4.2 Routing Protocol
4.2.2 Inter and Intra Rack Communication
The concept of air masses is important because air masses help to categorized world climate types. In regions where one air mass is dominant all year, there is little seasonal variation in weather, for example at the tropics and at the poles. Areas such as the British Isles, where air masses constantly interchange, experience much greater seasonal and diurnal (daily) variation in their weather. (Blij and Muller, 2005).
If air remains stationary in an area for several days, it tends to assume the temperature and humidity properties of that area. Stationary air is mainly found in the high pressure belts of the subtropics and in high latitudes. The areas in which homogenous air masses develop are called source regions. Air masses can be classified according to the latitudes in
Mirror image in southern hemisphere
Polar front
Polar tropo
Mid-‐latitude trospospause Tropical
tropopau
Polar cell
Forrel cell
Cumulonimbus clouds Hadley cell
which they develop which determines their temperature – Artic (A), polar (P) or tropical (T) and the surface over which they develop, which affects their moisture contents – maritime (M) or continental (C).
The five major air masses which affect a location at various times of the year are as follows:
1. Artic Maritime Air Mass (AM) 2. Polar Maritime Air Mass (PM) 3. Polar Continental Air Mass (PC) 4. Tropical Maritime Air Mass (TM) 5. Tropical Continental Air Mass (TC)
3.3 Meso Scale
3.3.1 Land and sea breeze systems: Land surfaces and water bodies displays sharply contrasting thermal responses to energy input. Land surfaces heat and cool rapidly, whereas water bodies exhibit a more moderate temperature regime. During day, a land surface heats up quickly and the air layer in contact with it rises in response to the increased air temperature. This rising air produces a low pressure cell over the coastal land or island. Since the air over the adjacent water is cooler, it subsides to produce a surface high pressure cell. A pressure gradient is thereby produced, and air in contact with the surface now moves from high pressure to low pressure. Thus, during the day, shore-zone areas generally experience air moving from water to land. This is called sea breeze.
Fig 6: Sea Breeze
At night, when the temperature above the land surface has dropped significantly, the circulation reverses because the warmer air (and lower pressure) is now over the water. This result in air moving from land to water. This is called land breeze.
Fig 7: Land Breeze
When the system generates, sea and land breezes, it produces a circulation cell composed of the surface breeze, rising and subsiding air associated with the lower-and higher pressure areas respectively, air flow aloft in the direction opposite to that of the surface. Although it modifies the wind and temperature conditions at the coast, the effect of this circulation diminishes rapidly as one move inland. Note also that we use the word breeze. This accurately depicts a rather gentle circulation in response to a fairly weak pressure gradient. The sea/land breeze phenomena can easily be overpowered if stronger pressure systems are nearby. (Williams, Conrad & Long, 2005).
3.3.2 Mountain/Valley Breeze Systems
Mountain slopes are subject to the reversal of day and night local circulation systems. This wind circulation is also thermal, meaning that it is driven by temperature differences between adjacent topographic features. During the day, mountains terrain facing the sun tends to heat up more rapidly than the surrounding slopes. This causes low pressure to develop, spawning an up sloping valley breeze. At night, greater radiative loss from the mountain slopes cools them more sharply, high pressure develops, and a down sloping mountain breeze results. The wind that blows up the valley is also known as an anabatic wind while the down valley wind is called the Katabatic wind, which are usually gentle but much stronger if they blow over glaciers or permanently snow covered slopes.
3.3.3 Fohn
The fohn is a strong, warm and dry wind which blows periodically to the lee of a mountain range. It occurs in the Alps when a depression passes to the north of the mountains and draws in warm, moist air from the Mediterranean. As the air rises it cools at the DALR of 1oC per 100m. If
the rising air will cool more slowly at the saturated adiabatic lapse rate (SALR) of 0.5oC per 100m. This means that when the air reaches 3000m it will have a temperature of 0oC instead of the -10oC had latent heat not been released. Having crossed the Alps, the descending air is compressed and warmed at the dry adiabatic lapse rate (DALR) so that if the land drops sufficiently, the air will reach sea level at 30oC. This is 10oC warmer than when it left the Mediterranean. Temperatures may rise by 20oC within an hour and relative humidity can fall to 10 percent.
(Williams, Conrad, Long, 2005).
This wind, also known as Chinook on the American prairies, has considerable effects on human activities. In spring, when it is most likely to blow, it melts snow and enable wheat to be grown. Conversely, it warmth can cause avalanches, forest fires and premature budding of trees.
4.0 CONCLUSION
The overall pattern as explained by the tricellular model is affected by the apparent movement of the overhead sun to the north and south of the equator (0oC). This movement causes the seasonal shift of the heat equator, the ITCZ, the equatorial low pressure zone and global wind systems and rainfall belts. Any variation in the characteristics of the ITCZ i.e. its location or width can have drastic effect on the surrounding climates, as seen in the sahel droughts of the early 1970s and most of the 1980s.
Categories of world climatic types are determined by air masses, seasonal variations and daily changes in weather are equally determined by air masses. Five major air masses affect the weather of a location.
They include: artic maritime air mass (AM), polar maritime (PM), polar continental (PC), tropical maritime (TM) and tropical continental (TC).
All these air masses have effects on the locations where they have dominance.
Local winds connote the winds that are peculiar to a relatively small area and are of local importance. They are seasonal and often confined to the lowest part of the atmosphere which result to differential heating and cooling of land and sea.
5.0 SUMMARY
In this unit, the tricellular model of atmospheric circulation has been discussed. There is a strong relationship between the equator and ITCZ and the atmospheric circulation as described by the tricellular model in understanding the atmospheric circulation pattern.
The synoptic system in understanding the circulation of atmospheric pressure or wind system with focus on the trade winds (air masses) and how they control or impact on seasons was also discussed.
SELF-ASSESSMENT EXERCISE
Schematically present the tricellular model of atmospheric circulation and describe the features of the model.
6.0 TUTOR-MARKED ASSIGNMENT
1. Explain the tricellular model of atmosphere circulation.
2. Mention the major air masses which affect a location at various times of the year.
3. Differentiate between land and sea breeze systems.
7.0 REFERENCES/FURTHER READING
Briggs, D. & Smithson, P. (1985). Fundamentals of Physical Geography. London: Unwin Hyman Ltd.
Blij, H. J., Muller, P.,O., Williams, R. S., Conrad, C. T., & Long, P.
(2005). Physical Geography: The Global Environment. Canada:
Oxford University Press.
MODULE 5 THE DYNAMICS OF PRESSURE AND WIND