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If the "weight" of the atmosphere at a specific point on the surface is the barometric pressure and defines a high or low at that point, how can you have a low at the surface and a high aloft or vice versa at basically the same point? Also, if that is possible is there a different interpretation to the terms high and low when describing the upper atmosphere.
Walt G.
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Great question and this one definitely requires some thinking. When we talk about the pressure at the surface, the pressure is measured here where we live. This pressure is reduced to a sea level pressure so we can look at data across the country on a constant level. If we used actual pressure, then places like Salt Lake or Denver would always have lower pressure because we are higher up in elevation.
Pressure or the weight of air pushing down is measured by using a barometer. When we talk about low pressure or low pressure moving in, the pressure is dropping (becoming lower) and bringing in generally cloudier or wetter weather. With low pressure, the air is rising, the air is cooling and condensing to its dewpoint and forming clouds.
High pressure aloft isn't the best term to use but we hear it a lot. More accurately when we look at maps above the ground we look for things called ridges or troughs or height rises and falls. A ridge of high pressure or an upper level ridge is just an area a loft where the heights are rising, making for generally better weather.
When we measure the levels above the surface we use what are called height charts. These charts tell us this. At what HEIGHT in the atmosphere is the pressure equal to 500 millibars or hectopascals. Millibars and hectopascals are the same, they are units of pressure just like inches of mercury. So at what level or height (in the vertical here, above our heads) can we find where the pressure is a certain reading. A city like Salt Lake might have a higher height level than Oklahoma City on a given day. So on an upper level chart or map, say a 500 mb chart, everyone and every city is at 500 mb. BUT the heights of those towns are different. Click on the right, there's a link to a 500 mb chart.
Aloft, or higher in the atmosphere you can have these areas of higher heights or ridges, or what's called an upper level high. To deem it as having a higher pressure isn't as accurate. It's commonly referred to as upper level high prsesure, but really, it's an upper level area where the heights are higher if that makes sense.
At the surface we can still have the formation of a low or commonly, a thermal low and still have the upper ridge. Thermal areas of low pressure form when temperatures in an area heat up, this heated up air rises up and the pressure below is lowered. Thermal lows can still form under a ridge higher up.
If the features in the upper atmosphere are strong enough, like in the winter time when we have a very intense jet stream, the surface features will couple. Meaning that a big ridge aloft will also coordinate with a big surface ridge or surface high.
Yes, there's a different interpretation of high or low pressure at the surface and high or low pressure in the upper atmosphere, you are correct there.
This is a pretty interesting topic for sure. On the right you will find some links to where you can learn more about high and low pressure along with what constant height charts are and why we use them. Also, many thanks to Dr. J. Shafer at Lyndon State College in VT for having some insight on this one as well.
Answered by KSL Meteorologist Dina Freedman.
