What makes atmospheric pressure

We utilise Google Analytics cookies to help us to improve our website by collecting information on how it's used. The cookies collect information in a way that does not directly identify anyone. Atmospheric pressure reduces with altitude for two reasons, both of which are related to gravity. Those lower down have to support more molecules above them and are further compressed pressurised in the process. Emissions and atmospheric metrology.

Environmental monitoring. Our research and measurement solutions support innovation and product development. We work with companies to deliver business advantage and commercial success. Close Search Search. Back to Research Engineering Dimensional metrology Electrochemistry Electronic and magnetic materials Mass and mechanical measurement Temperature and humidity. Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky.

We live at the bottom of the atmosphere, and the weight of all the air above us is called air pressure. Above every square inch on the surface of the Earth is That means air exerts High in the atmosphere, air pressure decreases. With fewer air molecules above, there is less pressure from the weight of the air above.

This is, in part, because the Earth is not equally heated by the Sun. Areas where the air is warmed often have lower pressure because the warm air rises.

These areas are called low pressure systems. Places where the air pressure is high, are called high pressure systems. A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses, forming clouds and often precipitation.

Today we know this effect as the Coriolis Force and as a direct consequence, great wind spirals are produced which we know as high and low pressure systems. In the Northern Hemisphere, the air in low pressure areas spirals counterclockwise and inward — hurricanes, for instance, are Coriolis mechanisms, circulating air counterclockwise.

In contrast, high pressure systems the air spirals clockwise and outward from the center. In the Southern Hemisphere the direction of the spiraling of the air is reversed. So why do we generally associate high pressure with fair weather and low pressure with unsettled weather? Since cool air has less of a capacity to hold water vapor as opposed to warm air, clouds and precipitation are caused by cooling the air.

Any droplets that might lead to the formation of clouds would tend to evaporate. The end result tends to be a clearer and drier environment. Conversely, if we decrease the air pressure, the air tends to rise into the higher levels of atmosphere where temperatures are colder. As the capacity to hold water vapor diminishes, the vapor rapidly condenses and clouds which are composed of countless billions of tiny water droplets or, at very high altitudes, ice crystals will develop and ultimately precipitation will fall.

Of course, we could not forecast zones of high and low pressure without employing some sort of device to measure atmospheric pressure. Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere. To measure that weight, meteorologists use a barometer. It was Evangelista Torricelli, an Italian physicist and mathematician who proved in that he could weigh the atmosphere against a column of mercury.

He actually measured pressure converting it directly to weight. The instrument Torricelli designed was the very first barometer. The open end of a glass tube is placed in an open dish of mercury. Atmospheric pressure forces the mercury to rise up the tube. At sea level, the column of mercury will rise on average to a height of