Weather and Climate, Seasons of Nepal, Monsoon and Asian Monsoon

Weather and Climate

Weather:

Weather is defined as the day to day state of the atmosphere, consisting of the short-term variation of mass and energy exchange within the atmosphere and between the atmosphere and the earth, surface variables are called the weather. Weather results from the process that attempts to equalize the difference in the distribution of the net radiant energy from the sun. It plays the vital role in the composition and the functioning of the ecosystem. The difference atmospheric variables and earth surface variables of producing weather are temperatures, precipitation, humidity wind velocity and the direction as well as to the soil moisture, drought condition etc.

Climate:

At a region, the climate is defined as the summary of the past weather events that have occurred at that location. Climate is typically described by the statistics of the atmospheric and the surface variables like temperature, precipitation, humidity, wind velocity and the direction as well as by the soil moisture content, drought conditions etc. statistics may be in terms of the long-term average or the other measure such as minimum of maximum temperature, length of growing seasons frequency of flood etc.

It determines the internal vegetation suitability for the human habitation and status of biodiversity. It influenced on the human welfare event range from immediate effects of the weather events to complex responses associated with the climate change. Flood drought hurricane and the heat wave as an immediate effect to melt of the polar ice rise in the sea level expansion of the desert and world hunger associated with the climate change.

Seasons of Nepal

Nepal is located towards the south of the Himalayan range. It has varying topography. This varying topography has resulted in the various season. As the Nepal is located in the subtropical region of the northern hemisphere, the usual season like spring, autumn, rainy can be identified.

As the country is dominated by the summer monsoon, the above way of classifying the season seems to be vague and a bit meaningless. Hence, it is more appropriate to identify the season on the basis of the temperature and the precipitation.

The climatic factors in this region show a marked variation in the temporal scale. The climate of Nepal can be divided into four seasons.

A: Pre-monsoon season (mar-may)

This pre-monsoon season is dominated by the dry climate westerly wind. The atmospheric condition is dusty and windy. The precipitation phenomenon is due to the convective phenomenon. The temperature gradually increases from March till May resulting the hotter days with increasing heat amounts. High temperature results a local convection activity is the hottest part of the country where the maximum temperature is in the range of 39-degree centigrade to 44-degree centigrade.

B: Rainy (monsoon) season (June-Sep)

This rainy season is associated with the south-easterly monsoon wind. Generally, monsoon arrives over the eastern part on the June 10 and within a week, it covers the whole country. When the monsoon wind approached, precipitation gets started with the high precipitation on the windward side and low precipitation on the leeward side. The precipitation in this season accounts for almost 80% of the total amount of rainfall. The moisture condition of an atmosphere is found to be high and the temperature is found to be low in the comparison to the pre-monsoon season. The solar insolation is low during this period because of the cloudiness in the sky.

C: Post monsoon season (oct-now)

This season starts just after the retreat of the monsoon i.e south-easterly wind and again westerly wind prevails. This season receives the very little precipitation which is characterized by the fine weather with the sunny days.

D: Winter season (Dec-Feb)

In the season of the winter westerly wind dominates. The wind comes from the Siberian high-pressure region and so it is characterized by the cold weather. The westerly winds are laden with the moisture from the Mediterranean and the other seas in the southern Europe. This moisture gets precipitated in the Nepal in the form of the winter precipitation. This phenomenon is termed as the western disturbance causes the precipitation in the higher region in the form of snowfall. The southern part of the country is mild but northern part is very cold. Examples are the formation in the atmospheric reduces the solar radiation.

Monsoon and Asian monsoon

Monsoon :

The term monsoon is derived from the Arabic word Muslim which means the season or wind shifting from one direction to the other direction.

In the meteorology, the monsoon is defined as the directional shifting of the wind from one direction to the other direction. In summer there is a warm and the moist wind blowing from the ocean towards the land while during the winter a cold and the dry wind originating on the land seaward. Monsoon circulation involves the changes in the 180 degrees in the direction of the wind.

Various theories have been proposed for the origin of the monsoon they are described as follows:

1: Thermal theory

Halley proposed this thermal theory. According to this theory, the monsoon is considered to be the gigantic conventional system produced by the differential seasonal heating of the continental and the oceanic areas. During winter, the huge land mass of the Asia cools more rapidly than the surroundings areas with the result that the strong high-pressure center develops over the continental and the pressure over the adjacent ocean is relatively low. Therefore, there is an outflow of air from the continental landmass towards the low latitudes. In the summer the temperature and the pressure conditions are reversed. In the summer, the huge landmass of the Asia heats quickly and develops a strong low pressure whereas pressure over the adjacent ocean is high. The warm and the moist air over the ocean flows from the high-pressure to the low-pressure system. The saturated warm and the moist air is an unstable and moves towards the up by the influenced of the high moisture of the Asia and the atmospheric turbulence. This air cools as it moves upward leading to condensation and the heavy rainfall is expected.

2: Planetary wind migration theory

John proposed that the seasonal migration of the planetary wind causes the reversal in the direction of the wind.

Asian monsoon

Especially there are two types of the Asian monsoon. They are:

1: Winter monsoon

2: summer monsoon

Brief description of the Asian types of the monsoon is as follows

1: Winter monsoon:

During the winter, the intense cooling of the huge land mass of the Asia leads to the formation of the high-pressure system over the continent. The center of the usually strong anticyclone lies in the eastern Siberia and in the Peshawar in about 1035 to 1040 Mb. In the adjacent, ocean is warmer than the extremely cold continent. The whole of the north pacific and in the equatorial zone of the Indian ocean, there is a low-pressure center. Since it is summer in the Australia there is also the low-pressure center from which there is an outflow of air that used to spread towards the south and the southeast over the Korea, china, and the japan. So, the continental polar air sometimes becomes very steep which results in the high velocity of the oceanic low-pressure center termed as the winter season or the dry monsoon.

2: Summer monsoon:

In the summer, the temperature and the pressure condition of the Asian continent are reserved. In this season, the huge land mass of the Asia heat quickly and a strong low-pressure center develops over there. This development is reinforced by the inter-tropical convergence of which reaches its maximum poleward migration to a positive over the southern Asia. Thus, the low pressure attracts warm and the moist air from the ocean. It may be pointed out that the tropical air masses which are unstable and full of moisture cannot by themselves yield precipitation. Precipitation results only when the moisture-laden winds are forced to the rise by the landform barrier or any atmospheric turbulence.

References:

.S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)