Pressure — Page 26, Lesson 24

I want to walk you through how pressure varies — first with height, then with temperature, and finally over the course of a day. Let’s start with height. As you climb upward, there is less air above you pressing down. So the weight of the overlying air reduces, and pressure falls with height. That much is straightforward. But the rate at which pressure changes with height — that’s called the barometric lapse rate — is not constant. The barometric lapse rate reduces as altitude increases. In other words, the height change you need to produce a 1 hPa pressure change gets larger as you go higher. The table on page 9 of your book gives the exact numbers, but the key idea is that pressure falls more slowly per foot of climb at high altitude than it does near sea level. Now, temperature has a dramatic effect on the pressure lapse rate. Warm air causes pressure to fall slowly with height — that means the pressure lapse rate decreases. Cold air causes pressure to fall rapidly with height — that means the pressure lapse rate increases. So at any given height above the surface, the pressure will be higher over a region of warm air and lower over a region of cold air. This is an important fact that we will come back to when we study altimetry and upper winds. Your book also gives you a feel for the numbers. It shows how temperature affects the height difference for a 1 hPa change in pressure. These values come from a formula described in the chapter on the atmosphere. At mean sea level in the International Standard Atmosphere, a 1 hPa change corresponds to about 27 feet. At 20,000 feet, that same 1 hPa change corresponds to about 50 feet. At 40,000 feet, it corresponds to about 100 feet. So you can see the pattern: the higher you go, the more vertical distance you need for the same pressure change. Now let’s talk about diurnal variation — that’s the change in pressure that occurs during the day. This variation is small: about 1 hPa in temperate latitudes, but it can be as much as 3 hPa in the tropics. Even though it’s small, you need to take it into account when you look at pressure tendency as an indication of changing weather. The shape of this daily variation is shown in Figure 2.6. The cause of this diurnal variation is difficult to explain, but it is probably due to a natural oscillation of the atmosphere that has a period of about 12 hours. That oscillation is maintained by the 24-hour variation of temperature — the daily heating and cooling cycle. So to summarise: pressure falls with height, but the rate depends on both altitude and temperature. Warm air gives a slower pressure fall with height; cold air gives a faster one. And even on a calm day, pressure goes through a small but measurable 12-hour cycle that you need to account for when interpreting pressure trends.