Often in forecast discussions we talk about "height rises" and "height falls" as a way of quickly summing up what is going on with the overall weather pattern. It turns out that the changes in the height of pressure surfaces aloft are related to changes in the wind and temperature, which in turn have ramifications for the potential for rain and storms.
First, let's remind ourselves about what these "heights" are measuring. Remember that the air pressure decreases as you go up in height. In meteorology it's often convenient to talk about the pressure structure aloft, so we talk about the atmosphere in terms of how far up you have to go (above sea level) until the pressure has decreased to a certain amount. We refer to these values as the "heights" of various pressure levels. 500mb is a common pressure level for us to talk about, as it's about "halfway" up the troposphere.
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| "Heights" of various pressure levels are simply how high you have to go above the ground until the pressure falls to that particular value. |
Every time I show an upper-level chart (like the 500mb chart), we're looking at a map of how high 500mb is above sea level across the country. When we talk about troughs and ridges, we're talking about areas of lower and higher heights, respectively, of the 500mb surface.
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| Sample 500mb map showing where 500mb is relatively lower (troughs) and where 500mb is relatively higher (ridges). The "heights" are a description of how high or low the 500mb surface is above the ground. |
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| 500mb map from the SPC mesoanalysis at 17Z, Apr 25, 2011. The 500mb heights are contoured with black lines. Areas where there have been height rises over the previous 12 hours are shaded and contoured in red. Areas where there have been height falls over the previous 12 hours are shaded and contoured in blue. Notice a trough (low heights) over New England with a big ridge (high heights) over the Rocky Mountain corridor in the west. As that trough moves out and the ridge (with its higher heights) moves in, we see a large area of hight rises being shown over much of the eastern US. |
But we can look a little bit deeper into connections between these heights and temperature. It turns out that heights and temperature are intimately connected. It's pretty common knowledge (or, I think it should be...) that as you warm air it expands and as you cool air it contracts. If you throw sealed containers onto a fire, they can explode because heating the contents causes them to expand. It's basic chemistry.
The same thing happens on a larger scale in the atmosphere. When we have warming in some level of the atmosphere, it causes the air to expand outward from the center of that warming. As the air expands, it changes the pressure structure. For instance, let's imagine our atmosphere like I showed in the first figure above, but this time add warming in the lower atmosphere.
As the air below heats up, it expands outward, increasing the pressure above it. This causes the heights of the pressure surfaces aloft to increase. You can think of the expansion due to warming "pushing" up the heights of the pressure surfaces above it. Therefore, height rises are usually associated with warming below.
The opposite happens when there is cooling. As an area of air cools, it contracts and the pressure decreases on the air above it. This contraction pulls the heights of the pressure surfaces above down, leading to height falls. So, height falls are usually associated with cooling below.
Things get more complicated if the warming or cooling is in the middle of the atmosphere. For instance, if there is warming in the middle of the atmosphere, the air expands--both up AND down. this has the affect of increasing the heights above the center of warming, but decreasing the heights below. So it can get rather complicated.
But, let's go back to our basic picture. Remember that height falls tend to precede troughs which can be associated with storms. In particular, lets think about situations where we might be expecting thunderstorms. To get thunderstorms, we have to have an atmosphere that is (conditionally) unstable. To destabalize the atmosphere, we have to increase the lapse rate (that is, make the temperature cool off rapidly with height). Two basic ways to do this are to either increase the heat (and/or moisture) near the surface, or to cool off the atmosphere above the surface.
So let's consider a forecast discussion that says, for example, "mid-level height falls are occurring over a region with very warm surface temperatures and high dewpoints". If we know that there are mid-level height falls, then from our discussion above we know there should be cooling going on below those mid-levels. This is a great case for instability to increase, because we have a warm moist surface, but the height falls imply that the temperatures are cooling off just above the surface. That will increase the lapse rates and make it easier for thunderstorms to form in the more unstable environment.
What about the opposite statement -- "height rises are occurring over an area with warm surface temperatures and high dewpoints". Initially you might think that the warm, moist surface would be good for thunderstorms. However, if height rises are occurring, that implies that there is warming going on just above the surface. This is not good for increasing the instability--it's doing nothing to steepen the lapse rates in the right direction, and could even create an inversion (where temperature increases with height).
So that's a simple look at what height rises and height falls mean. You can look at them as just signals of approaching ridges and troughs. But, they also have importing implications for how the temperature is changing aloft which, in turn, can affect chances for thunderstorms.
