We breathe it. We feel it. We see its effects. Without it we wouldn’t be here.
The atmosphere is a layer of gases (78% Nitrogen, 21% Oxygen, 0.9% Argon, 0.04% Carbon Dioxide, plus others) that blankets the earth, extending to a depth of a few 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 where all our weather happens. It extends from the surface to a height of around 16 km in the tropics, and 8 km 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 1,013.15 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. Around half of the mass of the atmosphere is contained in the lowest 5.5 km. In the lowest couple of kilometeres, where the air is thickest, 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. Higher up in the troposphere the fall in pressure with height is much less.
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 because 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 16 km in the tropics and only 8 km at the Poles.
You will be familiar with the surface pressure charts, which plot the pressure values at a certain height, namely sea-level. In meteorology, however, we are also interested in the reverse, i.e. the actual heights at which certain pressure values are found, as these are important parameters in generating our weather. For example, as we rise from a surface of say 1010 hPa at sea-level and climb upwards, pressure will drop and we will arrive at 850 hPa at a certain height, roughly 1450 m. The exact height varies from day to day and season to season, with values ranging from 1600 m in summer heatwaves to 1300 m in cold winter spells. Likewise, we look at the heights of other pressure-levels, usually 950, 700, 600, 500, 300, 200 hPa, etc. These actual heights are measured by radiosonde instruments attached to weather balloons, which are launched at least twice a day at several hundred locations around the globe. The reason for using pressure levels 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.
We’re not only interested in the height of these pressure-levels (called Geopotential), but also other factors, such as their temperature, dewpoint, wind speed and direction, etc. 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.
Areas of cold air will have low geopotential, as the air will be denser and hence the spaces between the pressure-levels are small. Likewise, areas of warmer air will have higher geopotentials, as the air is less dense and the pressure-levels are spaced further apart. 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 temperature at a particular pressure level is very important. In winter we keep a close eye on the 850hPa temperatures, as these are a good indicator of the whether it will rain or snow. When we’re in a cold airmass, the 850hPa geopotential is a lot lower than the standard 1450m, and the temperatures can be down to -10 °C or lower, leading to snow. When the 850hPa temperature 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 a forecaster’s 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