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Constant Pressure vs. Constant Elevation

Posted on by MichaelV (Otsego)

Both at the surface and in the upper atmosphere, meteorologist constantly refer to “high” and “low” pressure systems. However, we look at them from two different perspectives.

At ground level, we seek air pressure values as they relate to “sea level” which provides us with a picture of the weather patterns at the surface. Using sea level (elevation = zero) as the common baseline we are able to make meaning of different pressure values between stations. So, on all surface charts, the elevation of the “surface” is considered zero feet.

The lines drawn on surface charts connecting areas of equal pressure are called isobars. “Iso” means equal and “bar” is the unit by which we measure pressure. Therefore, an isobar is a line representing the location where the pressure is equal (the same) along that line.

When we examine the atmosphere however the altitude at which any particular pressure value occurs will vary from reporting station to reporting station.

These changes in altitude represent different air densities in the atmosphere. Recall that as air temperature decreases the air’s density increases.

This means the altitude where any particular pressure occurs will be lower in the atmosphere when the air is colder. Conversely, higher air temperatures result in lower densities which increases the altitude of pressure levels.

This is why, as a rule, the atmosphere decreases in height from the equator toward the poles.

Therefore, we draw lines on constant pressure chart to represent the altitude at which that particular pressure occurred. These lines are called isoheights, lines of equal height.

By convention meteorologists simply refer to isoheight lines as ‘contours’. These lines are analogous to topographic charts. In essence, upper air charts show the atmosphere in three dimensions.

By looking at these contours we observe patterns of higher heights (called ridges) and lower heights (called troughs). These ridges and troughs drive the weather we experience at the surface.

Wind flowing from a ridge toward a trough is decreasing in height above the surface. Conversely, wind flowing from a trough into a ridge is increasing in height.

Between the colder, more dense air and the warmer, less dense air is the location of the greatest change (gradient) in heights of any particular pressure level.

The lakeshore flood warning continues until tomorrow morning at 8am.  Our warm(er) 60° temps may be gone for the year unless we get a heat wave which I am not seeing through the long term.  We will see lake effect rain showers today with chances of rain right on through the weekend.

We have less than two weeks before we lose an hour of daylight in the evening as we switch back to Eastern Standard time.

 

Category: Michigan Weather Forecast, Weather ready nation

21 thoughts on “Constant Pressure vs. Constant Elevation”

  1. I heard the sun is going to shine about mid day! Yeah!! Tuesday it almost felt like winter. Good thing was it was just rain. 🙂

  2. 715pm Currently 45* degrees out at thee YARDofBRICKS NE of GR damp feels like 37* degrees yes winter coat weather in October who would of thought….INDY

    1. And yet the days average temp is still warmer than normal. How can that be? Just ran some errands, no coat and still wearing shorts. Perfect temps out.

  3. 1230 pm Currently 44* degrees out at thee YARDofBRICKS NE of GR temps falling like the leafs a blowing with the furnace a rolling …INDY

  5. BREAKING NEWS!!! The European model shows 1 to 3 inches of snow on Halloween that would be scary lol…Could it be the coldest Halloween ever?? INDY

    1. Bring it and yet another month will end up with below normal temps despite all the warm hype! Incredible!

    2. The record snow fall for Grand Rapids on Halloween is 1.5″ in 1917. As has been stated for what they are calling budget issues the site I go to for much of my long term weather history is down. They say it is short term but we shall see.
      Slim

      1. That just maybe broken …Models coming in possible snow of 3+ inches ….One thing for sure its going to be cold Halloween week …Thanks Slim ….INDY

  6. So the atmosphere is higher at the equator than at the poles, about how much higher? Hundreds or thousands of feet? Mile or more? Interesting stuff.

    1. At sea level the average depth is around 20 kilometers at the equator and around 6 kilometers at the poles.
      Slim

      1. Wow, that’s almost 4 times the height. Guess I never really considered that. Obviously it’s not feasible to build launch pads at the poles, but it would seem to be beneficial to launch rockets as far away from the equator as possible. You still have to overcome gravity, but with far less height in the atmosphere comes far less resistance from friction thereby reaching escape velocity faster and with less fuel. Prob something I’m missing.

  7. Ohh forgot with the coldest Halloween ever possible coming big news..It’s currently 48* degrees with wind blowing leafs of the wet trees feels like 38* degrees perfectttt FALL day out at thee YARDofBRICKS NE of GR …Best weather of the year is here ….INDY

  8. The cold and snow is building to our north and about to move in on west Michigan colder and snowy pattern coming next week could it be?? Prepare now sharpen them snow shovels baby …. Possible the coldest Halloween ever ? Slim will let us know the facts and the when’s whats’s on that one soon shuuu… Loving it!! Have a super Tuesday…INDY

  9. Good morning here at my house I recorded 0.21” of rain yesterday and overnight. At the Airport they reported 0.36” for yesterday. The H/L at GRR yesterday was 62/48. At this time it is 48 both here at my house and the airport. The sky is cloudy and there is some but not much wind at this time. For today the average H/L is 58/40 the record high is 83 in 1953 and the record low is 20 in 1895. Last year the H/L was 46/35.
    Slim

  10. we have 46 degrees with clear skies this morning – .31 of an inch of rain in the last 24 hours…. 2.97 inches thus far for the month.

    1. It is now cloudy here but right at sun rise there was just enough clear skies for a nice red sun rise.
      Slim

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