Those living in northwestern parts of the subcontinent, i.e., Pakistan and Northwestern India, know that the region experiences two types of rain during the year. One kind of precipitation is associated with cold fronts and long periods of light rain, usually for days in the winter months. The second type of precipitation occurs during the late summer months and is heavy and usually lasts only for hours. This sharp variation in these two rainy seasons is due to the different systems responsible for these rains.
The first rain-bearing system that brought rain in winter is known as Western Disturbance while the weather system responsible for rain in summer months is known as the Monsoon. These systems are quite different in their behavior, origins, and time of impact. In this article, we will briefly discuss the top differences between weather systems. But before digging deep into the topic, let’s first know what these two rain-bearing systems are.
Western Disturbance
As the name depicts, Western disturbances are extra-tropical systems flowing west to east. It usually originates in the Atlantic Ocean and the Mediterranean Sea and travels for days before reaching the Western South Asian region. Western disturbances form due to high-pressure areas that develop over Eastern Europe during winter months. Due to high pressure, Siberian thrust diverts to the southern region, interacting with a warm moist environment available over the Mediterranean Sea thus providing a favorable environment for developing extra-tropical storms known as Western Disturbances. These systems then move eastward towards Pakistan, Northern India, Tibbet, and sometimes towards Bangladesh.
Monsoon
Monsoon’s name is derived from the Arabic word “Mausim” which means reversal of seasonal winds. The monsoon phenomenon is associated with the reversal of winds which brings wet weather to various parts of the world during the summer months. The most prominent example of a monsoon region is South Asia. In summer during the hot weather, low-pressure areas build over the Thar desert pulling the wet winds from the Indian Ocean from June to September. The nature of monsoon is abrupt, and a lot of rain can fall in a few hours creating flash flood-like situations in urban centers.
Major Differences Between Western Disturbances and Monsoon
Western Disturbances and monsoons are two different weather phenomena having differences including:
1- Origin
The origin of western disturbances (WDs) and the monsoon is quite different. WDs originate from the Atlantic Ocean and, to a lesser extent, the Mediterranean Sea. It is then intensified by the interaction of Siberian Thrust diverting to the Mediterranean Sea due to high-pressure areas formed over Eastern Europe. WDs then move towards to reach Pakistan/Northwest India.
Monsoon develops due to low-pressure areas that form over the Thar desert of Pakistan/India due to severe hot weather during Asian Summer. As the temperature over the Indian Ocean is comparatively cooler and pressure is high, this creates a perfect pulling effect for moist winds. These moist winds then interact with the Himalayan mountains and rise high. As the winds rise, they cool and thus cloud formation takes place, resulting in heavy rains over the South Asia Sub-continent.
2- Nature of Precipitation
The nature of rain/precipitation varies a lot between both these systems. WDs produce generally light to moderate rain over a prolonged period and for a larger area. As WDs are most active during winter months, therefore, a major portion of precipitation falls as snow over mountainous areas. Meanwhile, the Monsoon develops during the summer season, and the sport of moist winds is abundant during its development, therefore, a lot of rain can fall over a short duration of time. The nature of monsoon varies greatly region-wise and topography-wise. Coastal and hilly areas receive more rainfall during monsoon compared to plain areas.
3- Duration of Impact
WDs impact the Northern Sub-continent during the whole year with intervals. However, the intensity of WDs varies greatly during seasons. The peak of WD intensity comes in the winter and spring seasons and its intensity is lesser during peak summer months.
In contrast, the monsoon season in the sub-continent is four months long and usually ranges from 1st June to 30th September. However, it is followed by North East Monsoon which usually lasts from October till December. In other months, there are no monsoon rains.