Question on Humidity in Hot and Cold Areas

[gene098]

Hey guys,

 

This has been a really big mystery to me and my classmates for a while so we wanted to ask more experienced people on what the reasons are.

 

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If hot air holds more moisture than cold air does, which causes hotter areas to be more humid, like Texas and Flordia.

 

Reference) “Warm air can hold more water than cool air.”

From Scientific American

 

1) Then how come there are many areas where it is hot and not humid?

 

 

Reference) “Dry regions, where evaporation and evapotranspiration exceed the annual mean precipitation, cover about 40% of Earth’s land surface and affect the livelihood of nearly two billion people, primarily in developing countries (Safriel and Adeel, 2005). Dry regions are primarily found in the subtropics,”

From Yale Climite and Energy Institute, Xavier Levine

 

 

Here Mr.Levine explains that the reason there are many areas where it is hot and not humid is due to…

“evaporation and evapotranspiration exceed the annual mean precipitation” but why would that be the case?

 

2) However from the first reference, hot air holds more moisture and the water that is heated and goes into the air doesn’t just disappear…but goes into the air, so shouldn’t it be humid no matter what?

 

3) Where else would the water go then?

 

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Then assuming that we accept what Mr,Levine says , the subtropics is where “evaporation and evapotranspiration exceed the annual mean precipitation”

= why subtropics are dry

 

And it does ring true that Florida and Texas are above that region as we can see in this image...

 

 

Reference) https://upload.wikimedia.org/wikipedia/commons/b/b0/World_map_indicating_tropics_and_subtropics.png

 

4) However, Arizona which is known for being hot AND dry just like the subtropics Mr Levine describes, doesn’t lie in the subtropics range…

So whats the explanation for that?

Reference)

[swansont]

2) However from the first reference, hot air holds more moisture and the water that is heated and goes into the air doesn’t just disappear…but goes into the air, so shouldn’t it be humid no matter what?

If there isn't all that much water around to go into the air, it will not be humid.

 

4) However, Arizona which is known for being hot AND dry just like the subtropics Mr Levine describes, doesn’t lie in the subtropics range…

So whats the explanation for that?

Reference)

 

 

On average, Arizona gets ~7 inches of rain per year, while Florida gets ~50 inches. Texas gets ~35

 

http://www.betweenwaters.com/etc/usrain.html

[Sriman Dutta]

Only high temperatures is not what causes greater humidity. There are a lot of factors which determine the overall climate of a place.

[Country Boy]

Your quote only says that hot air can hold more water. That does not say that it always does.

[Velocity_Boy]

Humidity is just the amount of water molecules in the air. So more humidity = more ambient H2O. More H20 in the air during a hot day will feel hotter to us than would the same temp. in a less humid environment. This is why we have a "heat index" that is useful for a "real feel" temperature measurment. Increased humidity will also make it seem colder, than it would feel in an envrironment with the same ambient air temp but with lower humidity.

 

Humid areas are found near large bodies of water, or near tropical and equatrorial regions. Also, low lying areas below a couple thousand ft. elevation tend to be more humnid than mountainous areas, where the atmospheric pressure is a bit less, there is usually more wind, and the O2 content is slightly lowered.

Edited April 13, 2017 by Velocity_Boy

[gene098]

If there isn't all that much water around to go into the air, it will not be humid.

 

 

Thanks, but what i wanted to know is,

If there is 7 in of rain in Arizona, and all of it is evaporated into the air, then the air should be humid right?

Its not like the evaported water goes into the air and then just dissapears right?

If there isn't all that much water around to go into the air, it will not be humid.

 

 

 

On average, Arizona gets ~7 inches of rain per year, while Florida gets ~50 inches. Texas gets ~35

 

http://www.betweenwaters.com/etc/usrain.html

 

is this the main reason Arizona is dry?

 

And if so, why does Arizona only get that little rainfaill?

Edited April 14, 2017 by gene098

[swansont]

 

Thanks, but what i wanted to know is,

If there is 7 in of rain in Arizona, and all of it is evaporated into the air, then the air should be humid right?

Its not like the evaported water goes into the air and then just dissapears right?

 

 

No, of course it doesn't disappear. Right after rain it's generally more humid than at other times. But the air moves around. Some of the water vapor condenses and eventually falls as rain somewhere else. Areas that are more humid on average have sources of water that are continually adding water to the atmosphere.

[druS]

gene098 thanks for giving me a chance at my first post!

 

Some quick background on humidity. Warmer air can carry more moisture - more accurately (from recollection) the water vapour content can be higher, in absolute terms, when air is warmer. Generally though we refer to "relative humidity" which is a comparison of how much water is in the air compared to how much water the air can carry. It relates to the partial pressure of the water vapour in the air.

 

To give an idea; a very rough reading from a psychrometric chart gives us warm, humid air (say, in Florida) - I am using 80% RH at 30 C - which provides 14 g of water per 1 kg of air. Note that this air only needs another 3g of water to reach 100% RH and be either raining or hitting dew point (moisture coalescing on the ground).

 

Let this same air blow over to Arizona, with no other changes, where it is, say 40 C. The same 14g of water per kg of air is now only 40% RH (give or take) because it's hotter and it needs much more water vapour to reach it's full carrying capacity. This is the same amount of absolute water in the air - but it now needs nearly three times as much water, another 9 g, to reach 100% RH. The chance of rain is minute (non existent) compared to the same air when it was in Florida. So instead of hindering evaporation, as it was in Florida, the air is now dry and sucking water out of the environment.

 

Same air. Same amount of absolute water in the air. Relative humidity completely different.

 

Back to your definition of "dry region", I suspect you may have inadvertently made a circular reference. Your source was trying to define dry regions, in effect, as those areas which evaporate more than they gain in rainfall. By definition they will be less humid - as we have seen even if they have the same amount of absolute water in the air.

 

Google up a phychrometric chart and have a play with it.

Edited July 6, 2017 by druS

[DrKrettin]

Humid areas are found near large bodies of water, or near tropical and equatrorial regions.

 

Is that generally true? I live on a small island surrounded by the Atlantic ocean, very near the Tropic of Cancer. The humidity is constantly extremely low, except for when it rains, which is hardly ever.

[druS]

 

Is that generally true? I live on a small island surrounded by the Atlantic ocean, very near the Tropic of Cancer. The humidity is constantly extremely low, except for when it rains, which is hardly ever.

 

Due to the rotation of the earth we have predictable "wind" at high level - atmospheric conditions. Where there is little to obstruct these conditions they prevail down to the ground/sea level.

 

These conditions can then stabilise over the ocean and more or less stay that way travelling from the west until they hit land. This means that wind conditions on the west coast of continents (and islands) tend to follow those conditions. The local weather patterns have less of a chance to build. Here in Australia, in the west (let's say Perth), there is a consistent wind we call "the Fremantle Doctor". It "blows through" local weather patterns a lot of the time. The air has more or less stabilised over the ocean at moderate humidity, hits the warmer land reducing the humidity, and rain is rare. Or more accurately rain happens in winter when the lower temperatures have the opposite effect and the air drops below dew point creating rain, if there was enough water vapour in the air and if the temperature is low enough.

 

Over in Sydney, east coast, the wind has been disrupted across the land. That consistent wind is still happening but much higher up in the atmosphere where it has much less impact on weather on the ground. In summer those local weather patterns provide little/no wind in the morning, and a building sea breeze in the afternoon, from the SE. As the air over coastal sea has been warmed up it holds more moisture. This travels past Sydney to the mountains which pushes the air up higher - it gets colder, below dew point - storms form and bounce back over Sydney and the coast.

 

East coast Australia has much more rain than west coast, and the rain falls in different seasons.

 

I'll bet you will find that California is drier than Florida, and that the same sort of thing is happening in Africa. The oceanic islands don't have enough land mass to build dominant distinctive weather patterns from their own conditions, so they follow much the same as general ocean areas.

 

Seasonal differences have an impact as well as the atmosphere tends to be higher pressure over the equator and lower pressure over the poles. Think about a spinning top and centrifugal force - strong in the middle, weak at the top and the bottom. In winter atmospheric lows form from the poles and can move away with predictable weather conditions (storm fronts, troughs etc). In Summer high pressure regions at the equator create lows in between the equatorial high pressure regions that are powered by the hot ocean and create monsoons/hurricanes (northern hemisphere tropics) or cylcones (southern hemisphere tropics) in Summer. Those weather patterns also change humidity and rain.

[DrKrettin]

Google up a phychrometric chart and have a play with it.

 

That had me fooled for a while - I thought I had to learn a new word.

[Sicarii]

I have a related question about humidity.

 

Looking at current weather forecast on Google, I see that % humidity in following three places is:

 

Houston, TX - 47% humidity, 93dF temperature

Jacksonville, FL - 55% humidity, 92dF temperature

Los Angeles, CA - 48% humidity, 82dF temperature

 

Is this saying LA is more humid than Houston? If so, why does it feel like Houston is more humid, generally speaking?

[StringJunky]

I have a related question about humidity.

 

Looking at current weather forecast on Google, I see that % humidity in following three places is:

 

Houston, TX - 47% humidity, 93dF temperature

Jacksonville, FL - 55% humidity, 92dF temperature

Los Angeles, CA - 48% humidity, 82dF temperature

 

Is this saying LA is more humid than Houston? If so, why does it feel like Houston is more humid, generally speaking?

Being hotter in Houston, that means you're sweating a lot more to cool down plus the humidity. This will likely result in having wetter skin.

Edited July 14, 2017 by StringJunky