Solar Radiation, Heat Balance and Temperature | Class 11 Geography Notes

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air column above a unit area on the Earth's surface. It is a fundamental concept in meteorology and geography because it influences wind patterns and weather systems. At sea level, the average atmospheric pressure is approximately 1,013.2 millibars (mb). This pressure results from the gravitational pull acting on the air molecules, making the air denser and heavier near the surface. As altitude increases, the atmospheric pressure decreases because the density of air reduces with height. This decrease is rapid in the lower atmosphere, roughly about 1 mb for every 10 meters increase in elevation, but the rate is not constant everywhere due to temperature variations and other factors. Atmospheric pressure is measured using instruments such as the mercury barometer and the aneroid barometer. To compare pressures from different locations accurately, pressures measured at stations above sea level are reduced to sea level values, eliminating the effect of altitude. This standardization allows meteorologists to draw isobars, which are lines connecting points of equal pressure on weather maps, facilitating the study of horizontal pressure distribution and wind movement.

📊 Diagram: Table on page 1 (5×3) showing standard pressure and temperature at selected levels of elevation.

🧪 Activity: Consult Practical Work in Geography — Part I (NCERT, 2006) to learn about mercury and aneroid barometers.

🔗 Connection: This section leads to the study of how pressure varies horizontally and vertically, which is essential to understand wind formation and atmospheric circulation.

Table on page 1 (5×3)