Atmospheric Pressure And Pressure Levels Explained

If you are a regular visitor to this site then you have probably heard many references to terms such as the "850hPa temperatures" or "500hPa troughs" and wondered what exactly they mean, iWeather Online's synoptics forecaster Fergal Tierney writes.

The atmosphere is a layer of gases that blankets the earth, extending to a depth of several hundred kilometres. It contains many different layers, such as the troposphere, stratosphere, mesosphere, thermosphere, etc., but the layer we are most interested is the one nearest the earth - the Troposphere, as this is the one we live in and is where all our weather happens. It extends from the surface to a height of around 16kms in the tropics, and 8kms near the Poles. The top of the troposphere is called the Tropopause, and it marks the boundary between the troposphere and the next layer above it - the Stratosphere.

Atmospheric Pressure is defined as the pressure exerted on the surface of the Earth by the weight of the atmosphere above it, and is measured in units of hectoPascal (hPa), which is equal to the more traditional millibar (mbar). The average atmospheric pressure at the surface is 1,013 hPa, but of course we all know that it varies widely from day to day and place to place, from a maximum of 1085.7hPa (Mongolia, 2001) to 870hPa (Typhoon Tip, Western Pacific, 1979). Areas of low pressure generally bring bad weather, and vice versa. The distribution of these pressure areas is shown on the familiar synoptic charts that we all see on the TV, in the newspapers, etc., and show lines connecting areas of equal pressure (isobars), as well as various fronts, which are the boundary between different airmasses.

Because pressure is defined as the weight of the atmosphere above a point, the higher up you go in the atmosphere, the lower the pressure will be, as there is less air above you. In the lowest layers of the troposphere, the pressure drops by 1 hPa for about every 8 metres increase in altitude, so at an altitude of say 200 metres, the pressure will be around 25 hPa lower than at ground level, etc. The exact decrease in pressure with increasing altitude depends on the density of the air, which itself depends on the temperature (and to a lesser extent, the moisture content) of the air. So as cold air is more dense than warm air, pressure will drop more quickly in a cold airmass than a warm one. This is why the troposphere extends to around 16kms in the tropics and only 8kms at the Poles. The image below shows how pressure (and temperature) vary with increasing altitude throughout the troposphere and lower stratosphere, with the average heights of the important pressure levels marked in metres, as per the Standard Atmosphere. (The Standard Atmosphere is an internationally agreed model of the average atmosphere, but depending on the season and location, actual values will vary greatly from this average).

In meteorology, our weather doesn't only happen at the surface but right throughout the depth of the atmosphere. Features several kilometres up are the driving forces that generate low or high pressure systems at the surface, so it is important that we get an idea of the state of the atmosphere at these upper levels too. The actual heights of certain pressure levels at any given time is a crucial indicator of what sort of processes are at play, and how things will evolve into the future. Instead of defining the pressure at a certain height, as is done with surface charts (which show the pressure at sea level), upper air charts actually show the heights (geopotentials) of certain pressure levels, such as the 500hPa chart below. The reason for this is that it makes the physical atmospheric equations* a whole lot simpler to solve if altitude is defined in terms of pressure instead of metres.

So it is usual that areas of cold air will have low geopotential, as the air will be denser. Likewise, areas of warmer air will have higher geopotentials, as the air is less dense. This is the basis of meteorology. The atmosphere is constantly trying to balance the temperature difference between the hot equator and the cold poles. Warm air moves poleward, cold air moves in the opposite direction. Throw in the effects of land masses, mountains, warm and cold seas, solar heating, etc. and you can see that the system becomes very complex. The movements of air poleward and equatorward occur in periodic pools, such as the one we're experiencing now. Arctic air is flowing southwards over western Europe, which is why we have an area of lower geopotential (upper low or Polar Vortex) over us. At the same time warm air is flowing northwards over Greenland, which is what has increased geopotential and caused the blocking high pressure system in the area.

Along with geopotential, the temperature at a particular pressure level is also very important. Lately we have been keeping a close eye on the 850hPa temperatures, as these are a good indicator of the whether it will rain or snow. As we're in a cold airmass, the 850hPa geopotential is a lot lower than the standard 1450m, and the temperatures are up to 15°C colder than the standard +5°C. At less than -40°C, the 500hPa temperatures are over 20°C colder than the standard -21°C. All of these factors are giving us our snow showers. If/when the 850hPa temperature ever gets above around -5°C then we can expect rain again, although this can be complicated by the layer of cold air stagnant over the snow-covered terrain.

So you can see how complex Peter O'Donnell's (iWeather Online forecaster) job of formulating forecasts is, and it's remarkable how anybody can get it right any of the time, given the complexity of the ever-moving atmosphere (see the quote below). But the next time you hear someone talking about "850hPa temperatures" or "500hPa troughs", then you will have an idea of what they're talking about.

"Consider a rotating spherical envelope of a mixture of gases, occasionally murky and always somewhat viscous. Place it around an astronomical object nearly 8000 miles in diameter. Tilt the whole system back and forth with respect to its source of heat and light. Freeze it at the poles of its axis of rotation and intensely heat it in the middle. Cover most of the surface of the sphere with a liquid that continually feeds moisture into the atmosphere. Subject the whole to tidal forces induced by the sun and a captive satellite. Then try to predict the conditions of one small portion of that atmosphere for a period of one to several days in advance." - Author Unknown

* An example of one such equation: