In meteorology, atmospheric pressure It is something very important to take into account when predicting and studying the behavior of the climate. Clouds, cyclones, storms, winds, etc. They are largely conditioned by changes in atmospheric pressure.

However, atmospheric pressure is not something tangible, something that can be seen with the naked eye, therefore there are many people who understand the concept, but do not really know what it is.

## What is atmospheric pressure?

Even if it seems not, the air is heavy. We are not aware of the weight of the air since we are submerged in it. Air offers resistance when we walk, run or ride in a vehicle, because, like water, it is a medium through which we travel. The density of water is much higher than that of air, that is why it is more difficult for us to move in water.

Somehow, air exerts a force on us and on everything. Therefore, We can define atmospheric pressure as the force exerted by atmospheric air on the earth's surface. The higher the height of the earth's surface relative to sea level, the lower the air pressure.

## In what units is atmospheric pressure measured?

It is logical to think that if the atmospheric pressure is due to the weight of the air over a certain point on the earth's surface, we must assume that the higher the point is, the lower the pressure will be, since the amount of air per unit is also less. above. Atmospheric pressure is measured as is speed, weight, etc. It is measured in atmospheres, millibars, or mm Hg (millimeters of mercury). Normally the atmospheric pressure that exists at sea level is taken as a reference. There it takes a value of 1 atmosphere, 1013 millibars or 760 mm Hg and a liter of air weighs 1,293 grams. The unit most used by meteorologists is that of millibars.

## How is atmospheric pressure measured?

In order to measure the pressure of a fluid, the pressure gauges. The most widely used and easiest to use is the open tube manometer. It is basically a U-shaped tube that contains a liquid. One end of the tube is at the pressure to be measured and the other is in contact with the atmosphere.

At Measure air or atmospheric pressure using barometers. There are barometers of various types. The best known is the mercury barometer that was invented by Torricelli. It is a U-shaped tube with a closed branch in which the vacuum has been drawn, so that the pressure in the highest part of this branch is zero. In this way, the force exerted by the air on the liquid column can be measured and the atmospheric pressure measured.

As we have mentioned before, the atmospheric pressure is due to the weight of the air over a certain point of the earth's surface, therefore, the higher this point is, the lower the pressure, since less is the amount of air that there is. We can say that the atmospheric pressure decreases in altitude. For example, on a mountain, the amount of air in the highest part is less than that on a beach, due to the difference in height.

Another more exact example is the following:

The sea level is taken as a reference, where atmospheric pressure has values ​​of 760 mm Hg. To check that the atmospheric pressure decreases in height, we go to a mountain whose highest peak is about 1.500 meters above sea level. We carry out the measurement and it turns out that at that height, the atmospheric pressure is 635 mm Hg. With this little experiment, we check that the amount of air at the peak of the mountain is less than that at sea level and, therefore, the force exerted by the air on the surface and us is less.

## Atmospheric pressure and altitude

An important point to keep in mind is that atmospheric pressure does not decrease proportionally in height since air is a fluid that can be highly compressed. This explains that the air closest to the ground surface is compressed by the air's own weight. That is, the first layers of air close to the ground contains more air being pressed by the upper air (the air on the surface is denser, since there is more air per unit volume), therefore the pressure is higher on the surface and does not decrease proportionally since the amount of Air does not decrease steadily in height.

In this way we can say that being close to sea level, making a small ascent in height causes a large drop in pressure, while as we get higher, we need to go much higher to experience a decrease in atmospheric pressure to the same extent.

Air density at height

### What is the pressure at sea level?

The atmospheric pressure at sea level is 760 mm Hg, the equivalent of 1013 millibars. The higher the altitude, the lower the pressure; in fact it is reduced by 1mb for every meter we go up.

## How does atmospheric pressure affect our body?

Normally there are changes in atmospheric pressure when there are storms, atmospheric instability or strong winds. Climbing in height also affects the body. Mountaineers are the people who suffer the most from these types of symptoms due to changes in pressure as they climb the mountains.

The most common symptoms are headache, gastrointestinal symptoms, weakness or fatigue, unsteadiness or dizziness, sleep disturbances, among others. The most effective measure against the appearance of symptoms of mountain sickness is the descent to lower altitudes, even if they are only a few hundred meters.

Many mountaineers suffer from headaches when they climb too high.

## Pressure and atmospheric instability or stability

Air has a somewhat simple dynamic and is related to its density and temperature. Warmer air is less dense and colder air is denser. That is why when the air is colder it tends to descend in altitude and the opposite when it is warmer. This air dynamics causes changes in atmospheric pressure causing instability or stability in the environment.

## Stability or Anticyclone

When the air is colder and descends, the atmospheric pressure increases since there is more air on the surface and therefore, it exerts more force. This causes a atmospheric stability or also called anticyclone. A situation of anticyclone It is characterized by being a zone of calm, without winds since the coldest and heaviest air slowly descends in a circular direction. The air rotates almost imperceptibly clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

An anticyclone on an atmospheric pressure map

## Cyclone or squall

On the contrary, when the hot air rises, it reduces the atmospheric pressure and causes instability. It is called cyclone or storm. The wind always moves in a preferential direction to those areas with lower atmospheric pressure. That is, whenever an area has a storm, the wind will be greater, because being an area of ​​less pressure, the wind will go there.

A storm on an atmospheric pressure map

Another aspect to keep in mind is that cold air and hot air do not mix immediately due to their densities. When these are on the surface, the cold air pushes the hot air upwards causing drops in pressures and instability. A storm is then formed in which the area of ​​contact between hot and cold air is called front.

## Weather and atmospheric pressure maps

The weather maps they are made by meteorologists. To do this, they use the information they collect from weather stations, airplanes, probe balloons and artificial satellites. The maps produced represent the atmospheric situations in the different countries and areas studied. The values ​​of some meteorological phenomena such as pressure, wind, rain, etc. are shown.

The weather maps that interest us at this time are those that show us atmospheric pressures. On a pressure map lines of equal atmospheric pressure are called isobars. That is, as the atmospheric pressure changes, more isobar lines will appear on the map. Fronts are also reflected in the pressure maps. Thanks to these types of maps it is possible to determine what the weather is like and how it will evolve in the next few hours with a very high degree of reliability, up to a limit of three days.

Isobar map

In these maps, the areas with the highest atmospheric pressure show an anticyclone situation and the areas with less pressure show storms. The hot and cold fronts are determined by symbols and predict the situation that we will have throughout the day.

## Cold fronts

The cold fronts are those in which cold air mass replaces hot air. They are strong and can cause atmospheric disturbances such as thunderstorms, showers, tornadoes, high winds, and short snowstorms before the cold front passes, accompanied by dry conditions as the front advances. Depending on the time of year and its geographical location, cold fronts can come in a succession of 5 to 7 days.

Cold front

## Warm fronts

The warm fronts are those in which a mass of warm air gradually replaces the cold air. Generally, with the passage of the warm front, the temperature and humidity increase, the pressure drops and although the wind changes, it is not as pronounced as when a cold front passes. Precipitation in the form of rain, snow or drizzle is generally found at the beginning of a surface front, as well as convective rains and storms.

Warm front

With these basic aspects of meteorology, you can already know well what atmospheric pressure is and how it works on our planet. In order to know well what meteorologists tell us in weather forecasting and to be able to analyze and understand our atmosphere more.

Find out everything about the barometer, the instrument with which atmospheric pressure is measured:

Related article:
Barometer

Rudolph Gabriel David said

What pressure is there at the height at which commercial aircraft travel?

Is there or do you know any graph that shows the variation of pressure from the sea to the exit of the atmosphere?

Thank you
Rodolfo

Saul Leyva said

Very good article. Congratulations. I answer my question.

Aries said

Excellent thanks. Greetings from Chile.