Question on upper level wind altitudes?

[SCUBAdude]

When reading discussions about a particular storm I frequently see reference to winds at a given level quoted in millibars. Like say, 200mb winds, or 500 mb winds.
My question is, at what altitude (in feet) are the different winds relative to millibars.
I'm assuming that the higher up you go the lower the mb wind reference and the closer to the surface the higher.
TIA,
SD

[senorpepr]

When reading discussions about a particular storm I frequently see reference to winds at a given level quoted in millibars. Like say, 200mb winds, or 500 mb winds.
My question is, at what altitude (in feet) are the different winds relative to millibars.
I'm assuming that the higher up you go the lower the mb wind reference and the closer to the surface the higher.
TIA,
SD

You pretty much have it down. Basically, the 500mb level is the height at which the pressure would be measured at 500mb. Because of atmospheric processes, the 500mb height (and any other pressure) can change from day to day.

Here is a 500mb chart:

Using that chart, we can see the 500mb height in Miami is 590 decameters (or roughly 19,356 feet).

Here are some rough averages for the standard atmospheric levels:

Code: Select all

925mb  2,500ft
850mb  5,000ft
700mb 10,000ft
500mb 18,000ft
400mb 24,000ft
300mb 30,000ft
250mb 35,000ft
200mb 40,000ft
150mb 45,000ft
100mb 50,000ft

[SCUBAdude]

Thanks senorpepr, that code at the bottom of your reply was what I was looking for. But your reply has raised another question.
You said that due to atmospheric processes that the actual height of a given pressure will change from day to day. So does that mean that in the case of a hurricane that a given pressure, lets say 500mb, would be higher up in the atmosphere due to the air being evacuated from the surface thus a piling on effect. Or would it be closer to the surface due to the redistribution of that air out and away from the center of the storm in the upper levels.

Thanks for bearing with me,
SD.

[senorpepr]

Thanks senorpepr, that code at the bottom of your reply was what I was looking for. But your reply has raised another question.
You said that due to atmospheric processes that the actual height of a given pressure will change from day to day. So does that mean that in the case of a hurricane that a given pressure, lets say 500mb, would be higher up in the atmosphere due to the air being evacuated from the surface thus a piling on effect. Or would it be closer to the surface due to the redistribution of that air out and away from the center of the storm in the upper levels.

Thanks for bearing with me,
SD.

Actually, it would be the 200-300mb levels that would be higher than normal, for the exact reason you stated. As the parcel of air raises from the ocean's surface, it reduces the mass of air (the pressure) in the lower troposphere. However, this mass cannot be lost. It is transferred to the upper troposphere (200-300mb). Since all this air collects in one spot, it creates high pressure. Since the high pressure aloft needs to find lower pressure to establish equilibrium, it begins rotating in an anticyclonic fashion and, thus, you have the anticyclone aloft.

As for the mid-troposphere (700-500mb), there isn't a sizable pressure change.

[SCUBAdude]

I think I've got it now. So in a hurricane pressure heights (actual altitude of a mb reference) in the lower troposphere fall closer to the surface because the air is being evacuated to the upper atmosphere. And upper level heights rise because of the presence of that very same air.
That would mean that the 925mb height would actually be at sea level in a very strong hurricane with a central pressure equal to that or below.
Right?

Also, during a storm I frequently see winds reported at (850mb ?) and then a reduction is made for actual surface winds. Does that mean that they are using 850 as a referencer to 5000ft. or do they actually fly down to an altitude that measures 850mb to take a wind reading?
Basically I'm wondering if mb levels like in the chart you posted above are used as a reference of altitude?

[senorpepr]

I think I've got it now. So in a hurricane pressure heights (actual altitude of a mb reference) in the lower troposphere fall closer to the surface because the air is being evacuated to the upper atmosphere. And upper level heights rise because of the presence of that very same air.

Bingo.

That would mean that the 925mb height would actually be at sea level in a very strong hurricane with a central pressure equal to that or below.
Right?

Right. For instance, during Hurricane Wilma, the dropsondes that were used when Wilma was at her peak did not have 925mb data (a standard reporting level) because 925mb pressure did not exist. The lowest standard reporting level was 850mb.

Also, during a storm I frequently see winds reported at (850mb ?) and then a reduction is made for actual surface winds. Does that mean that they are using 850 as a referencer to 5000ft. or do they actually fly down to an altitude that measures 850mb to take a wind reading?
Basically I'm wondering if mb levels like in the chart you posted above are used as a reference of altitude?

Well, here’s the deal: aircraft normally don’t fly based on altitude, but rather pressure. “Altimeters” on aircraft are nothing more than a barometer that is set while the aircraft is on the ground, compared to known station elevation.

When an airplane takes off, it climbs to a standard atmosphere level. Usually the longer flights will fly at 300mb to 200mb (30,000 – 38,000 feet). The shorter flights (i.e. Kansas City to Saint Louis) will fly at something like 500mb (24,000 feet). Recon does the same thing. They set their controls to fly at 925mb in weak storms (2,500 feet), 850mb in developing storms (5,000 feet), and 700mb in hurricanes (10,000 feet).

Here’s the scary part: the controls are set to fly at this standard pressure reading. If the pressure drops, the aircraft drops, and vice versa. While recon is investigating a hurricane, it will maintain 700mb, regardless if the actual height is 10,000 feet or not.

That is one reason to look at the HDOBs that the crew transmits every ten minutes. That will tell you the pressure altitude (PA), or the height the altimeter thinks the aircraft is flying at, and the radar altitude (RA), or the height the radar knows the aircraft is flying at.

Here is a graph from recon mission 25 for Hurricane Katrina.



The bottom graph is simply the measured wind speed, in miles per hour, at flight level. (Mind you, those need to be adjusted to the surface. On this mission, Katrina was barely a category five.)

However, the top graph is the flight data – representing the aircraft’s elevation in feet. The blue line is the aircrafts pressure altimeter. Notice how when the plane is at operational altitude (in this case, 10,000 feet), it stays at that level? The purple line is the actual altitude, as measured by radar. Notice that the aircraft drops 2,000 to 3,000 feet during eye passage? The second eye penetration, the crew actually made two eye crossings to obtain more information about the maximum winds.

This graph also illustrates why the plane must fly higher during stronger storms. Of course, the closer to the surface, the better data you can get, but in storms like Katrina, 2,000 to 3,000 feet are lost during the eye passage. Imagine if the crew was flying at 5,000 feet. Now they are only a 2,000 to 3,000 feet from the ocean and all it takes is one downburst and they will be swimming.