The atmospheric pressure changes continuously due to several factors. The most important factors among them are changes in temperature, water vapor content and rotational movement the earth. Hot air expands and exerts low pressure. Cold air contracts and exerts high pressure. In the months of January and June, there is significant difference in these conditions, and therefore there is difference in air pressure also in both these conditions.

In the month of January, center of low air pressure develops in the south – western coastal region and near Andaman – Nicobar islands group in India. While in northern India, condition of high air pressure is seen in mountainous parts and in north – western Rajasthan. The situation is absolutely opposite in the month of July. Due to high temperature, center of low pressure develops in north – western Rajasthan, western Haryana and Punjab, while in the south, the coastal parts and Andaman – Nicobar region show the condition of high air pressure as they become less heated up due to the effect of the ocean.