Western disturbance is an extra tropical rain bearing system which is very important in the sense that the winter rainy of least 15 countries in dependent on it.
The disturbance is so called because it disturbs the normal, settled, sunny and stable weather prevails in the regions it affects. The term western is used because all the countries which it affects lie east to it.
It has its origin in the Mediterranean sea which, by the way is more than 3000 miles to the northwest of North Central India where its effect can be recorded.
The path of this rain bearing system is very well marked and well known to the meteorologists. Its path is so reliable that if it is affecting Lebanon or Syria or Israel, that lie very close to it, it can be predicted with more than 90 percent accuracy that how many days it will take to reach Islamabad or even New Delhi. It is point to be noted that both these capitals are located more than 2-3 thousand kilometers to the south west of it.
For instance if it is currently giving rain at Tel Aviv in Israel, then it can easily be predicted that when it will hit Amman in Jorden, then Baghdad in Iraq, then Teheran in Iran, then Kabul in Afghanistan, then Islamabad in Pakistan and finally New Delhi In India. Beyond New Delhi, however, it loses its grip and rainy figures fall sharply.
The rain it produces is of uniform pattern and for three to four days the sky is overcast. It is a slow moving system unlike monsoon rain which is like a heavy cloudburst for a few hours and it is clear afterwards. In western disturbance case, sky is overcast for days together with light to moderate intermittent rain pouring day and night.
Effects on Temperature and Humidity of the Area it affects
An approaching western disturbance raises the temperatures of the area it is going to affect by 10-15 F on the average and the humidity rises considerably. Then comes the rain which at some occasion can also give hail. After it passes away, there is a sharp fall of temperature and within 24 hours, temperatures can drop as much as 20 F. Cold winds begin to blow and humidity falls sharply.
In short, western disturbance is a rain bearing system originating in the Mediterranean Sea affecting countries as far away as India.