All, esp. meteorologists,

It's tornado season in the Southern States, hurricane season in the Carribbean and here in the Uk some sultry weather has lead to massive thunder storms. The usual explantion for what is powering this severe weather is that the latent heat of water vapour in the atmosphere is 'released' as the water condenses. OK, so far.

But how does it work? Water vapour can be condensed by cooling it, but that requires removal of energy from the system. It will also condense if the pressure falls, but that also occurs only if energy is removed. Condensation neither adds nor subtracts from the system's energy total, but how does it 'release' energy to build tornados, hurricanes or thunderstorms?

John

Not a meteorologist.

At first glance, you may be just having problems with "system". One way of looking at it is that there are three parts to it.

1. Warm air
2. Moisture in the warm air
3. Colder air- surrounding, or adjacent to the warmer air (1).

The temperature difference between (1) and (3) sets up processes that strive to remove this difference, i.e. wind and normal weather. However, if you have lots of (2) in (1), energy can be transferred from the water to the air, through the release of latent heat. This maintains the temperature difference between (1) and (3) for a much longer time, and you get severe weather.

It's like the difference between a normal battery and a Duracell.

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It's this "release of latent heat" bit that gets me - no heat is released!
Water vapour is maintained as vapour until something takes energy from it!

But I think I see your point about the latent heat allowing a heat/pressure gradient to be maintained longer and greater than if there were no vaopour in the air.

Respect to Praed, but any real meteorologists to sort me out?

John

No worries JohnD.

Heat is released. Water vapour (gas) condenses into water droplets (liquid).

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Not a meteorologist, but I did once write about this stuff for beer money.
A rising parcel of air enters regions of progressively lower pressure with altitude, and therefore expands. In expanding, it does work on the surrounding air and therefore cools. If it is unsaturated ("dry"), it cools at a characteristic rate, the dry adiabatic lapse rate, which is around 10°C/km. If it is saturated ("moist"), the cooling causes water condensation, which releases latent heat that offsets the cooling process. So there's also a moist adiabatic lapse rate which is less than the dry adiabatic lapse rate: typically about half.
So moist air cools half as quickly as it rises, and therefore retains a lower pressure, and therefore is more buoyant than dry air. We're therefore more likely to see strong convective instability when moist air rises, which accounts for those billowing towers that develop on top of cumulus clouds. More rising air draws in more air at ground level, and does more work aloft.

Grant Hutchison

Edit: I quite understand that you'd rather hear this from a meteorologist. Googling on "moist adiabatic lapse rate" (or "saturated adiabatic lapse rate") and "thunderstorm" will turn up any number of lecture notes and information pages prepared by meteorologists, which will (I hope!) say pretty much what I describe above.

Heat is the transfer of energy. Latent heat is the energy absorbed or released when a substance changes phase. So something taking energy from another thing is the same as the other thing "releasing heat."
Clouds form when water condenses into water droplets. That's a phase change, so the energy of latent heat is released into the surrounding environment, powering the storm.

Edit: I'm also not a meteorologist, and didn't give as detailed an explanation as grant.

Heat is released. You seem to be confusing heat with temperature...the two are not the same thing.

I’m not a meteorologist but:

Warm air rises, because it’s less dense then the surrounding air

At higher altitudes atmospheric pressure is less so the air mass expands

This expansion of the air mass causes it’s temperature to drop.

As the temperature decreases the amount of water vapor the air can hold is reduced so it’s forced to condense into particles.

To do this the water vapor must give up heat, this heat acts to offset the cooling caused by the expansion of the air mass.

The overall effect of this adiabatic effect is to carry energy upwards away from the surface and where it drives storms.

That's the Bad Meteorologist's Number One Error. See http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

In sum, it's got nothing to do with the air. Air does not have a "capacity" for holding water vapour. If you have an evacuated flask and put some liquid water in it, a characteristic amount of water vapour will evaporate above it at a given temperature. Add some air, and it won't make any difference to the amount of water vapour above the liquid in the flask. Cool it down, and some of the water vapour it will condense. The presence or absence of air will make no difference to the quantity.

Meh, you’re splitting hairs. The temperature of the water vapor is directly determined by the temperature of the air around it. If it were any different it would soon equalize due to the constant exchange of energy in both directions. The temperature of the air therefore *does* determine how much water vapor is present, and that’s all that is essential to understanding the explanation.

If you wanted to extend that and explain why the temperature of the air is important more would be required.

The error is really in that word "hold". Air doesn't have a capacity to "hold" water. The water simply finds its own equilibrium according to temperature. Indeed, hot water will produce hot vapour which will increase the temperature of the air above it.

Grant Hutchison

Is there a significant difference between the effects of various droplet sizes of water in the air in terms of the transfer of latent heat. Usually some non-water catalyst is envolved with the initiation of the formation of water droplets and discussions about radiative forcing seem to distinguish between the effects of water versus water vapor versus clouds. I imagine that the analysis of steam (no droplets if pure) when considering the effects of various droplet sizes mixed in ith the steam is beyond comprehension.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

I found this explanation very helpful:

"Tropical cumulus production and thunderstorm production are driven by air density. Air density is a function of temperature (affecting density directly) and evaporation (water vapor is lighter than air).

A thunderstorm is both a self-generating and self-sustaining heat engine. The working fluids are moisture-laden warm air and liquid water. Self-generating means that whenever it gets hot enough over the tropical ocean, which is almost every day, at a certain level of temperature and humidity, some of the fluffy cumulus clouds suddenly catch fire. The tops of the clouds streak upwards, showing the rising progress of the moisture laden surface air. At altitude, the rising air exits the cloud, replaced by more moist air from below. Suddenly, in place of a placid cloud, there is an active thunderstorm.

Self-generating means that the thunderstorms arise spontaneously as a function of temperature and evaporation. Above the threshold necessary to create the first thunderstorm, the number of thunderstorms rises rapidly. This rapid increase in thunderstorms limits the amount of temperature rise possible.

Self-sustaining means that once a thunderstorm gets going, it no longer requires the full initiation temperature necessary to get it started. This is because the self-generated wind at the base, plus dry air falling from above, drive the evaporation rate way up. The thunderstorm is driven by air density. It requires a source of light, moist air. The density of the air is determined by both temperature and moisture content (because curiously, water vapor at molecular weight 16 is only a bit more than half as heavy as air, which has a weight of about 29).

Evaporation is not a function of temperature alone. It is governed a complex mix of wind speed, water temperature, and vapor pressure. Evaporation is calculated by what is called a “bulk formula”, which means a formula based on experience rather than theory. One commonly used formula is:

E = VK(es – ea)
where

E = evaporation
V= wind speed (function of temperature difference [∆T])
K = coefficient constant
es = vapor pressure at evaporating surface (function of water temperature in degrees K to the fourth power)
ea = vapor pressure of overlying air (function of relative humidity and air temperature in degrees K to the fourth power)
The critical thing to notice in the formula is that evaporation varies linearly with wind speed. This means that evaporation near a thunderstorm can be an order of magnitude greater than evaporation a short distance away.

In addition to the changes in evaporation, there at least one other mechanism increasing cloud formation as wind increases. This is the wind-driven production of airborne salt crystals. The breaking of wind-driven waves produces these microscopic crystals of salt. The connection to the clouds is that these crystals are the main condensation nuclei for clouds that form over the ocean. The production of additional condensation nuclei, coupled with increased evaporation, leads to larger and faster changes in cloud production with increasing temperature.

So increased wind-driven evaporation means that for the same density of air, the surface temperature can be lower than the temperature required to initiate the thunderstorm. This means that the thunderstorm will still survive and continue cooling the surface to well below the starting temperature.

This ability to drive the temperature lower than the starting point is what distinguishes a governor from a negative feedback. A thunderstorm can do more than just reduce the amount of surface warming. It can actually mechanically cool the surface to below the required initiation temperature. This allows it to actively maintain a fixed temperature in the region surrounding the thunderstorm."

The full essay is here.

(Note to mods: Willis Eschenbach has stated he is happy to have his work reproduced anywhere online)

Actually, that brings up another question...do surface tension effects make for any significant differences in the heat capacity of water at different droplet sizes? I would expect merging of droplets to form ones with higher volume for the surface area and lower curvature of the surface would increase temperature slightly...and for a similar effect to produce a little excess heat as water vapor condenses onto droplets, perhaps slowing condensation a bit at small droplet sizes...

Only in part. It's also determined by the evaporating water, as well as the nature and amount of radiative energy flowing into and out of the system. In fact, at higher relative humidity rates, the water content of the air does more to drive the air temp than the air temp does to drive the water!

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"Toward no crimes have men shown themselves so cold- bloodedly cruel as in punishing differences of belief." - James Russell Lowell