I keep seeing that Hurricane Milton has record pressure but I don’t understand it. What does pressure of 905 mb mean and why is it important for understanding the size and scale (and potential devastation) of a hurricane?
In: Planetary Science
Barometric pressure is just another way to say atmospheric pressure, which is determined by the weight/density of the air above you/a location.
Hurricanes are/cause low pressure zones in the atmosphere, the lower the pressure drops, the more intense the storm.
Barometric pressure is basically a measurement of how “heavy” the air is. High pressure is typically associated with good weather. Low pressure is typically associated with storms. Air will want to move from high pressure to low pressure to equalize them, meaning differences in pressure cause wind.
Now with some of those basics out of the way, how this relates to hurricanes. Normal barometric pressure at sea level is a little over 1000 mb. Over warm bodies of water like the Gulf of Mexico, Caribbean Sea, and the South Atlantic, the water evaporates more quickly. This warm air rises. This will create lower pressure as there is effectively less air there, meaning air from nearby will get sucked over to replace that. That air can also then become warmed and start rising. If this starts to happen quickly enough, that process can start repeating over and over and start to form a tropical depression. This will create a lower pressure center, where the warm air keeps rapidly rising, more air getting pulled in which keeps warming and rising. All of this excess moisture in the upper atmosphere will form clouds and eventually rain. And this whole system become a spinning cyclone with higher pressure on the outside spinning around the lower pressure which keeps bring in more air that will rise and form more clouds and more rain.
The lower the pressure of the eye, the more this process repeats meaning the more moisture is accumulating in those clouds. This process typically continues as the storm stays over warm water and continues to lower the pressure, which increases the wind speeds around it. The barometric pressure of the eye will determine how strong the hurricane is, with the lower the pressure, the stronger the hurricane. A category 1 will usually be between 980 and 990, a category 2 between 965 and 979, etc. A category 5 will be less than 920. Hurricane Helene had a pressure of 938 mb when it made landfall. Hurricane Beryl earlier this year was at 935 mb when it made landfall.
So Milton with the pressure of 905 means it is very low, and will have very strong winds. All of that said Milton looks like it is going to brush by the Yucatán which will weaken it a little bit as the parts of it over land won’t be able to keep sucking up more water. Most projections say Milton will be down to a category 3 when it hits Florida.
I think of it like temperature to give me kind of an estimate of severity. Just drop the first number. So in the 60-80s, thats probably fine. 20-30s? Yeah that’s real cold so that’s a bad hurricane. 0-10degrees? That’s too damn cold/that’s too bad a hurricane.
The pressure in mb helps determine what Category the hurricane will be (along with other factors)
Cat 1: >980 mb
Cat 2: 965-979 mb
Cat 3: 945-964 mb
Cat 4: 920-944 mb
Cat5: <920 mb
Air has weight. You don’t notice it, but each square inch of you body has about 15 (14.7) pounds of air pushing in on you if you were at sea level. Because the wind blows, the amount of weight of the air changes from day to day (or even hour by hour). The amount of air weight is called barometric pressure.
It was origonally measured by filling a glass tube full of mercury and turning it upside down. They measured how high the mercury filled into the tube. You can actually do the same with water, but you need about a 3 story building. The average was around 30 inches and ultimately the “standard” was set at 29.92 which is still used today for airplanes and pilots. Metric system came along and they set the standard to 1000 millibars (which was later adjusted to 1013).
Low pressure systems tend to draw moisture and wind into them with resulting storms. High pressure pushes wind and moisture away resulting in hot dry weather.
Hurricanes are extremely low pressure systems where the low pressure correlates to high winds and high storm surge. 905 is VERY low and only is found in the strongest hurricanes ever recorded.
In highschool, the science teacher lit a cigarette and blew a puff of smoke into an empty 5 gallon water jug. He pointed at the hazy jug and said “that’s low pressure”. Then he put his mouth on the jug, blew in and the smoke cleared. He stood up and said “that’s high pressure”.
Almost 50 years later and I still remember.
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