Weather 101: Barometric Pressure

I said I want to try to keep this pretty simple, although they are used interchangeably, there are slight differences between air-pressure and barometric pressure but if I were to point out every difference between the two it would get complicated. That is not my intention here, I picked this topic because where we live, the air pressure is much lower than most areas of the United States, and a large percentage of the people who live here don’t even realize it.

Barometric pressure is a term you cannot get away from hearing especially during Hurricane season. It is one of the ways a Hurricane’s intensity s measured, the other is by wind speed.

The thing about hurricanes is that they all happen at sea-level, so to compare the intensity of a hurricane you have to know what normal pressure at sea-level is. Normalized sea pressure is measured at 1013mb. We use this unit of measure called millibars (designated by the abbreviation mb) to determine barometric pressure.

Some of the most intense hurricanes have recorded pressure readings as low as 882mb! Last year Hurricane Dorian recorded a pressure reading of 910mb. Remember, Normalized sea pressure is measured at 1013mb.

Meteorologists substitute air pressure in millibars for altitude. This is because there is a direct correlation between altitude and pressure. Let’s look at a snapshot of the conditions at a remote weather station this morning slightly west of Durango near Tech Center Drive.

Temperature 38°F 3°C

Humidity 63%
Wind Speed WNW 1 MPH
Barometer 1013.55mb
Dewpoint 26°F (-3°C)
Visibility NA
Last update 16 Mar 08:14 AM MDT

Here is the Colorado surface map. I have circled the barometric pressure reading on the map in blue.

Capture

That 1014mb calibrated reading aligns well with what the remote sensor in town showed! So our pressure readings today would be said to normal.

1014mb, does this seem high to you since we know normalized sea-pressure is 1013.25mb?

It is. That’s because 1014mb is not the actual barometric pressure! Do you want to know what the actual pressure is? At that current location in the reading above, at 6495 feet elevation, the pressure is 796.78mb! However, the pressure is calibrated to sea level so there is more of a base measuring platform to track the pressure, this is called QNH calibration. Uncalibrated, raw data, is referred to as QFE. I am not going to use those terms anymore, just remember that what you see on weather maps, and on the news is the calibrated reading.

Whenever I share this with people they want to know the actual air pressure where they live, here is an approximate guideline for normal uncalibrated pressure at our various altitudes.

6,000 feet 812mb

Durango in town 797mb

7,500 feet  767mb

8,000 feet 752.5mb

8,500 feet  738mb

Purgatory base  730mb

Purgatory top elevation  675mb

Wolf Creek top elevation  646mb

Telluride top elevation 614mb

So what does this have to do with the weather and weather terms I use?

Does anyone remember seeing anything like this on my website, or various Ski websites?

500mbhght

This is the upper air pattern for the incoming storm. You will always hear “the dark blues (lower pressure heights) indicate the potential for stormier than normal weather”. This is tracked at the 500mb level!  500mb=18,280 feet. This is a key component in tracking storms across the Continents and Oceans.

How about the jet stream? This is the 250mb forecast.

jet

The jet stream can add lift, which when it flows favorably over our area, can enhance precipitation, rates, and accumulations. The jet stream usually tracks between 200mb (38,000 feet) and 300mb (30,000 feet). When I refer to “jet enhancement” in my posts I usually show the 250mb (34,000 feet)  wind forecast.

This is just a sample of how forecasters incorporate pressure into their forecasts.

I will post my storm update early Tuesday morning then Tuesday afternoon I will do another Weather 101 segment. Thanks for following and continuing to support the site!

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