As the summer heats up and it seems to get hotter and hotter every day we all wait eagerly for the monsoons, scanning the papers to see if it will arrive on time. They have just hit southwest India and are now slowly moving northwards. Once the rains come to Mumbai the newspapers carry information on how much rain has fallen and if the water reservoirs that store water for the city are close to becoming full. Economists attempt to predict what the monsoon will be like and what effect it will have on the country.

The monsoon rains are vitally important to both farmers (for their crops) and to city people for drinking water.

But what are these rains and where do they come from? Though we think of the monsoons as rain, meteorologists define the monsoon as a seasonal shift in wind direction. The word 'monsoon' itself originally comes from the Arabic word 'mausim', meaning season, and does not mean heavy rain.

In a real monsoon-type climate, seasonal wind shifts typically cause a drastic change in the general precipitation (as rain is scientifically called) along with temperature patterns. The monsoon is also associated with dry weather, since the 'wet' monsoon phase of warm, moist air is seasonally replaced by a 'dry' monsoon of cool, dry air. This phenomenon is the dominant feature of low-latitude climates stretching from West Africa to the western Pacific Ocean .

The annual monsoon cycle takes place as a result of a seasonal and geographical variation in heating at the surface of land and water by sunlight. Different parts of the earth's surface heat up and cool down at different rates depending on their ability to absorb solar radiation and the time of year. Oceans, which can absorb sunlight at varying depths and reflect less back to the atmosphere, store energy more efficiently than land and so retain heat longer than a landmass. Land surfaces gain or lose heat at a quicker rate due to the shallowness of their absorbing surfaces. Warm air rises while cold air sinks. Those of us who live on the coast may have noticed a phenomenon known as 'land-sea breeze': on a sunny day the land warms more quickly than the ocean. As the hot air rises over the land, cooler air replaces it over the water so the wind blows from the sea into the shore. At night, however, the land cools at a quicker rate than the water, so the wind shifts, blowing from the land out to the warmer sea.

Monsoons are a larger version of this. In a continent surrounded by oceans, heat builds up on land, which, over time results in warmer air of lower density (seen as areas of low pressure). Meanwhile, denser air associated with high pressure build-up over ocean surfaces. Over time, these temperature and pressure imbalances build up and form currents, wind and ocean currents that result from air flowing from high to low pressure areas of warmer and colder air and water, contributing to the global energy balance.

Just as heat differences develop between land and water surfaces, the variation in space and time of solar heating due to the earth's tilt create seasonal heat imbalances. The hemisphere receiving the most direct radiation (during the summer months) experiences more total heating -- more energy is gained from the sun than is lost to space. The winter hemisphere is at the same time experiencing net cooling. As part of a global compensation, heat is transported from warmer to cooler areas by ocean and wind currents. Since the areas of heat surplus and deficit change throughout the year, as in the sea breeze example, the direction the heat flows in must also change. Countries and geographical regions dominated by the monsoon experience the most pronounced of such seasonal wind shifts. In South Asia , our rainy season, typically beginning in June, is preceded by nearly two months of scorching temperatures, cooled only with the commencement of the rains brought by winds coming from over the ocean from the southwest. January is the peak of the dry season with winds coming from the dry northeast.

In countries like India , which has a monsoon, these cycles are deeply ingrained in our culture and economy. In fact, the economy can be said to revolve around the monsoon because our agriculture depends on the monsoon and India is an agricultural country whose economy depends on farms and crops; two-thirds of our population earn a living from agriculture and nearly a quarter of all economic activity is related to it. Of this agriculture activity nearly 60% depends on the monsoon rains. Because of this, economists are trying to predict what India 's economy in the year ahead will be like as the monsoon engulfs the country.

What's important to farmers, therefore also to economists and planners, is not just the total amount of rain the monsoon brings but also the timing of the rain. (Of course, too much rain and flooding becomes a serious problem.)

It's mostly June and July that need good steady rains since that is the sowing season for most rain-fed crops. A late monsoon will affect the sowing of crops such as rice and cotton, which are planted around this time of year, June and July, and become ready to be harvested by the end of the year in October and November. If it rains for a little while at the beginning of the monsoon then fizzles out most of these plants will die; if there are large gaps between heavy rains some plants weaken and then drown or start rotting when it rains heavily again. One crop that illustrates the importance of the rains is sugarcane; a good monsoon is vital for sugarcane. In a year with bad monsoons, India can go from a sugar exporting country to a sugar importing one. For sugarcane, a late monsoon can be devastating even if the total quantity of rain is enough.

A bad monsoon hits rural incomes. During a bad monsoon, since farmers know they will make less money, they spend less. They buy fewer things and pay less salary to their labourers. Less spending means fewer goods need to be made. So less work for the factories, which make less profit, and so they start firing people, leading to unemployment. Since many factories and head offices of industries are in big cities, cities get badly affected by higher unemployment. Banks also feel the pinch, as farmers don't pay back their loans. As people have less hope that money will be coming in, construction also suffers since people will invest less in houses and other buildings. So a bad monsoon has a domino effect which can affect the entire country.

In India , most of the farmland that can be irrigated is already irrigated. So any planned growth in agriculture has to come from rain-fed agriculture; there is not much room for irrigated agriculture to grow further. One of the major reasons the government is trying to push its river linking project is to bring irrigation to parts of the country where irrigation has not so far been possible. Environmentalists are against this mega project for many reasons, but in any case rain-fed farming is a lot better for the environment than irrigated farming. Though irrigation helps crops grow where they might not have been able to earlier, and also in greater quantities, it also damages the soil making it less productive due to salinisation, a process where irrigation water leaves behind salts as it dries. Irrigation also leads to waterlogging and leaching out of soil nutrients that plants need to grow. Hence irrigated fields also need lots of fertiliser. Dams and canals built to provide irrigation water also bring their own problems.