11/05/26 Thursday 4:30 am
These “Winter Weather Series” posts will be lengthy, but I will try to make them easy to understand and interesting to read. Rather than skimming through it, read it when you have time to absorb it. With what looks like a very snowy winter ahead, I will be using these terms a lot
Millibars vs Inches of Mercury
A term forecasters and meteorologists throw around a lot is millibars. They use them to track approximate rain vs. snow levels and to track storms. They use them to track the Polar Vortex, high-pressure ridges, and blocking patterns.
It’s not complicated; millibars indicate air pressure. Most modern weather stations convert air pressure to sea-level equivalent (MSL). So the true air pressure is effectively hidden behind the translation.
You can measure air pressure in two ways. Either millibars (MB) or inches of mercury (inHg). Inches of mercury is the more common measure to most people; millibars are more common with forecasters.
Some of us are old enough to remember that it was common to own a barometer. They were decorative and would sit on a fireplace mantel or an office desk; if the pressure was low, the dial would say “Stormy”; if high, it would say “Clear” or “Pleasant”.
When you watch hurricane coverage on the news, you hear about Hurricane intensity. Intensity is indicated by categories 1 through 5. Meteorologists determine categories using central pressure (millibars) and observed wind speeds from hurricane hunter aircraft. Hurricanes become major hurricanes when the central pressure drops to 964 millibars (MB).
Air pressure lowers with elevation; we live at elevation. “Standard” sea level is 1013.25 MB or 29.92 (inHg). As I said, most weather stations convert to sea-level pressure automatically regardless of your elevation, so it’s misleading.
My weather station indicates 30.15 (inHg). I live at 7450 feet; my true air pressure is 22.89 (inHg). In millibars, sea-level pressure is 1013(MB). At my elevation, my actual reading is 775.40(MB).
Why It Matters With Our Weather
Storm Tracking
Forecasters use (MB) readings to track storms and help determine snow levels. Here is a map we call “the 500(MB):
Look at the MB lines from top to bottom: 564, 567, 570, 573, etc. This indicates high pressure with “pleasant” weather.
Here is an example of low pressure (stormy)
We have two examples. The huge storm in the west, and the smaller storm in the east. See the circles of lines? That monster in the Northwest shows 513, 516, 522, 528, etc. 513(MB) is a very, very strong storm. It’s going to absolutely pound Washington and Oregon.
Snow Levels
Here is a GFS wishcast for November 19th.
I chose this because it shows all of the snow levels I monitor for storms. 552, 546, 540. These are also referred to as “thickness” levels.
At 540(MB), the rain/snow line is all the way down to 5,000 feet (elevation)
546(MB)=6,500 feet, so if you live in town, Durango, you will get snow when 546 is overhead.
552(MB)= 8,000 feet.
558(MB)=9,500 feet.
564(MB)=11,000 feet
For the most part, that is all you need to know when you look at my forecast maps. I am going to make these Winter Weather Series accessible on the home page of the site. It won’t be right away, but it is on the list.
Next up during this calm weather break: an El Niño update