Barometric pressure in Philly 28.29 ??

[riverwalker]

Hurricane Central?@twc_hurricane

Barometric pressure in #Philly now 28.39"; this breaks their all-time low pressure record of 28.43" set in March '93 superstorm #Sandy

[ffr]

1. That's a lot of water vapor

I'm sure they're forecasting heavy precipitation in your area.

Be safe.

[dumbledork]

8. Yeah, that's a really low baro...but it doesn't have anything to do with water vapor

or precipitation. They are different areas of meteorology.

[calimary]

10. Welcome to DU, dumbledork!

Reminds me of what Mad-Eye Moody always told Harry Potter, in his warnings about the Dark Lord and all his little friends: "CONSTANT VIGILANCE!!!!!!!!!!" Certainly relevant for us, too.

Glad you're here. We need you! There's an election to win.







Now get to work.

[ffr]

12. Are you sure? I've always understood it to be based on atomic weight

And it is the way we studied it in Meteorology and flight school. A barometer, along with weather balloons, can test surface and aloft air pressures, giving an indication of how much water vapor is above a location. And since water vapor is lighter than air, changes in barometric pressure readings over time, can indicate an increase or decrease in water vapor.

Water vapor - H2O - is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 18 atomic units. With 18 atomic units water vapor is lighter than diatomic Oxygen with 32 units and diatomic Nitrogen with 28 units.

Note! Water vapor in air will replace other gases and reduce the total density of the mixture. Dry air is more dense than humid air!

- Engineeringtoolbox.com

[dumbledork]

15. Yes, it is (oddly) true that water is lighter than air (in its gaseous state), but

it still doesn't have anything to do with pressure. Suppose you take a hermetic vessel containing some air (or even pure N2 if you wish)...you can change its pressure with heating or cooling (Charles' Law) or by mechanical adiabatic compression/expansion (Boyle's)...the absolute humidity doesn't change, in
that essentially isentropic system. It is, all that being said, true that high pressure in the atmosphere is usually associated with low water vapor levels but that is dependent on lots of other factors including inversions.

[ffr]

16. Thanks, I think I learned a little, but how can I...

equate potential heavy precipitation, like with this storm/hurricane, with the low barometric readings people are showing?

Wouldn't it just be easier to say, in layman's terms, 'that's a lot of water vapor above you' when answering DU threads?

[dumbledork]

18. Well, the amount of gaseous water (vapor) almost always decreases with altitude, mainly due to

the normal lapse rate (lower temperature)...it just condenses out when the temp reaches the dew point
and turns into visible moisture ...that's why your flight school people told you that you can only
get structural icing in the presence of it. (Clouds are an example).

The thing about the low pressure is that it results in convection around the circulation which does indeed raise air with dissolved water vapor to whatever altitude has its dew point equal to the temp when clouds (and likely condensation/precipitation) occur.

Probably the best way to address them is to say "that low pressure sure is sucking in some moisture"

[KittyWampus]

2. My head aches just thinking about that

[msanthrope]

3. My sinuses and jaw ache--it feels funny here. nt

[DinahMoeHum]

4. Yep, believe it. Latest advisory from NHC. . .

says minimum central pressure at the storm is 940 millibars = barometer at 27.7 inches.

http://www.nhc.noaa.gov/text/refresh/MIATCPAT3+shtml/292058.shtml

[Canuckistanian]

5. Wow, 958mb

That's low

[kentauros]

11. Okay, that's a number I understand

And yes, it's pretty damn low!

[Glimmer of Hope]

6. I think I have had a post-mirgraine hangover for days now. Maybe this is why.

Blah.

[elleng]

7. Heard similar, but lower, current low in Delaware.

Daughter in NJ near Philly, in high rise apartment, scared; windows rattling.

[calimary]

9. But there is no climate change...

That's just something some dumb ol' libruls made up.

America - For the Love of God, WAKE UP!!!!!!!!!!

[RobertEarl]

13. That's mighty low

The center of the storm is headed right for you. To the north east of you temps are up to 12 degrees warmer and to the south 10 degrees cooler.

There is a strong west wind to the south. A strong east wind to the north. This is almost like a very wide, slow speed tornado.

[BumRushDaShow]

14. As of the NWS Mt. Holly 5:54 pm update

for Philly, the pressure was -

28.35 in (959.9 mb)

So it went even lower.

[uponit7771]

17. English please... :-)..break it down for us and use grunts sparingly ..... tia