Barometer, telescope and compass on a wooden table

Atmospheric pressure explained

Atmospheric pressure is the force exerted by the atmosphere on the Earth's surface. It is caused by differences in air density and gravity. Also known as barometric pressure, it is measured in hectopascals (hPa) or millibars (mbar) and corresponds to 1013.25 hPa or 1013.25 mbar for a standard atmosphere.

What is atmospheric pressure?

Atmospheric pressure is a measure of the force exerted by atmospheric gases on a surface. It is greater at sea level than in mountainous regions, but it tends to decrease with increasing altitude. The explanation for this difference lies in the air masses above us, which are greater at sea level than in the mountains.

How to calculate atmospheric pressure?

Clipedia | Atmospheric pressure: introduction

The variation of atmospheric pressure is one of the factors that control the movement of air in the atmosphere and, consequently, the weather.

This force acts as a filter that selects the types of air that can move aloft in the atmosphere. Thus, high pressure favours cold air flows towards higher latitudes, while low pressure favours warm air flows towards lower latitudes. The variation of atmospheric pressure is one of the factors that govern the movement of air in the atmosphere and, ipso facto, the weather.

Atmospheric pressure is measured with an instrument called a barometer. There are different types of barometers, but the principle of operation is the same: they use the force exerted by atmospheric gases to estimate the surface air pressure.

From a theoretical point of view, starting from an average state of the atmosphere (different from its actual condition), it is possible to determine the atmospheric pressure using the international barometric levelling formula.

If we take sea level as the reference altitude h0 and an average state defined by theICAO standard atmosphere (pressure 1013.25 hPa, temperature 15 °C = 288.15 K, vertical temperature gradient 0.65 K per 100 m), the international formula for barometric levelling is obtained:

International formula for barometric levelling

This formula allows the calculation of the pressure at a certain altitude, without the need to know the temperature and the vertical temperature gradient. However, the practical application of this formula is limited, as it does not reflect reality.

This is because the warmer the air, the lower its density, which results in a decrease in air pressure. In addition, air pressure is also influenced by weather conditions, such as warm and cold air fronts.

Which device measures atmospheric pressure?

The device that measures atmospheric pressure is the barometer. This instrument measures the atmospheric pressure and allows its comparison with a normal pressure of 1013.25 hPa or mbar (1 hPa = 1 millibar).

The advantages of measuring atmospheric pressure with a barometer can be summarised as follows:

  • High atmospheric pressure is associated with good weather, good days and good weather;
  • low air pressure is associated with poor weather, bad weather and bad weather.