I'm just a 21 year old weather graphic geek. Most of you know Max.
By: trHUrrIXC5MMX , 6:38 PM GMT on March 14, 2013
I'll be working on some weather topics every once in a while, as trying to explain some things which help me out in my career....
By dictionary definition, pressure is the physical force applied against or onto an object, as being stressful or crushing in many cases.
In the atmosphere air determines the pressure, the exact weight of the air concentrated above a certain point tells what the pressure is.
Air has density, believe it or not, therefore it has pressure, just like water.
There are different levels of stress in different altitudes in the atmosphere, in general as you go ascend the pressure decreases.
There are different ways on measuring pressure, mmHg (inches of mercury, as when you see it in weather.com), psi (pounds per square inch), pascals and bars (e.g. 100 bar).
1 bar (it's really 1.01325 bars) equals to the mean sea level pressure (or surface pressure)
So where does the "milibars" come into play? Well, a milli-bar is a 1000th of a bar (bar /1000), so our surface pressure in millibars is 1013.25 mb which equals to 29.918" (inches of mercury, mmHg)
Of note: millibars=hPa
^ It might sound confusing to you, but that's what it is
High pressure systems and low pressure systems are called so because of their air weight pressure.
Low pressure systems are so because they have a lower pressure than the surrounding areas at the surface. The lower pressure, the less crushing the force is. In hurricanes, by getting deeper into the topic, the pressure becomes a serious matter.
Image 1: Powerful Major Hurricane Wilma as being the strongest hurricane ever in the Atlantic Ocean. The NHC did not believe the storm had attained a pressure of 882mb after the Hurricane Hunters reported, it had to be calibrated first...
"BASED ON DROPSONDE AND FLIGHT-LEVEL DATA FROM AN AIR FORCE PLANE
JUST RETURNING FROM ITS MISSION IN WILMA...THE MINIMUM CENTRAL
PRESSURE IS ESTIMATED TO BE 882 MB...26.05 INCHES. THIS IS THE
LOWEST PRESSURE ON RECORD FOR A HURRICANE IN THE ATLANTIC BASIN"
A pressure of 882 mb (-131.25mb from the mean surface pressure) has been the lowest pressure ever recorded in a low pressure system.
Remember that low pressure systems full everything in towards them, that's what causes the low pressure values. Think of a vacuum cleaner.
That's still far from the all time's pressure record set by Typhoon Tip in 1979 of 870 mb.
In the other hand, high pressure systems, expand the air away from them, "cleaning the area" what's why in most cases is clear and sunny outside, however, it could be warm or cold. High pressures cover a wider area than a low pressure. The flow from a high pressure is much denser and heavier which allows to pull anything away.
If you were to place a 1070mb high pressure system over you, it would equal to 2,235,000 pounds of weight per foot. Which would crush you to nothing, killing you :(. Yes that is how much weight the air can hold, even more. No doubt that density can clear away any clouds or thunderstorms.
As I mentioned before, the higher you go up into the atmosphere the lesser amounts of pressure there are. That is called Pressure Altitude
Let's start below sea level (thousands of feet below)
Image 2:Mariana Trench.
The Mariana Trench is so deep, over 36,000 feet (6.8 miles), that it's very difficult to explore... Why? The pressures in the deepest areas are so high: over 1,086,000 mb! Which can totally dissolve anything in a matter os seconds. The pressure there is extremely high and highly lethal to any living organism. Anything there must be strong enough to support the heaviest weights of the water and gravity.
Image 3: Surface
At the surface, the pressure is much lower than that at the Mariana Trench.
1013.25 mb or hPa. This is the sea level pressure.
Image 4: Burj Khalifa... the tallest man-made building in the world. 2,722 feet!
Going a little higher. The Burj Khalifa is nearly 3,000' above sea level, the pressure is less than that at the surface. Over 900 hPa or mb the pressure would be.
I want to go there one day...acrophobia is not a problem for me..
Image 5: Mount Rainier seen over Tacoma, WA.
A jump to nearly 15,000 feet would take you over the summit of Mt. Rainier. The pressure there is nearly 570 mb, and decreasing with more height.
Image 6: Mt. McKinley
At 20,000ft you'll be over the tallest mountain in North America. Pressure near 465 mb.
Image 7: Mount Everest. The highest peak on earth.
A big jump to nearly 30,000ft (5.68 miles up) would take up to the top of the Himalayas. The pressure there is very low, 300 mb.
Anything above this is much below 230 mb.
If you want to find an additional pressure value not listed here or making a conversion you can go here to find out yourself.
Pressure is key to determine how strong a cyclone or anticlone is.
For tropical cyclones, pressure can depend heavily on the size of a storm and how strong it is. In recent years it was proven that larger cyclones have lower than normal pressure. By lower than normal I mean pressures that stronger storm usually have.
Hurricane Ike, Alex, Irene, Isaac, and Sandy were examples of large storms which had lower than normal pressures to what their wind intensity was originally. That is why the SSHS is only now for measuring winds, not pressure or surge. We now very well what happened with Ike and the SSHS surge.
Just remember that low pressure systems are "suckers" and high pressure "snow plowers".
Hurricane Season 2013
By the way...I'll be still updating my chart every once in a while, please keep sending me your thoughts about this season if you haven't yet, you don't have to do it now if you want until May or June.
My 2013 Hurricane Season Outlook
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