Climatology is the study of the long-term state of the atmosphere, or climate. The long-term state of the atmosphere is a function of a variety of interacting elements. They are:
· Solar radiation
· Air masses
· Pressure systems (and cyclone belts)
· Ocean Currents
· Topography
Solar radiation is probably the most important element of climate. Solar radiation first and foremost heats the Earth's surface which in turn determines the temperature of the air above. The receipt of solar radiation drives evaporation, so long as there is water available. Heating of the air determines its stability, which affects cloud development and precipitation. Unequal heating of the Earth's surface creates pressure gradients that result in wind. So you see, just about all the characteristics of climate can be traced back to the receipt of solar radiation.
Air masses as an element of climate subsumes the characteristics of temperature, humidity, and stability. Location relative to source regions of air masses in part determines the variation of the day-to-day weather and long-term climate of a place. For instance, the stormy climate of the midlatitudes is a product of lying in the boundary zone of greatly contrasting air masses called the polar front.
Pressure systems have a direct impact on the precipitation characteristics of different climate regions. In general, places dominated by low pressure tend to be moist, while those dominated by high pressure are dry. The seasonality of precipitation is affected by the seasonal movement of global and regional pressure systems. Climates located at 10o to 15o of latitude experience a significant wet period when dominated by the Intertropical Convergence Zone and a dry period when the Subtropical High moves into this region. Likewise, the climate of Asia is impacted by the annual fluctuation of wind direction due to the monsoon. Pressure dominance also affects the receipt of solar radiation. Places dominated by high pressure tend to lack cloud cover and hence receive significant amounts of sunshine, especially in the low latitudes.
Ocean currents greatly affect the temperature and precipitation of a climate. Those climates bordering cold currents tend to be drier as the cold ocean water helps stabilize the air and inhibit cloud formation and precipitation. Air traveling over cold ocean currents lose energy to the water and thus moderate the temperature of nearby coastal locations. Air masses traveling over warm ocean currents promote instability and precipitation. Additionally, the warm ocean water keeps air temperatures somewhat warmer than locations just inland from the coast during the winter.
Topography affects climate in a variety of ways. The orientation of mountains to the prevailing wind affects precipitation. Windward slopes, those facing into the wind, experience more precipitation due to orographic uplift of the air. Leeward sides of mountains are in the rain shadow and thus receive less precipitation. Air temperatures are affected by slope and orientation as slopes facing into the Sun will be warmer than those facing away. Temperature also decreases as one moves toward higher elevations. Mountains have nearly the same affect as latitude does on climate. On tall mountains a zonation of climate occurs as you move towards higher elevation.
The purpose of classification is to organize a set of data or information about something to effectively communicate it in an informative way. Classification helps synthesize information into smaller units that are more easily understood. When considering the Earth's climate, there is such an enormous amount of information that one has to break it down into areas of commonality to easily understand it. Climatologists have therefore created several ways to organize the wealth of information about Earth's climate to bring order and understanding to it.
There are three fundamental types of classifications used in climatology. First there are empirical systems of classification that are based on observable features. The Koeppen system discussed below is an empirical system based on observations of temperature and precipitation. These are two of the easiest climate characteristics that can be measured, and probably the ones with the longest historical record. It's fairly easy to collect air temperature readings with a thermometer and precipitation with some sort of collecting device that can measure the amount of precipitation. Climates are grouped based on annual averages and seasonal extremes.
Genetic classification systems are those based on the cause of the climate. A genetic system relies on information about the climatic elements of solar radiation, air masses, pressure systems, etc. The important point here is that we assume we know what causes climate. Though atmospheric science is progressing everyday, we still have a long way to go before we have a complete understanding of the workings of our climate. These are inherently the most difficult classifications to create and use because of the multitude of variables needed.
Applied classification systems are those created for, or as an outgrowth of, a particular climate-associated problem. The Thornthwaite classification system is one based on potential evapotranspiration and thus groups climates based on water requirements. Research conducted by C.W. Thornthwaite and his associates attempted to formulate a water budget technique that assessed water demand under different environmental conditions. His classification system grew out of the issue of trying to predict the supply and demand for water in different climate regions.
The Koeppen system is one of the most widely used systems for classifying climate because it is easy to use and data requirements are minimal. For information about the system and description of the map below see:
· Brief Guide to Koeppen Climate Classification System
Figure CS.1 World Climate patterns according to Koeppen(FAO)
The tropical rain forest climate supports one of the most lush and diverse environments on Earth. Its location near the equator dominates all aspects of the climate. Year-round warm temperatures and copious rainfall characterize the rain forest climate.
Figure CS.2 Rain forest of Uganda
(Source: FAO Used with permission)
The rain forest climate is generally found straddling the equator and along tropical coasts that are backed by mountains and exposed to the trade winds. The climate tends to be restricted to low elevations (below 1000 meters) because at higher altitudes temperatures are too cool. Large regions of rain forest climate are found in the Amazon River basin of South America, the Congo River basin of Africa, the east coast of Central America and Madagascar. Malaysia, Indonesia, and the Philippines are dominated by rain forest climate.
The equatorial location of the tropical rain forest places it in a region of high annual insolation. High sun angles throughout the year make for high annual temperatures with very little seasonal variation. Located in the heart of the Intertropical Convergence Zone and near mE and mT source regions, high annual precipitation is experienced in all months.
The low latitude location of the rain forest promotes constant high temperatures throughout the year. Being located near the equator, the incidence angle of the noon sun is always high. In addition, the direct rays of the sun pass over the climate twice throughout the year creating two periods of maximum insolation. Given that the circle of illumination bisects the equator, day length tends to be nearly the same day-after-day.
Annual temperatures in the rain forest average between 20o - 30o C (68o - 86o F). Annual temperature range rarely exceeds 3o to 4o F. In fact, the daily range of temperature is often larger (10o - 12o F) than the annual range in temperature. The larger daily ranges are due to the sunny mornings and cloudy afternoons of cooling rain.
Figure CS.3 Iquitos, PeruIquitos, Peru's climograph displays the distinguishing characteristics of the rain forest climate: high annual temperatures and ample rainfall.
The rain forest is noted for its copious rainfall occurring in all months of the year. Over 200 cm (80 in) of precipitation annually falls in the rain forest. Abundant precipitation occurs in each month and is fairly evenly distributed between high and low sun seasons. However, some locations have one month of highest precipitation. Precipitation occurs on more than half the days and is largely generated by convective uplift of warm, moist equatorial air (mE). A distinctive diurnal pattern of cumulus cloud development in the morning, precipitation in the early afternoon, followed by dissipating clouds towards the late afternoon is typical. Thunderstorms are usually concentrated in small areas, so their duration is short but intense. Coastal locations and islands on the poleward limits of the rain forest experience hurricanes, but they do not occur near the equator or inland.
Figure CS.4 Convective thunderstorms over Brazil (July 2002)(Source: Copyright 2002 EUMETSAT)
Humidity in the rain forest can be oppressive with dew point temperatures ranging from 15oC - 20oC (59oF- 68oF). Since humidity is so high during the day, when cooling occurs at night, early morning radiation fogs form and heavy dew drips from the rain forest vegetation. These condensation products evaporate into the air as the Sun rises, thus increasing the air's humidity. Under these conditions, the air is oppressive and sultry most of the day and well into the evening. The rate of evaporation and transpiration are exceedingly high requiring a correspondingly greater amount of precipitation to support satisfactory conditions for plant growth due to the high temperature.
The tropical monsoon climate experiences abundant rainfall like that of the tropical rain forest climate, but it is concentrated in the high-sun season. Being located near the equator, the tropical monsoon climate experiences warm temperatures throughout the year.
The monsoon climate is found along the coastal regions of southwest India, Sri Lanka, Bangladesh, Myanmar, Southwestern Africa, French Guiana, and northeast and southeastern Brazil.
The major controlling factor over the monsoon climate is its relationship to the monsoon circulation. Recall that the monsoon is a seasonal change in wind direction. The "classic" monsoon circulation of Asia exhibits an onshore flow of air (air moving from ocean towards land) during the summer or high-sun season, and offshore air flow (air moving from land toward water) during the winter or low-sun season. The change in direction is due to the difference in the way water and land heat.
Changing pressure patterns that affect the seasonality of precipitation also occur in Africa. During the high-sun season, the ITCZ induces rain while the subtropical high creates dry conditions. The monsoon climate of Africa, and South America for that matter, are typically located along tradewind coasts.
Figure CS.4 Climograph for Mangalore, India
Lat/Long = 12.53o N, 74.52o EAverage Annual Temperature (oC) = 27.05Annual Temperature Range (oC) = 3.6Total Annual Precipitation (mm) = 3409.2Summer Precipitation (mm) = 3115.9Winter Precipitation (mm) = 293.3
Like in the tropical rain forest climate, temperatures remain high all year in the monsoon climate. As shown in the climograph for Mangalore, India (Figure CS.4), the average annual temperature is 27.05 oC, but only has an annual temperature range of 3.6 oC. The monsoon climate's temperature range is somewhat similar to that of the rain forest, but it exhibits a slightly different temporal pattern. In the rain forest we noted two periods of maximum temperature in association with the migration of the Sun's vertical rays. The monsoon climate tends to have its highest temperature just before rainy period. Once the rainy period starts, clouds block incoming solar radiation to reduce monthly temperatures.
Figure CS.5 Comparison of monthly temperature in the rain forest (Iquitos) and monsoon climates (Mangalore).
Seasonality of its precipitation is the hallmark and most well-known characteristic of the monsoon climate. Many think that the term "monsoon" means wet weather, when in fact it describes an atmospheric circulation pattern. Though the annual amount of precipitation is quite similar to that of the rain forest, monsoon precipitation is concentrated into the high-sun season. Maritime equatorial and maritime tropical air masses travel from the ocean on to land during the summer, where they are uplifted by either convection or convergence of air to induce condensation. Locally, orographic uplift is an important mechanism for promoting precipitation. As air travels into the Indian subcontinent, it is uplifted by the Himalayas, causing cloud development and precipitation.
The low-sun season is characterized by a short drought season when high pressure inhibits precipitation formation. In the case of the Asian monsoon, the replacement of the thermal low with the subsidence of the Siberian High suppresses uplift. Air masses that dominate this period are dry given their continental origin (cT, cP) or stability (mTs).
Figure CS.6 Impala on the Savanna.(Source: FAO)
The Tropical Wet/Dry climate is located on the poleward sides of the tropical wet climates, positioned between them and the tropical dry climates. This location places the climate at an intermediate position between the ITCZ and the Subtropical High. As a result, the climate experiences a distinct seasonality to its precipitation like that of the tropical monsoon climate. Also known as the "Savanna" climate, it supports a ground cover of drought resistant grasses with scattered trees, but not enough rainfall to make agriculture a viable, life sustaining activity.
The Tropical Wet/Dry climate lies at latitudes of about 5o - 10o and 15o - 20o. Broad expanses of the savanna exist in north and south central Africa, The Llanos of Venezuela, Campos of Brazil, much of northern and eastern India, western Central America, the Caribbean Islands, and south Florida, Myanmar (Burma) and the Indo-Chinese Peninsula
Figure CS.7 Climograph for Dakar, Senegal Click image to enlarge
Its position at about 15o North or South latitude places the Tropical Wet/Dry climate in a zone between the alternating influence of the Intertropical Convergence Zone (wet season) and the (dry season) subtropical high. The subsidence of the subtropical high suppresses precipitation, creating clear skies allowing much insolation to the surface. Converging air into the ITCZ, in combination with convection, forces air to rise, causing condensation, cloud development, and precipitation.
Located at a higher latitude, the variation in insolation is greater and hence this climate has a larger annual temperature range than the other tropical wet climates. However, the average annual temperature is similar to that of the other tropical wet climates. Mean monthly temperatures in the Tropical Wet/Dry climate range from 18oC (64.4oF) to above 25oC (77oF). Like the monsoon climate, the maximum temperature tends to occur in late spring to early summer prior to the onset of the rainy season. There may be a secondary maximum after the rainy period. Annual temperature ranges increase as one moves poleward through the climate. Daily temperature ranges are greatest during the dry season and a bit larger than the rainy tropical climates. Diurnal temperature ranges of 10oC to 15oC during winter are not uncommon. During the summer, high daily temperatures, small temperature ranges, and high humidity create the same uncomfortable conditions as those found in the rainy tropics.
Figure CS.7 A Baobab tree, with its thick trunk and large edible fruit, Dakar, Senegal.(UN/DPI Photo #187250C by Evan Schneider)
The Tropical Wet/Dry climate is the driest of the tropical wet climates. Like the monsoon climate, it has a distinct seasonality to its precipitation. However, its wet season is much shorter and receives far less precipitation than the monsoon climate. The seasonality of precipitation is related to the migratory movement of the ITCZ and Subtropical High. During the high sun season the ITCZ dominates, causing convergent uplift, along with the ever-present convection to promote the production of precipitation. During this period, warm and moist mE and mT air masses dominate.
During the low sun season the ITCZ moves out and the subtropical high moves in suppressing precipitation and initiating the long drought period. The exceedingly warm and dry cT air masses dominate during this time of the year.
Variability of precipitation makes it very difficult for agriculture. Nairobi, Kenya averages 86 cm (33.9 in) of rainfall, but from year-to-year can vary from 50 (9.7 in) to 150 cm. (59 in) The drier the Savanna location, the more unreliable the precipitation. Rains are essential in greening the Savanna, and animals migrate with the seasonal rhythms in search of water and pasture.
Figure CS.8 Wildebeest in Masailand, Kenya...
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