I recently came across this question while talking with a colleague about “my” high pressure systems. It is one of these things you always use in your field but if you have to explain it, it is suddenly not that easy anymore.
When talking about high and low pressure systems (as we every now and again do in atmospheric sciences…), one has to be aware that these features not only have a horizontal extend but also a vertical one. Before we start with a surface pressure map and then work our way up into the so called free atmosphere, lets have a quick look at the units of pressure. Historically there are many different units to measure the pressure in, here I only use Hectopascal (hPa) a standard unit in meteorology. The standard sea level pressure (also called 1 atmosphere (1 atm); about 1 bar) is 1013.25 hPa.
Fig. 1 shows the sea level pressure for July 15, 2010 in Europe and parts of Africa and Asia. Regions with a high pressure system (normally marked by a capital ‘H’) show values above the standard surface pressure, regions with a low pressure system (‘L’) show values below it. At July 15, a distinct surface low is sitting over the UK and another one east of Scandinavia. In between a high pressure ridge is located. Everything in Fig. 1 is in Hectopascal, so it’s fine! But what happens if we leave the surface and ask how, e.g., a high pressure system looks higher up in the atmosphere?
It is commonly known that pressure declines exponentially with altitude. So as soon as one moves away from the surface, each elevation has another “normal” pressure. We could just go ahead and use pressure anomalies relative to the normal pressure for each altitude. High pressure systems have positive anomalies, low pressure systems negative ones. But in meteorology we prefer to look at certain features not on a certain altitude but on a certain pressure level. Especially the 500 hPa level (it is located at approximately 5 km altitude) is often used as reference level for detecting high and low pressure systems and other, more special features, such as blocking. But here we run into a problem because the pressure at 500 hPa is by definition always pretty exactly… 500 hPa. So what do we do?
Instead of asking what is the pressure at a certain elevation we ask: what is the elevation of a certain pressure level.
In a high pressure system, pressure is higher than normal. Since the pressure decreases with altitude, a certain pressure value (e.g., 500 hPa) can be found at a higher elevation, compared to normal conditions. Fig. 2 shows the geopotential height (more or less the altitude) anomaly of the 500 hPa pressure level. So basically, it shows how much higher or lower than normal the 500 hPa pressure is located in the atmosphere. We can clearly see a “trough” and a “peak” representing two areas of low and high pressure, respectively. It is the same day as in Fig. 1 and we can still identify the low pressure system over the UK, the high pressure ridge has become a fully developed high pressure system, while the second low pressure system east of Scandinavia is less distinct. But the unit is that of altitude, therefore, meter.
I made this awesome 3D plot to hopefully visualize in an understandable way what we do when presenting you with high and low pressure systems in meter. But 3D plots are inconvenient in almost every other aspect. They are hard to make and even harder so if you want them to look good (this one took me hours…). More importantly, they are very imprecise to read. Yes, there is a low and a high pressure system in that plot, but it is almost impossible to exactly say how strong they are or where they are located. So we normally map the information from the third dimension into colors. It is actually already done in Fig. 2: higher pressures are reddish, lower pressures are bluish. So the only thing left to do is to turn the map until we look at it from the top.
Until we finally end up with something like this. Red represents high, pressure blue low pressure. And yet it is in meter.
If not stated otherwise all pictures/figures are by Lukas Brunner and licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
I acknowledge the Wegener Center for Climate and Global Change (WEGC), University of Graz, Graz, Austria for providing their radio occultation data which where the basis of all plots, if not stated otherwise.