In meteorology, atmospheric pressure is a crucial factor in predicting and studying weather patterns. Clouds, cyclones, storms, winds, and other weather phenomena are largely determined by changes in atmospheric pressure, which are also linked to events like tornadoes . Furthermore, understanding how atmospheric pressure works helps us better understand our environment and the climate.
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?
Although it may not seem like it, air has weight . We are not aware of the weight of air because we are immersed in it. Air offers resistance when we walk, run, or travel in a vehicle because, like water, it is a medium through which we move. The density of water is much greater than that of air, which is why it is more difficult to move in water. This dynamic of air is also related to the pressure gradient.
In some way, air exerts a force on us and on everything. Therefore, we can define atmospheric pressure as the force exerted by the atmosphere on the Earth's surface. The higher the altitude of the Earth's surface above sea level, the lower the air pressure, which can also affect phenomena such as cyclones.
In what units is atmospheric pressure measured?
It's logical to think that if atmospheric pressure is due to the weight of the air above a certain point on the Earth's surface, we must assume that the higher the point, the lower the pressure, since there is also less air above it. Atmospheric pressure is measured in the same way as speed, weight, etc. It is measured in atmospheres, millibars, or mmHg (millimeters of mercury) . The atmospheric pressure at sea level is usually taken as a reference. There, it has a value of 1 atmosphere, 1013 millibars, or 760 mmHg, and one liter of air weighs 1,293 grams. The unit most commonly used by meteorologists is the millibar.
How is atmospheric pressure measured?
Manometers are used to measure the pressure of a fluid . The most common and easiest to use is the open-tube manometer. It is essentially a U-shaped tube containing a liquid. One end of the tube is at the pressure to be measured, and the other end is in contact with the atmosphere.
Barometers are used to measure air or atmospheric pressure. There are various types of barometers. The most well-known is the mercury barometer, invented by Torricelli . It consists of a U-shaped tube with a sealed end that has been evacuated, so the pressure at the top of this end is zero. This allows the force exerted by the air on the column of liquid to be measured, thus determining atmospheric pressure, which is essential for weather forecasting.
As we mentioned earlier, atmospheric pressure is due to the weight of the air above a certain point on the Earth's surface. Therefore, the higher this point is, the lower the pressure, since there is less air present. We can say that atmospheric pressure decreases with altitude. For example, on a mountain, the amount of air at the summit is less than at a beach, due to the difference in height.
Another more exact example is the following:
Sea level is used as a reference point, where atmospheric pressure is 760 mm Hg . To verify that atmospheric pressure decreases with altitude, we go to a mountain whose highest peak is about 1.500 meters above sea level. We take a measurement and find that at that altitude, the atmospheric pressure is 635 mm Hg. With this small experiment, we verify that the amount of air at the mountain peak is less than at sea level and, therefore, the force exerted by the air on the surface and on us is less.
Atmospheric pressure and altitude
An important point to consider is that atmospheric pressure does not decrease proportionally with altitude because air is a highly compressible fluid. This explains why the air closest to the ground is compressed by its own weight. In other words, the lower layers of air near the ground contain more air because they are pressed down by the air above (air at the surface is denser, as there is more air per unit volume). Therefore, the pressure is higher at the surface and does not decrease proportionally because the amount of air does not decrease at a constant rate with altitude.
Thus we can say that being close to sea level, making a small ascent in altitude causes a large decrease in pressure , while as we are higher, we need to ascend much more to experience a decrease in atmospheric pressure to the same extent.
What is the pressure at sea level?
Atmospheric pressure at sea level is 760 mm Hg , equivalent to 1013 millibars. The higher the altitude, the lower the pressure; in fact, it decreases by 1 mb for every meter of altitude gained.
How does atmospheric pressure affect our body?
Atmospheric pressure typically changes during storms, atmospheric instability, or strong winds. Ascending to higher altitudes also affects the body. Mountaineers are particularly susceptible to these symptoms due to the pressure changes as they climb. These changes are significant, especially when related to altitude sickness. Furthermore, it's essential to consider that altitude sickness is influenced by the reduction of oxygen at high altitudes, which is also related to the levels of available oxygen.
The most common symptoms are headache, gastrointestinal issues, weakness or fatigue, dizziness or lightheadedness, and sleep disturbances , among others. The most effective measure when altitude sickness symptoms appear is to descend to lower altitudes, even if only by a few hundred meters.
Pressure and atmospheric instability or stability
Stability or Anticyclone
When colder air descends, atmospheric pressure increases because there is more air at the surface, and therefore, it exerts more force. This creates atmospheric stability, also known as an anticyclone. An anticyclone is characterized by calm conditions and no winds, as the colder, denser air slowly descends in a circular motion. 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
Conversely, when warm air rises, it lowers atmospheric pressure and causes instability. This is called a cyclone or low-pressure system . Wind always moves in a preferred direction towards areas of lower atmospheric pressure. That is, whenever an area has a low-pressure system, the wind will be stronger because, being an area of ​​lower pressure, the wind will blow in that direction. This phenomenon can be studied in more detail by analyzing synoptic maps.
A storm on an atmospheric pressure map
Another aspect to consider is that cold and warm air do not mix immediately due to their different densities. When they meet at the surface, the cold air pushes the warm air upwards, causing a drop in pressure and instability. A low-pressure system then forms, and the area of ​​contact between the warm and cold air is called a front. This is essential for understanding how weather phenomena form.
Weather and atmospheric pressure maps
Weather maps are created by meteorologists. They use information gathered from weather stations, airplanes, weather balloons, and satellites. These maps represent atmospheric conditions in different countries and regions. They show the values ​​of various meteorological phenomena such as pressure, wind, and rainfall.
The weather maps that interest us at this time are those that show atmospheric pressure. On a pressure map, lines of equal atmospheric pressure are called isobars. That is, as atmospheric pressure changes, there will be more isobars on the map. Pressure maps also show weather fronts. Thanks to these types of maps, it is possible to determine the current weather and how it will evolve in the coming hours with a high degree of accuracy, up to a limit of three days.
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
Cold fronts are those in which a mass of cold air replaces warm air . They are strong and can cause atmospheric disturbances such as thunderstorms, showers, tornadoes, strong winds, and brief snowstorms before the cold front passes, followed by dry conditions as the front moves in. Depending on the time of year and geographic location, cold fronts can occur in succession every 5 to 7 days.
Cold front
Warm fronts
Warm fronts are those in which a mass of warm air gradually replaces cold air. Generally, as a warm front passes, temperature and humidity increase, pressure decreases, 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 usually found at the beginning of a surface front, as well as convective showers and thunderstorms.
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.