Characteristics of Tropical Monsoon Climate-Seasonal, Temperature, Rainfall, Variation

Climate is the average state of the atmosphere near the earth’s surface over a long span of time. It refers to many elements including temperature, precipitation, humidity, air pressure, wind movement, and direction. The geographical location (latitude, coastal or continental position) and physical settings (e.g. mountains) influence the climate of any country. Let’s find more characteristics of a tropical monsoon climate.

Bangladesh extends from 20˚34΄N to 26˚38΄N latitude and from 88˚01΄E to 92˚41΄E longitude. It is bordered by the Himalayas to the north and by the Bay of Bengal to the south.

As the tropic climate passes through the country, a tropical climate prevails. The influence of the monsoon wind is so strong that as a whole, the climate of Bangladesh is known as a tropical monsoon climate.

Characteristics of a tropical monsoon climate

In brief: characteristics of a tropical monsoon climate

• Two dry seasons with a low rainfall:
• Cool and dry (winter)
• Hot and dry (summer)
• Rainy season with very high rainfall
• High annual temperature (about 26˚C)
• Seasonal distribution of rainfall
• High annual rainfall (more than 1800 mm)
• High humidity

Seasonal Variations

The season is the climatic type, at any place, associated with a particular time of the year. The change of season is mainly due to the change in the angle of the earth’s axis in relation to the position of the sun at a particular place.

Bangladesh is called the land of six seasons (Sadaritu). It has a tropical climate because of its physical location. The Bengal calendar year is traditionally divided into six seasons:

• Grisma (Summer)
• Barsa (Rainy)
• Sarat (Autumn)
• Hemanta (Late autumn)
• Shhit (Winter)
• Basanta (Spring).

Each season lasts on average two months, but some seasons merge into other seasons, while others are short.

More broadly, Bangladesh has three distinct seasons:

• The hot and dry pre-monsoon season, from March to May
• The rainy monsoon season, from June to October
• The cool and dry winter season, November to February.

The seasons of Bangladesh regulate its economy, communications, trade and commerce, art and culture, and, in fact, the entire lifestyle of the people.

The influence of the tropical monsoon climate is clearly evident in Bangladesh during the rainy season and the cool, dry winter season.

Characteristics of the hot and dry pre-monsoon season

• High temperature
• The occurrence of thunderstorms

April is usually the hottest month in the country. After April, increasing cloud cover lowers the temperature. Wind direction changes from time to time in this season, especially during its early part.

Rainfall which takes place during this time accounts for 10 to 25 percent of the annual total. This rainfall is caused by thunderstorms.

Characteristics of the rainy monsoon season

• Very high humidity
• Heavy rainfall
• Long consecutive days of rainfall
• South to southwesterly winds.

The rainy season coincides with the summer monsoon. The rainfall of this season accounts for 70 to 85 percent of the annual total. This is caused by the tropical depression that enters the country from the Bay of Bengal.

Characteristics of the cool and dry winter season

• Low temperature
• Cool west or northwesterly winds
• Clear skies
• Low rainfall.

The average temperature in January varies from 17˚C in the northwest and northeast of the country to 20˚C-21˚C in the coastal areas. The minimum temperature in the extreme northwest in late December and early January can be as low as 3˚C to 4˚C.

Resource Skills Task

Look at the date below. Construct a climate graph for Dhaka, and on it label the features of a tropical monsoon climate.

Dhaka	J	F	M	A	M	J	J	A	S	O	N	D
Temp ˚C	20	24	28	31	29	30	29	29	29	28	25	21
Rainfall	0	0	0	21	428	348	553	282	361	368	13	0

Temperature variations

January is the coldest month in Bangladesh. However, the cold winter air that moves into the country from the northwestern part of India loses much of its intensity by the time it reaches the northwestern corner of the country.

Average temperatures in January vary from about 17˚C in the northwestern and northeastern parts to 20˚-21˚C in the coastal areas. In late December and early January, minimum temperatures in the extreme northwestern and northeastern parts of the country, such as Rangpur and Sylhet, are as low as 4˚C to 7˚C.

As the winter season progresses into the pre-monsoon hot season, temperatures rise, reaching the maximum in April, which is the middle of the pre-monsoon hot season.

Average temperatures in April vary from about 27˚C in the northeast to 30˚C in the extreme west of the country. In some places in Rajshahi and Kushtia districts, the maximum temperature is during the latter part of the pre-monsoon season.

On average April, the temperature decreases slightly during the summer months, which coincides with the rainy season. Widespread cloud cover lowers the temperature during the later of the pre-monsoon season.

Average temperatures in July vary from about 27˚C in the southeast to 29˚C in the northwest of the country.

Rainfall Variations

Water that is condensed from the aqueous vapor in the atmosphere and falls in drops from the sky to the earth is called rain, and the total amount of rain that falls in a particular area within a certain time is called rainfall.

The single most dominant element of the climate of Bangladesh is rainfall. Because of the country’s location in the tropical monsoon region, the amount of rainfall is very high.

The rainfall in Bangladesh varies, depending upon season and location. Winter (November through February) is very dry and accounts for only less than 4% of the annual rainfall, rainfall, Rainfall in this season varies from 20 mm in the west and south to 40 mm in the northeast, which is caused by the westerly disturbances that enter the country from the northwestern part of India.

Rainfall in the pre-monsoon hot season (March-May) accounts for 10-25% of the total annual rainfall. The rain in this period is caused by conventional storms (thunderstorms) or nor-westers (locally called a Kal Baiskhkhi).

The average rainfall of this season varies from 200 mm in the west-central part of the country to 800 mm in the northeast region such as Sylhet and Chittagong divisions. Higher rainfall in the northeast is caused by the additional effect of the orographic uplifting provide by the Meghalaya plateau.

The rainy season (June-October) accounts for 70 to 80% of the annual rainfall, which varies from 70% in the eastern part of the country to about 80% in the southwest, and 85% in the northwest.

The amount of rainfall during the season varies from 1000 mm in the west-central part of the country to over 2000 mm in the south and northeast. Rainfall in this season is caused by weak tropical depressions that are brought from the Bay of Bengal into Bangladesh by the monsoon winds.

Again, higher rainfall in the northeast is caused by the additional optional uplifting effect of the Meghalaya plateau. After the withdrawal of the wet monsoon, which usually occurs in mid-October, rainfall diminishes at a rapid pace.

The average annual rainfall in Bangladesh varies from 1500 mm in the west-central part to over 3000 mm in the northeast and southeast. In Sturm Valley and neighboring hills, the rainfall is very high.

At Sylhet the rainfall average is 4180 mm, near the foot of the abrupt Meghalaya Plateau at Sunamganj it is 5330 mm, and at Lalkhal 6400 mm, the highest in Bangladesh.

Resource Skills Task

1. Using the data below, construct climate graphs of the following three stations (1999) to illustrate the varying temperatures and rainfall amounts.
2. Describe the rainfall distribution at each station.

Sylhet	J	F	M	A	M	J	J	A	S	O	N	D
Temp ˚C	21	24	26	28	29	28	29	29	29	28	25	22
Rainfall	0	0	49	207	731	472	775	503	253	344	0	0

Total 3334

Rajshahi	J	F	M	A	M	J	J	A	S	O	N	D
Temp ˚C	18	22	27	32	30	30	29	29	28	28	24	20
Rainfall	0	0	0	9	144	472	348	349	354	155	1	0

Total 1862

Chittagong	J	F	M	A	M	J	J	A	S	O	N	D
Temp ˚C	21	25	27	30	29	29	29	29	28	28	26	22
Rainfall	0	0	0	0	463	879	491	848	203	201	2	107

Total 3194

Resource Skills Task

1. Look at a map in an atlas that shows the average annual rainfall for Bangladesh. Describe the distribution of the rainfall (comment on the areas which receive the highest and lowest amounts).
2. Look at a map in an atlas that shows the annual mean temperatures for Bangladesh. Describe where the highest and lowest temperatures are found

Factors that contribute to rainfall and cyclone

Monson rainfall

The word ‘monsoon is derived from the Arabic word ‘Mauism’ which means seasons. The wind that changes direction with the change of seasons is known as the monsoon wind.

Seasonal changes in this wing direction are caused by the differential heating and caused by the differential heating and cooling of landmass and oceans between summer and winter.

The monsoon wind blows from the northeast (towards the sea) in winter (the dry monsoon) and from the southwest (towards the land) in summer (the wet monsoon). 

During the winter months in the northern hemisphere, the land is colder than the sea and so a high-pressure system develops over the Asian landmass.

At the same time, a low-pressure system develops over Australia and the Indian Ocean where it is warmer. Winds from Asia’s high-pressure area blow towards the low-pressure system in the southern hemisphere.

These are the northeast monsoon winds. since the wind in this season blows forms the land to the ocean, almost dry conditions prevail and very little rainfall occurs at this time. They are cool and dry as they blow over the cool land and pick up a little moisture.

The converse happens during the summer months. The high temperatures of the Asian landmass create an area of low pressure. At the same time, high pressure develops over the comparatively cooler Indian Ocean.

This difference in pressure causes the winds to flow from the high-pressure area to the low-pressure area that is, from the ocean to the land area. This flow of wind is known as the summer monsoon circulation.

As a result, the summer monsoon winds bring in a huge amount of moisture and cause heavy rainfall, especially in Bangladesh and the neighboring states of India.

It enters Bangladesh in late May or early June.

Depression Rainfall

Depression is a region of low atmospheric pressure. A depression forms as warm, moist air from the tropics mixes with cold, dry polar air, producing warm and cold fronts.

The warm air being less dense rises above the cold air to produce an area of low pressure on the ground. The rising warm air cools rapidly, condensation occurs, clouds form, and rainfalls.

Depressions occasionally develop over the Indian Ocean or in the Bay of Bengal. The timing of such depressions coincides with the southwest monsoon wind. During such depressions, continuous heavy rainfall occurs in Bangladesh for 7 to 10 days. At times more than two to three weeks of rainfall may occur in Sylhet and Cox’s Bazar areas.

Thunderstorm rainfall

Thunderstorms are tropical storms with thunder and lightning accompanied by heavy rain or hail. On hot, humid days, the air near the ground is heated up. As a result, warm moist air expands and rises rapidly.

This rising air cools, and condenses into water vapor forming cumulonimbus clouds. This results in heavy rainfall (see Figure 2.3). These storm clouds are usually 5 km wide and 8 km high.

Usually individual thunderstorms in just one cell in a group of storms, which may be 30 km wide, lasting for more than five hours. A single-cell storm can also become a superstorm covering an area of 50 km. This produces large hailstones, strong winds, thunder, and lightning.

In Bangladesh, in early summer during March/April and late monsoon in October/November, this type of thunderstorm occurs in the evening with great intensity. This is popularly known as Kal Baishaki in the early summer and Ashshiner Jhor during the late monsoon. 

Cyclones

A cyclone is a tropical storm or atmospheric turbulence involving the circular motion of winds. Technically, a cyclone is an area of low pressure where strong winds blow around a center in an anticlockwise direction in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere.

Cyclones occurring in tropical regions are called tropical cyclones. The tropics can be regarded as the region lying between 30˚N latitude and 30˚S latitude. All the tropical seas of the earth, with the exception of the south Atlantic and southeast Pacific, give birth to this atmospheric phenomenon known as tropical cyclones.

Bangladesh is part of the humid tropics, with the Himalayas on the north and the funnel-shaped coast touching the Bay of Bengal on the south.

This peculiar geography of Bangladesh binges not only on the life-giving monsoon but also catastrophic cyclones, western, tornadoes, and floods. The Bay is an ideal breeding ground for tropical cyclones.

Cyclones develop over warm seas when the surface temperature is more than 27˚C. The air above the sea is heated, it expands and rises very rapidly creating an area of intense low pressure.

The warm, moist air spirals upwards, condenses, and forms clouds and rain. Air from the surrounding area spirals inwards to replace the rising air.

This spinning air can reach speeds of 200 km/hr and absorbs large amounts of moisture forming cumulonimbus clouds that result in heavy rain. The cold air then sinks. The center of the cyclone is calm and is known as the eye.

Among all the atmospheric disturbances, cyclones are the most destructive. The diameter of a cyclone may range from 300 km to 600 km.

The most striking feature of a cyclone is its ‘eye’. That can be seen clearly in the satellite pictures case of a well-developed cyclone, which is small and almost circular; it coincides with area’s lowest pressure and has a diameter ranging from 8 km to 50 km. warmer than the rest storm area, more violent storm, eye. winds are a very light eye, usually, not 25 30/hr rain practically absent, contrast, strongest heaviest occur just outside this central eye.

The life cycle of a cyclone ends soon after the cyclone reaches land (landfall) because it is cut off from its moisture source.

In addition to the waves associated with winds, abrupt surges of water known as storm surges are associated with cyclones. They strike the coast nearly at eh same time that the center of the storm crosses the coast.

In Bangladesh, the maximum value of this storm surge has been reported to be as high as 13m. Most of the damage during a cyclone is done by the storm surges, which sometimes wash over entire offshore and large areas on the coast.

The most destructive element of a cyclone is its accompanying surge. There is little that can withstand a great mass of onrushing water often as high as 6m. In Bangladesh, cyclones occur in April-May and also in September-December.

On average, five severe cyclonic storms hit Bangladesh every year and the accompanying surge can reach as far as 700 km inland. Surge heights increase with the increase of wind speed. Astronomical tides in combination with cyclonic surges lead to higher water levels and hence severe flooding.

Resource Skills Task

1. Study the map above. Name two high-risk areas to be hit by cyclones. What will the surge heights be in those areas?
2. How far inland is likely to be affected by a cyclone?
3. Is Dhaka likely to be affected by a cyclone? Justify your answer.

Cyclones in the Bay of Bengal

The funnel-shaped coast near the Bay of Bengal very often becomes the landing ground of cyclones. The Bay cyclones also move towards the eastern coast of India, towards Myanmar, and occasionally into Sri Lanka. But they cause the maximum damage when they come into Bangladesh. This is because of: 

• The low flat terrain
• The high density of the population
• Poorly built houses.

Most of the damage caused by the cyclones occurs in the coastal regions of Khulna, Patuakhali, Barisal, Noakhali, and Chittagong and the offshore island of Bhola, Hatiya, Sandwip, Mapura, Kutubdia, Maheshkhali, Nijhum Dwip, Urir Char, and other newly formed islands.

From 1981 to 1985, 174 severe cyclones (with wind speeds of more than 24 km/hr) formed in the Bay of Bengal. The month-wise occurrence is as follows:

Severe cyclones occur mostly during pre-monsoon (April-May) and post-monsoon (September-December) periods and they are the ones that cause the most destruction.

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