirocco. The tropical maritime air mass is greater in size and wider than CT. the north and south Atlantic, Indian and Pacific oceans are extensive sources of mT air. These air masses are saturated and universally moist. They are normally pushed beyond the source areas causing the high-temperature condition and heavy precipitation. (Barry and Perry, 1998).
The dry tropical air is usually linked with the driest and hottest conditions together with clear skies. It is comparable with the traditional cT designation. This mostly takes place in the deserts in both northwestern Mexico and the southwestern USA. In addition, these weather conditions can be emitted by strong developing winds, such that the brisk compressional heating gives rise to desert-like conditions.
Polar maritime air mass
MP is believed to be a large subset of the mP air mass. The weather conditions are often cloudy, humid, and cool with light precipitation. This normally appears through inland abduction of air from either the north Atlantic or North Pacific. It also occurs wherever a cP acquires moisture when passing through a cool water body or where a good overruns to the south. (Frederick and Edward, 2007).
MP air is less humid, less cloudy, and not as warm as the MM air. This forms either an independent warm front south of the MP air or as a modified mP air mass. This can also happen under mT influence during summer particularly on those days with a high cloud cover. MT air is similar to mT, it arrives in North America either through the Tropical Pacific ocean or through the Gulf of Mexico. This kind of weather is very humid and warm, it is particularly cloudy in summer and fully cloudy in winter. Convective precipitation is a common scenario, especially in the summer season. The moist air masses record small temperatures in summer. MP is the coolest mass in the summer with its afternoon temperatures remaining below twenty degrees Celsius.
Conclusion
It can also be noted that, during winter, recurrent East Pacific edging amplifies the northerly component of the mid-tropospheric height contours, causing the movement of Arctic air masses southeast into central North America. During summer, on the other hand, there is a reduced tropospheric height gradient causing the decrease in the area1 extent of the westerlies. This successfully restrains major southeastern movement of polar air. The other two dry air masses acquire a significant presence during summer. From the East, DM air reveals a north-south frequency decrease, which ranges from about 5 % in southern Florida to about 30% in New England. a large amount of the DM air instigates from the Pacific and moves to the east when the upper-level circulation shows a slight edge over the Rocky Mountains or a zonal pattern. (Frederick and Edward, 2007).
References
Barry, G. and Perry, A.(1998): Synaptic Climatology, London , Cambridge University Press.
Frederick K. and Edward J. (2007): The Atmosphere: an Introduction to Meteorology. 10th ed. Upper Saddle River, NJ: Pearson/Prentice Hall.