Indian Climate – Seasons

1. 1. The cold weather season or winter season,
   2. The hot weather season or summer season,
   3. The south-west monsoon season or Rainy season, and
   4. The season of the retreating monsoon or cool season.

The Winter Season of India (January – February)

• Commences in November and continues till March. January is the coldest month.
• Sun’s apparent path is to the south of equator.
• Clear sky, pleasant weather, low temperature, low humidity, high range of temperature, cool and slow north-east trade winds are chief Characteristics.
• The diurnal range of temperature, especially in interior parts of the country, is very high.

1. Temperature Conditions in Winter Season of India

• • By the end of December (22nd December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere. So, India which lies in the northern hemisphere experiences low temperatures. January and February are the coldest months over most parts of the country.
  • The isotherm of 20°C runs roughly parallel to the Tropic of Cancer.
  • South of this Isotherm the temperatures are above 20°C in southern India and here is there is no distinctly defined winter weather where as in northern India mean Temperatures are well below 21°C and the winter weather is distinct.
  • The extreme south India typically experiences temperatures near 30°C.
  • The mean minimum temperature is about 5°C over north-west India and 10°C over the Gangetic plains.
  • Dras Valley in Kashmir is the coldest place in India. The minimum temperature recorded at Dras was – 45°C in 1908.

a. The Northern Region

• • • This season usually begins in late-November in northern India. Lowest temperatures are observed in Punjab and Rajasthan. December and January are the coldest months in the northern plain. The night temperature may be quite low.
    • There are three main reasons for the excessive cold in north India during this season:
      • • States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
        • The snowfall in the nearby Himalayan ranges creates cold wave situation; and
        • Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the north-western parts of India.

b. The Peninsular Region

• • • The Peninsular region of India, however, does not have any well-defined cold weather season.
    • There is hardly any seasonal change in the pattern of the temperature in coastal areas because of-
      • • Moderating influence of the sea, and
        • Proximity to the equator.

2. Air Pressure And Winds in Winter Season of India

• • In winter months, the weather conditions over India are generally influenced by the distribution of pressure in the Central and Western Asia.
  • A high-pressure centre in the region lying to the north of the Himalayas develops during winter which gives rise to the flow of air from the north towards the Indian subcontinent, south of the mountain range.
  • These surface winds blowing out of the high-pressure centre over Central Asia reach India in the form of a dry continental air mass.
  • These continental winds come in contact with trade winds over north-western India. The position of this contact zone is not, however, stable. Sometimes, it may shift its position as far east as the middle Ganga valley with the result that the whole of the north-western and northern India up to the middle Ganga valley comes under the effect of dry north-western winds.
  • In south India, the air pressure is slightly lower (due to higher temperatures). As a result, winds start blowing from north-western high-pressure zone to the low-pressure zone over the Indian Ocean in the south. Due to a low pressure gradient, the light winds blow outwards with a low velocity of about 3-5 km per hour.
  • These land bearing winds being cold and dry don’t give rain over most parts of the country. However, these winds cause rain along the Coromandel Coast since they collect moisture on their way over the Bay of Bengal.
  • In Short,
    • • High air pressure prevails over large parts of north-west India due to low temperatures coupled with divergence induced by the ridge of the STJ.
      • Pressure is comparatively lower in south India.
      • The winds start blowing from high pressure area of north-west to low pressure area of south-east. The wind velocity is low due to low pressure gradient.
      • The path of the winds depend on pressure gradient and physiography.

a. Jet Stream and Upper Air Circulation

• • • Higher up in the atmosphere, a different pattern of air circulation is observed during the winter months –
    • All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east. These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands. These are known as the sub-tropical/tropical westerly jet stream.
    • Tibetan highlands act as a barrier in the path of these jet streams. As a result, jet streams get bifurcated –
      • • One of its branches blows to the north of the Tibetan highlands, while
        • the southern branch blows in an eastward direction, south of the Himalayas, Which later combines over China once again.

• • • It is believed that this southern branch of the jet stream exercises an important influence on the weather in  Climate in Winter Season of India.
    • The mean position of this Jetstream is about 25°N.
    • This Jetstream first appears over the northern parts of the Indian subcontinent in the month of October after the withdrawal of the summer monsoon and shifts progressively southwards with the advance of the winter season. Thereafter, it shifts back towards the north, weakens and disappears from the South Asian region with the establishment of the South-West monsoon in June. According to Dr.Koteshwaram, the disappearance of this jet from the south of the Himalayas paves the way for the burst of South-West Monsoon along the coast of Kerala.

b. Western Disturbances

• • • During the Winter Season of India, the weather is pleasant. The pleasant weather conditions, however, at intervals, get disturbed by due to inflow of depressions from west. These low pressure depression are called Western Disturbances.
    • These temperate cyclones originates in Mediterranean Sea and travel eastwards across West Asia, Iran, Afghanistan and Pakistan and enter the north-western parts of India with the help of the westerly Jetstream (discussed above).
    • On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. Since these extratropical cyclones reach India from the West, they are usually referred to as the western disturbances over the Indian region.

• • • Because of the high terrain, mountain ranges, most of these disturbances are in the occluded form (Mature Form) by the time they reach India.
    • They intensify over Rajasthan, Haryana and Punjab, these disturbances slow down and stagnate due the nearly closed in nature of the region with high hills.
    • With moisture feed from the Arabian Sea, these disturbances may intensify and move north-east. The tracks of these disturbances come farthest south to latitudes 22/23°N in the month of February.
    • After the passage of disturbance, widespread Fog and cold waves lowering the min. temperature by 5o C to 10^o C below normal are experienced. Haze is common in Morning and evening.
    • An increase in the prevailing night temperature generally indicates an advance in the arrival of these disturbances.

The western disturbances result in:

• • • A cold spell in north-western India as these depressions are followed by cold waves.
    • Widespread Fog and Cold waves in northern India.
    • Winter rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh, locally known as ‘mahawat’. Although the amount is meagre, it is highly beneficial for Rabi crops.
    • Snowfall in the higher altitudes of Kashmir and Himachal Pradesh and sometimes hail too. It is this snow that sustains the flow of water in the Himalayan Rivers during the summer months.

c. Tropical Cyclone

• • • Least tropical cyclone activity in this season due to –
      • • the low temperature of sea surface, and
        • location of ITCZ farthest south in this season
    • Storms which are born in Bay of Bengal moves toward Tamil Nadu and that originates in Arabian Sea moves towards either north or west.

3. Precipitation

• • The retreating winter monsoon blow from land to sea and do not cause much rainfall.
  • While crossing Bay of Bengal picks up some moisture and cause winter rainfall in Andhra Pradesh and Tamil Nadu and South-east Karnataka and Kerala.
  • The Western Disturbance also causes a little rainfall in north-west India.
  • The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains.

So, this is all about the Climate in winter season of India.

In the next post (Click here), we will study about the summer season in India.