Weather, climate and their elements

"Is it going to rain today?" "Will it be sunny for the match?" You are asking about the weather (mausam): the state of the air hour by hour and day by day. It can change a lot from one day to the next.

Climate (jalvayu) is the average weather of a large area over a long time, usually thirty years or more. A simple way to remember: weather is what you get today; climate is what you expect from the place.

Five elements make up weather and climate: temperature, precipitation, humidity, wind and air pressure. Let us take them one at a time.

1. Temperature. It changes between day and night and between seasons. The key factor is insolation: the energy from the Sun that the Earth receives. The Sun's rays hit the equator most directly and the poles at a low angle, so insolation, and temperature, fall from the equator towards the poles. That gives three zones (Fig. 3.6): the hot torrid zone, the temperate zones and the cold frigid zones.

2. Humidity (aardrata) is the amount of water vapour in the air. Water evaporates from land, rivers and seas and becomes vapour. Warm air can hold more vapour than cold air. On a humid day, clothes take ages to dry and your sweat does not evaporate, so you feel sticky and uncomfortable.

3. Precipitation. When air is full (saturated) of water vapour, the vapour turns back into water or ice and falls to the ground: drizzle, rain, snow, sleet or hail. Winds, mountains and seasons decide how much falls. Rain is the most common form. It cools a place, and most of our groundwater comes from it. A long time without rain makes a dry climate.

4. Air pressure (vayudab) is the weight of the air pressing on the ground. It is highest at sea level and falls quickly as you go up.

• Where it is hot, air warms up and rises: low pressure, with clouds and wet weather.
• Where it is cold, air is heavy and sinks: high pressure, with clear, sunny skies.

5. Wind is air moving from high pressure to low pressure. Think of letting air out of a balloon: it rushes from where it is squeezed to where there is room. The Earth's spin also bends the wind's direction.

Watch out: a wind is named after where it comes from, not where it goes. A westerly wind blows from the west.

How strong is the wind? (Table 3.1)

Land and sea breezes (Fig. 3.8). Land heats up and cools down faster than water.

• By day (especially in the afternoon), the land is warmer than the sea. Air over the land rises, making low pressure there, so cool air blows from the sea to the land: a sea breeze.
• At night, the land cools faster than the sea. Now the wind blows from the land to the sea: a land breeze. It is gentle, because the difference is small.

These breezes keep the climate of coastal places mild.