Tropical Climates

Question 1

What is the temperature like in Tropical Rainforest Climate (Af)?

Answer 1

High temperatures throughout the year (mean monthly and yearly temperatures are close to 27°C).

Question 2

What is the reason for the high constant temperature in Tropical Rainforest Climate (Af)?

Answer 2

They are a result of the high annual insolation received at these low latitudes due to the angle of incidence of Sun’s rays being high throughout the year and therefore less beam spreading and less atmospheric depletion.

Question 3

What is the temperature range like in Tropical Rainforest Climate (Af)?

Answer 3

Small annual temperature range (about 3°C at most).

Question 4

What is the reason for the small temperature range in Tropical Rainforest Climate (Af)?

Answer 4

Due to very little seasonal variation from the high angle of incidence of the Sun’s ray throughout the year also. Insolation is thus distributed evenly throughout the year with each day having approximately 12 hours of daylight/darkness.

Question 5

What is the annual precipitation like in Tropical Rainforest Climate (Af)?

Answer 5

High, usually exceeding 2000mm annually. Most storms are violent with heavy rain falling from cumulonimbus clouds, accompanied by lightning and thunder. The precipitation is also evenly distributed throughout the year with no dry season.

Question 6

What is the reason for the high evenly-distributed precipitation in the Tropical Rainforest Climate (Af)?

Answer 6

The position is near the equator which puts them under constant influence of the ITCZ. The high insolation received throughout the year at these locations promotes year long convectional uplift. The atmospheric instability is further reinforced by the convergence of trade winds. Therefore, the extensive uplift of air facilitates cloud formation and precipitation throughout the year.

Question 7

What is the diurnal pattern in the Tropical Rainforest Climate (Af)?

Answer 7

Cumulus cloud development in the morning, heavy rainfall in the afternoon and dissipating clouds by late afternoon.

Question 8

Why is the diurnal temperature range small in the Tropical Rainforest Climate (Af)?

Answer 8

Due to greater presence of cloud cover all year round. The convection of humid air promotes the formation of cumulus clouds that scatter much of the daytime insolation back to space. The cloud cover also retards cooling at night as it traps longwave terrestial radiation in the Earth’s atmosphere.

Question 9

Why are the temperatures in the Tropical Rainforest Climate (Af) not the highest in the Earth?

Answer 9

Much energy is expended on evaporation rather than heating the surface temperature.

Question 10

Where is the Tropical Rainforest Climate (Af) found?

Answer 10

Straddling the equator between 10°N and 10°S and restricted to lowlands. Singapore, Malaysia, Indonesia, The Philippines, Western Equatorial Africa, East Coast of Central America and Madagascar.

Question 11

What is the temperature like in the Tropical Savanna Climate (Aw)?

Answer 11

Temperatures are generally high, with mean monthly temperatures ranging from 18°C to 25°C. Although temperatures are high throughout the year, there is a short, slightly cooler season when the sun is overhead in the tropic in the opposite hemisphere. Temperatures also tend to drop slightly on the rainy season. Annual temperature range is between 3-10°C and it increases towards the poles as these areas experience a greater degree of seasonality.

Question 12

What are the diurnal temperature variations like in the Tropical Savanna Climate (Aw)?

Answer 12

Diurnal variations are greater during the dry season when the absence of clouds facilitates greater daytime heating and nighttime cooling. During this time, daily lows and highs might range from 15-30°C. During the rainy season, high humidity and cloud cover reduces the diurnal ranges to values similar to those of tropical wet regions.

Question 13

What is the reason for the diurnal temperature variations in the Tropical Savanna Climate (Aw)?

Answer 13

Due to the changing amount of cloud cover rising from the seasonal migration of the Hadley Cell. It is greater during the dry season due to the absence of the ITCZ because of the sub-tropical high pressure belt that suppresses cloud formation — leading to higher daytime and night-time temperatures.

Question 14

What is the precipitation like in the Tropical Savanna Climate (Aw)?

Answer 14

Precipitation is lower and unevenly distributed throughout the year. Rainfall may range from less than 500mm to 1500mm a year. There is a presence of a wet and dry season.

Question 15

What is the reason for the wet/dry season in the Tropical Savanna Climate (Aw)?

Answer 15

Due to the seasonal migration of the Hadley Cell which leads to alternating influence of the ITCZ (at the rising limb) and STHP (at the sinking limb). For a part of the year, influence of STHP creates zone of high pressure associated with sinking air masses and atmospheric stability that suppresses convection and gives rise to clear skies and high daytime temperatures. In the high sun season, the position of the overhead sun shifts towards the Tropic of Cancer in the northern hemisphere or the Tropic of Capricorn in the southern hemisphere and the ITCZ migrates accordingly, bringing a period of heavy rainfall to locations under its influence.

Question 16

What is the reason for the spatial variation in amount of rainfall received in a year in Tropical Savanna Climates (Aw) of different places?

Answer 16

Due to the different length of time at which it is under the influence of the ITCZ. Areas at the poleward limit of the savanna are only briefly affected and have a short wet season with low annual rainfall totals. Towards the equator the wet season lasts much longer since these areas remain under the influence of the ITCZ for longer periods of time and hence have a longer rainfall season with higher rainfall amounts.

Question 17

Where are Tropical Savanna Climates (Aw) located?

Answer 17

5° to 20°N and S of equator, between Af and BwH climates. E.g. Kanos and Lagos, both Tropical Savanna Climates but Lagos has higher total annual rainfall around 68mm (as compared to Kano’s around 32.2mm) and a longer wet season due to being closer to the equator. More locations include northern and eastern India, western Central America, Central Africa surrounding Congo basin in north and south centra Africa.

Question 18

What are the characteristics of Tropical Desert Climate (BWh)?

Answer 18

Low-latitude deserts are characterised by extreme temperatures. Annual temperature range is often 20-30°C, diurnal temperature range can be over 50°C, precipitation is usually low and unreliable but deserts are not totally dry. Rainfall may only occur once every two years or maybe once or twice in a year. When it does occur, rain tells to fall in short bursts.

Question 19

What are the locations of Tropical Desert Climate (BWh)?

Answer 19

Found in continental interiors, 15-25°N and S of equator near the Tropics of Cancer and Capricorn. Much of North Africa (the Sahara), Saudi Arabia to Iran, Pakistan and western India.

Question 20

What are the characteristics of Subtropical Hot Steppe Climate (BSh)?

Answer 20

High mean annual temperatures, large annual and diurnal temperature range, degree of seasonality in the precipitation regime with distinct summer-dry and winter-wet pattern, low precipitation with annual precipitation usually below 60cm.

Question 21

What are the locations of Subtropical Hot Steppe Climate (BSh)?

Answer 21

Exist around the periphery of hot deserts. E.g. Walgett, in interior New South Wales Australia, around Sahara’s periphery and in the Iran, Afghanistan, Turkmenistan and Kazakhstan region.

Question 22

Why is there a spatial variation in the period in which wet season occurs in Subtropical Steppe Climate (BSh)?

Answer 22

On steppe areas located on the equatorward side of subtropical deserts, precipitation occurs more often during summer months due to localised convection and some influence of the ITCZ. In contrast, steppe regions on the poleward side of subtropical deserts experience most precipitation in the winter in response to the passage of midlatitude low pressure systems.

Question 23

Why is there little total annual precipitation in both Subtropical Steppe (BSh) and Low Latitude Desert (BWh) Climates?

Answer 23

They lie under persistence influence of subtropical high which is associated with subsidence, which causes stable conditions in the troposphere and the lack of uplift inhibits cloud formation, resulting in clear skies and little precipitation. In addition, air subsiding from the subtropical high is adiabatically warmed which reduces the relative humidity of air, dropping to 10% or less. The extremely low relative humidity causes evaporation of what little surface water there is. The continental location of some subtropical deserts and steppe lands places them far from a source of moisture which contributes to their aridity. Some deserts occur in subtropical regions as narrow strips along the west coast of continents, adjacent to the cold ocean currents. The cold ocean current cools the air and contributes to stable atmospheric conditions.

Question 24

Why is there a large annual and diurnal temperature range in both Subtropical Steppe (BSh) and Low Latitude Desert (BWh) Climates?

Answer 24

Daytime summer temperatures in subtropical deserts and steppe lands can be extremely high. In fact, hottest locations around the world are found in these regions. During summer, combination of low humidity, high sun and clear skies allows high inputs of solar radiation to be absorbed at the surface. Furthermore, lack of soil moisture causes ground temperatures to be extremely high as little heat is expended in the evaporation of water. Hence daytime temperatures commonly reach as high as 45°C or higher. In contrast, the nights may be extremely cold since clear skies allow much heat to be lost. Without the absorptive blanket of clouds, longwave radiation emitted from the Earth readily escapes to space, chilling the night air.

Question 25

What are the characteristics of a Tropical Monsoon Climate (Am)?

Answer 25

High temperatures throughout the year (mean annual temperatures of 27°C), small annual temperature range (between 3°C to 6°C). Typically, the warmest months occur just prior to the main precipitation season. Total annual rainfall is similar and more in quantity to that of the Tropical Rainforest Climate (Af), 2000mm or more. However, distribution of precipitation is very uneven, with most of it concentrate in a few months — high precipitation during high sun season and low rainfall in the low sun season.

Question 26

What are the locations of a Tropical Monsoon Climate (Am)?

Answer 26

The tropical monsoon climate is usually found in coastal locations 5° to 25° N and S of equator. Examples include coastal regions of southwest India, Sri Lanka, Bangladesh, Myanmar, South-western Africa, French Guiana and northeast and southeast Brazil.

Question 27

Topography and Physical Context of India

Answer 27

Surrounded by three different water bodies, the Indian Ocean, the Bay of Bengal and the Arabian Sea. The country is divided from the Tibetan plateau by the Himalayan mountain range in the north and north-eastern range. The position of the mountain ranges and the direction of the seasonal winds are two main factors that determine the precipitation pattern in India. India experiences both dry (northeast winter monsoon ) and wet (southwest summer monsoon) monsoon seasons.

Question 28

Northeast Winter Monsoon in India

Answer 28

Winters are hot and dry and caused in part by northeast monsoon. Monsoon initiates when ITCZ shifts to the Southern Hemisphere due to the southern migration of the overhead Sun. Given that land loses heat faster than the ocean, land masses in the northern hemisphere remain colder than the surrounding oceans and oceans take longer time to cool down, remaining warmer than the land masses. Since the Indian Ocean has a higher temperature than the Indian subcontinent, air above the oceans will rise, creating a zone of low pressure. Meanwhile, the land masses will have a lower temperature and air above it will sink, creating a zone of higher pressure. Since winds blow from areas of low pressure to high pressure, winds will blow from continent to the ocean. As they blow from the northeast direction, they move over landmasses, picking up little moisture. This brings minimal rain to India in winter which explains the dry season during this period.

Question 29

Effects of the Northeast Winter Monsoon in India

Answer 29

Dry winds produce clear skies and sunny weather over most of India from November to May. Temperature can reach as high as 40°C during Indian winter. Little rain falls at this time of the year although slightly higher amounts of rainfall are noted in southeast India and eastern Sri Lanka. This is a result of the evaporated moisture from Bay of Bengal which is then blown onshore by northeast winds.

Question 30

Southwest Summer Monsoon in India

Answer 30

Between March and May, the overhead Sun starts to shift northwards towards the Tropic of Cancer. The ITCZ eventually shifts to lie just above India, south of the Himalayas. Maximum insolation occurs across the Indian subcontinent. As the land heats up faster than the ocean, the temperature of the ocean is lower than that of the Indian sub-continent due to specific heat capacities of land and the sea where the oceans heat up slower than the landmasses. Since the ocean is cooler, air above the oceans will sink, creating a zone of high pressure, and since the landmass has a higher temperature, air will rise above the landmass, creating a zone of low pressure. Winds will blow onshore (from ocean to land) from a south-westerly direction. As the winds blow from a south-westerly direction, it moves over water bodies such as the Indian Ocean and Bay of Bengal, picks up moisture, moves onshore into India and brings rain. This happens in summer from June to September which explains the distinct wet season during this period.

Question 31

Effects of the Southwest Summer Monsoon in India

Answer 31

Due to the intense insolation, evaporation of moisture from the sea takes place rapidly. This warm, moist air is forced to rise either by strong convection or orographic uplift to form clouds, then rain. A great deal of latent heat is released, further encouraging the convectional processes. Thus moisture laden winds blow onshore, bringing heavy rainfall to the subcontinent. Between May to October, the Indian subcontinent receives 80% of its annual rainfall from the monsoon.

Question 32

Jet Streams and the Indian Monsoons

Answer 32

Upper atmospheric circulation reinforces pressure conditions on the surface. Upper air convergence leads to lower air divergence and vice-versa. The Indian monsoons are affected by the westerly subtropical jet stream and the westerly subtropical jet stream (WSTJ) and the seasonal tropical easterly jet (TEJ).

Question 33

Westerly Subtropical Jet Stream (WSTJ) in the Winter

Answer 33

In winter, the WSTJ dominates the upper atmospheric circulation over India. Due to the height of the Himalayas, the subtropical jet stream splits into northern and southern branches. Upper air convergence of the southern branch leads to descending air at the surface that reinforces high pressure at the surface. This upper air convergence associated with the subtropical jet stream contributes to low-level, dry north-easterly winds that flow out of India in the winter.

Question 34

Tropical Easterly Jet (TEJ) in the Summer

Answer 34

In summer, the southern branch of the subtropical jet stream weakens as it migrates northwards due to the northward shift of the overhead Sun. The subtropical jet stream will thus be north of the Himalayas. A TEJ develops over India during this period. It is a unique and dominant feature of the northern hemispheric summer in southern Asia and northern Africa. The TEJ's position fluctuates between 5° and 20°N. On average, the height of this jet stream is 7.8km above ground and it moves between 30-45m/s. Unlike the subtropical jet stream, it flows from east to west. The establishment and maintenance of the TEJ is not fully understood but it plays an important role in initiating the processes leasing to the southwest monsoon. In summer, the upper air divergence associated with the equatorial jet over the parts of India reinforces low pressure conditions at the surface. The upper-level divergence associated with the equatorial jet helps to draw in rain-bearing south-westerly winds from the Indian Ocean. The southern branch of the subtropical jet stream persists even until early summer and the sudden onset of the wet monsoon season in India is typically associated with northward movement of the subtropical jet stream.

Question 35

Other defining characteristics of the Southwest Summer Monsoon

Answer 35

As the monsoon moves northwards, movement is not a smooth progression but a stepped one with the monsoon pausing or stagnating as it advances over India. The monsoon is not simply a prolonged period of exceptional rainfall for there are drier periods lasting weeks in June and August. The onset of the monsoon winds also affect other parts of Asia.

Question 36

Northeast Winter Monsoon in Africa

Answer 36

Just like India, winter monsoon in Africa is due to a shift of the Sun to the southern hemisphere in December/January. The ITCZ extends from about 5°N on the western edge and to 23°S at the eastern edge of the African content. An intense high pressure area develops over the north-eastern part of the continent as the land loses heat 3 times faster than the ocean. Winds then blow clockwise out of high pressure system in the northern hemisphere, forming northeast trade winds. These are known as the local harmattan wind which are dry and dusty trade winds. The wind blows over northern part of Africa to converge with the wetter southeast trades at the ITCZ. Similarly, on the western edge of the continent, south westerly winds passing over the ocean converge with the local harmattan wind (north east trade winds) resulting in conditions favourable for rainfall.

Question 37

Southwest Summer Monsoon in Africa

Answer 37

During summer in the northern hemisphere, the ITCZ shifts northwards to the Tropic of Cancer. An intense low pressure system develops in the north as ITCZ shifts, bringing intense heating and unstable conditions with increased rainfall particularly to the western and central-northern African region.

Question 38

Jet Streams and African Monsoons

Answer 38

The African summer monsoon is also affected by upper level winds. The TEJ extends from the Eastern Indian ocean to West Africa and flows between 20° and 30°N. During summer months, African Easterly Jet (AEJ) forms 10-15°N. The AEJ plays a crucial role in the West African monsoon system by enhancing ground level conditions.