Over India and the Asian land mass to the north of it, a summer low is replaced each year by a winter high. This happens because of the different ways that land and water react to temperature change. Ocean water mixes as it is heated. Turbulence and wind spread the growing heat through a layer that may be 50 meters deep. With more volume to heat than found on land, ocean water does not heat quickly, although it holds heat well.
Solid land conducts heat more slowly and the warmth does not penetrate as far, perhaps only one meter. It heats faster than water. So the air over land heats relatively quickly, and in summer it forms a low. This low is the low air pressure zone that forms when heat causes inland air to warm and rise, the way a hot-air balloon lifts when the air inside it is heated. The air is less dense, so it exerts less pressure.
Meanwhile, over the ocean, surface water becomes water vapor. Maritime air gains moisture. Then it is drawn in over the land to take the place of the air that has risen up and away over India. As it moves over the land the sea air cools, particularly as it rises over mountains. As the air cools, the water vapor it holds condenses and falls as rain.
In northern India, the great wall of the Himalayas holds back the moist air, producing an area with some of the highest rainfall on earth. As the drought of winter breaks, storms rage and rain pounds down. Forty feet of rain will fall on certain parts of India during the four months of the summer monsoon.
Around the world, winds flow as directly as possible from high pressure to low, allowing for the curvature caused by the rotation of the earth. Since it is the nature of wind to flow from high to low, it changes direction as the location of the monsoonic highs and lows change from wet to dry seasons.
In summer, from June to September, the low may bring India 90% of the rainfall of the entire year. But in the dry season, this low is replaced by a winter high, when the cooler air of winter sinks over the land, packing in close to the ground and causing high air pressure. Then the winds flow from the continental interior to the relatively warm low over the sea that has retained some of the heat of summer. This outflow is dry continental air. It can not rain. Throughout southern and eastern Asia the winter phase of the monsoon is associated with dry weather.
There is a monsoon weather system in northern Australia
as well, but there the rainy months are opposite. Northwesterly (that is, winds from the northwest) winds flow into northern Australia
from November to April bringing intermittent rain. Then the southeasterly trade winds flow outward from the central Australian desert from April to November, and the land dries. Asian and Australian weather are actually connected. Scientists have noticed a correlation between the amount of Australian rainfall and the depth of Himalayan snow.
Northeastern Mexico and the southwestern United States also have a monsoon that brings sudden rain during many summers. It can deliver 60% of the rainfall for a year to northern Mexico. Fierce storms may extend as far north as Utah. When they appear, visitors are astonished to see lightning in the desert, warring with the city neon, as the dry washes suddenly overflow with water that will soon sink beneath the sagebrush and sand and disappear. August is the month to see spectacular thunderstorms in Las Vegas.
