What phenomenon is generally indicated by an increase in night temperature during the winter months?
AThe arrival of a western cyclonic disturbance
BThe retreat of the monsoon
CThe formation of a high-pressure zone over the Himalayas
DThe onset of trade winds
Answer:
A. The arrival of a western cyclonic disturbance
Read Explanation:
Western Cyclonic Disturbances and Winter Temperatures
- Western Cyclonic Disturbances (WCDs) are extratropical storms that originate in the Mediterranean region.
- These disturbances travel eastwards across Northern India, bringing about a significant change in weather patterns during the winter months (December to March).
- Primary Impact on Temperature: An increase in night temperatures during winter in Northern India is a strong indicator of the approach or presence of a WCD.
- Mechanism: As a WCD approaches, cloud cover increases, which traps the outgoing longwave radiation from the Earth's surface. This greenhouse effect leads to warmer nights. The clouds also prevent excessive cooling by radiation into space.
- Other Associated Phenomena: WCDs are also responsible for:
- Winter Rainfall/Snowfall: They cause light to moderate rainfall in the plains and snowfall in the higher reaches of the Himalayas. This is crucial for winter crops like wheat (Rabi crops).
- Reduced Visibility: Fog and mist can also be associated with these disturbances, especially in the early stages.
- Changes in Wind Patterns: They can alter local wind directions.
- Origin: WCDs develop in response to the interaction of westerly winds with the subtropical westerly jet stream over the Mediterranean Sea.
- Geographical Relevance: The phenomenon is most pronounced in the northwestern parts of India, gradually affecting other northern regions as the disturbance moves eastward.
- Distinction from Monsoons: Unlike the summer monsoon rains, which are predominantly southwesterly and bring heavy rainfall, WCDs are associated with westerly or northwesterly winds and bring much-needed winter precipitation.