Do hurricanes remove moisture from the air for 100s of miles away?

This is based solely on my experience this year from Charley and now Frances. I’m about 500 miles away from Frances right now and, well, it just feels weird outside. Very low humidity, maybe 10 degrees cooler ‘feeling’ than normal. It felt very similar when Charley passed by, made what would have been a normal day quite autumn-like.

Is this all in my head or do these big tropical systems hundreds of miles away really affect local weather?

Just the opposite. They draw tropical moisture for hundreds of miles. Here take a look at the water vapor Frances is drawing in.

Even Frances doesn’t affect weather 500 miles away. It may seem drier to you because it is. There is some dry air near the SE coast along the Carolinas, which won’t last long, as Frances gets closer. I believe the extent of the convection caused by Frances is 380 miles.

The weather report before Frances got to the coast was that there was a dry region ahead of a front that was moving across the western part of the SE US. This dry region reduced the storm from a 4 to a 3 and then to a 2 by the time it came on shore by “starving” it of moisture. Maybe “starving” is too strong and it was just a reduced-moisture diet. Just the same, it was still a big and wet storm and it moved slowly so that it had more time to do damage by wind and heavy rains.

The pocket of dry air may have been a factor in reducing the strength of the hurricane, but I doubt it. It did not invade the hurricane, and thus would have had no bearing on it. The reason why it’s severity was lessened was because it went across some islands and lost a lot of its latent energy. It’s a possibility that it can become a hurricane again now that it is in the GOM, but so far it has shown no signs of increasing its strength (although the center is becoming slightly better organized).

Hurricanes draw vast amounts of moisture from the ocean and the atmosphere. Overall, this air is rising throughout the storm. This air then has to go somewhere, so it ends up sinking on the periphery of the storm. Sinking air warms and dries as it sinks (think air sinking on the leeward sides of mountains), so it is very possible. I’m not sure 500 miles is close enough to experience this, but it would be very possible.

However, I have not looked at the conditions currently associated with the system, so I can’t say for sure, and it’s my day off so I’m feeling a bit lazy :slight_smile:


As for dry air ahead of the storm, lower relative humidities would weaken the storm, as there is less latent heat energy present in a “lower moisture” atmosphere. Dry air would also increase evaporation in the mid-levels of the storm, and since evaporation is a cooling process, it requires energy. Condensation from the water vapor releases energy, and is what makes hurricanes what they are. This combined with the westerly wind shear, the possibility of increased interaction with the Bahamas (not terribly likely but possible), and the possibility that as the storm slowed, it dug up cooler ocean water, are all reasons why Frances could have weakened before landfall.

The only area in a hurricane in which the air is rising is the center, which usually will have a well-defined eye, precisely because the air is rising. (It’s an “eye,” because the center is not covered by clouds, obviously. There would be no eye if the area was clouded over.) Convection occurs throughout the storm, except in the center. In fact, the eyewall has the strongest winds and usually the heaviest rains. And the greatest sinking is in the eyewall. This is what sustains the storm: continuous rising air in the center with continuous sinking air at the eyewall. It’s the continuing cycle of this that causes the hurricane. It’s a complicated process, but those are essential factors.

If you check my link above, you’ll see that the water vapor does not extend out to 500 miles, except over the ocean, where it goes up to Canada. Here’s a close-up of the water vapor:

Here’s a radar image of the actual rain:

It’s been raining all day in Charleston, but this is not shown in the above image, apparently because it doesn’t go north enough.
Origin of hurricanes.

I don’t see how dry air ahead of a storm would have any effect on the storm. The latent energy is from the condensation from the evaporation from the ocean. I did not hear of any westerly wind shear in connection with Frances. The storm slowed but it was not stationary. It continued to move and, therefore, was not over the same spot on the ocean and did not stir up the ocean sufficiently to cool it. The main reason it weakened, IMO, is that it went over the islands. The same reason it weakened more when it traversed Florida (a land mass, as the islands are).

Wherever there are thunderstorms, the air is rising. In fact, the reason why the eye is clear is because of sinking air (as air sinks, it compresses and warms, which is why eye temperatures are the warmest in the entire storm (not necessarily at the surface, but anywhere in the vertical column).

As for the dry air ahead of the storm, I’m sorry I wasn’t more clear. Dry air ahead of the storm by itself isn’t a big deal, it’s if it gets ingested into the circulation where it causes problems, where it increases evaporation as mentioned above and in your links.

The westerly shear was more apparent when the storm was starting to weaken, and was visible in satellite images as well as in some online objective analyses. There was little convection/outflow on the western side of the storm at that time. It was supposed to have decreases as the storm got closer to land.

While I agree that the islands could have weakened the storm a bit, the fact that they are not concentrated landmasses, and the fact that the hurricanes don’t get their energy from a specific point (which you can argue that compared to the storm, the islands are merely small points in comparison). Finally, in regards to the dredging up cooler water, it was only a guess, but more likely as the storm slowed. The storm slowed to near 5 mph as it moved over a good chunk of the Bahamas, and the storm itself was very large, which would have given it a long timeframe for waves to churn up the water and bring up cooler water from below. It doesn’t take long for the water to get sufficiently churned, and only a few degrees cooler would play a non-inconsequential role.


Just the opposite. The eye is clear because of rising air. Above the storm is a weak high which causes the air in the center to rise. Since the lowest surface pressure is found at the eye, this causes a weak vertical shear, necessary for sustaining the hurricane. (Not to be confused with horizontal shear which will destroy a storm.)

The rest belongs in GD, but is ascertainable because the meteorologists know. I’ve stated my opinion and you yours.

I am a meteorologist. :slight_smile:

Here is a few links explaining why air sinks in the eye:

For the longest time I was in your position, I couldn’t understand why such low pressure in the eye was associated with sinking air. I hope the links help a bit.


My apologies. :o I said that it was complicated. :slight_smile:

Actually my post that brought up the dry air said that the dry air was ahead of a front across the southeastern US. As the Frances moved into this air it lost a lot of the major source of heat, namely the condensation of water vapor.

As to energy being gained by the storm from evaporation from the ocean followed by condensation of that vapor. I fail to see how this adds any net energy to the storm. The air in the storm has to provide 970 Btu/lb. to evaporate the water and that is energy lost by the storm. The storm gets the energy back when the vapor subsequenty condenses but I can’t see any net gain. Net input energy has to come from water vapor in the air that is provided by an energy source other than the storm, like the sun for example.

It’s best to let Visceraanswer this, but they derive their energy from convective instability and condensation of copious amounts of water that provide abundant rains. It is these changes in the state of water without an accompanying change in temperature which causes a gain in the heat of the system. This is known as latent heat or latent energy.

Viscera, just to get the eye straight, as I understand it now there is rising air in the eye, but in the immediate core the air is sinking, as it is in the eyewall. Is this correct?

It is just barely possible that, incredible as it might seem, I was mistaken about there being no net energy input to the storm from the evaporation of water from the ocean. On thinking about it some more it occurs to me that the energy to evaporate the water comes from the water in that the fastest, or most energetic water molecules are the ones that form the vapor. This energy is then taken into the air with the vapor and released as heat when the vapor condenses.

I allowed myself to get fooled by evaporative air conditioners in which the hot and dry air is taken in, water is evaporated and the air comes out cooler which makes it look as if the energy to evaporate the water comes from the air. I now think that analysis is incorrect. I think the correct version is that the evaporation cools the water in the tank and this cool water is pumped over the pads where the evaporation takes place and where the air is drawn through. The air is cooled by being drawn over the pads soaked with cooled water which is kept cooled through continued evaporation.