Introduction

The chapter opens with a simple observation: you drink more water in summer, and your uniform changes with the season. In north India you wear light clothes in summer and heavy woollen clothes in winter, but in southern India woollens are hardly needed. The northeastern states have mild winters except in the hills. These everyday differences point to something deeper — weather conditions vary from season to season, and these variations are driven by changes in the elements of weather: temperature, pressure, wind direction and velocity, humidity, and precipitation.

Weather is the momentary state of the atmosphere — what is happening right now, or over a day or a week. It changes quickly. Climate, on the other hand, is the average of weather conditions over a much longer period. Climate changes imperceptibly; you may notice a shift only after fifty years or even more. So the same elements — temperature, pressure, winds, humidity, rainfall — operate on two different time scales, giving us weather on the short term and climate on the long term.

The unit also introduces the monsoon, which you have studied in earlier classes. The word "monsoon" carries a specific meaning: it connotes a climate associated with the seasonal reversal in the direction of winds. India has a hot monsoonal climate, and this type of climate is the prevalent one across south and southeast Asia.

The unit as a whole covers two broad themes. First, weather and climate — their spatial and temporal distribution of temperature, pressure, winds and rainfall, the mechanism of the Indian monsoons, their onset and variability (again in both space and time), and the climatic types found in the country. Second, natural vegetation — forest types and their distribution, wildlife conservation, and biosphere reserves.