Weather Question?

Question

To settle an argument:
My friend says that the relative humidity can reach up to 200%, but I always thought that it rained even when it got above around 50%. He also said that the air pressure had more to do with precipation than humidity, but I think humidity is more closely related because it is the amount of water in the air, thus meaning it would dictate precipation. Who is right here?

Answer 1

With regards to weather, relative humidity does not rise above 100%.

You are correct. Relative humidity controls precipitation.

Relative humidity is affected by temperature and temperature is affected by pressure so indirectly your friend is also right.

Relative humidity is a way of expressing the amount of water in air. It is a measure of the ratio of the the actual partial pressure of water vapor versus the equilibrium vapor pressure of water at the current temperature. That is where temperature comes in.

If the temperature is reduced the equilibrium vapor pressure of water falls. The opposite occurs if the temperature is raised. If air temperature changes the actual amount of water vapor in the air does not change but the equilibrium vapor pressure does change therefore the ratio of the two changes and so the relative humidity changes.

You can see the impact of these effects in several different situations. Deserts are very dry in part because the air is hot. Hot air will have a very high equilibrium vapor pressure because the temperature is high but also has an actual vapor pressure similar to surrounding cooler areas; therefore, the relative humidity is very low, which is a way of saying very dry.

The effect of pressure can be seen as air rises up a mountain slope. As air rises the pressure falls. The fall in pressure causes the temperature to also fall, which is why the tops of mountains are cold. As the temperature falls the equilibrium vapor pressure of water falls, but the actual vapor pressure does not and so the relative humidity rises. If it reaches 100% a cloud will form. This is why the tops of mountains are often shrouded in clouds.

Answer 2

Humidity cannot reach 200%. There can no more be 200% humidity than you can put 2 gallons of water into a 1 gallon bucket.

Humidity can go over 100%, and by over I'm talking .01% over, and that can result in rain, but not always. Humidity can reach 100% and it still won't rain. The southern US regularly sees humidity in the 70-90%, and it stays that way all day. Humidity is basically the amount of water vapor that is in the air.

As far as pressure goes. "As the name says, a "high" is an area where the air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count. The pressure is high at the surface where air is slowly descending — much too slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally — but not quite always — associated with good weather."

"The air that descends in high-pressure areas has to get to high altitudes in some way, and its done by rising in areas where the pressure at the surface is low. As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there's enough water or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather."

Its been a while since I took Meteorolgy...

Answer 3

Your friend is closer. Where I live, the humidity levels literally get up to around 105%.

Humidity and pressure both effect precipitation, but there has to be some kind of dust or particle in the air for rain drops to form (like silver iodide in the case of cloud seeding), though shockwaves can also be used to "shock" the water out of the air; in times past, cannon have been used. If the humidity is above 100%, it means that the air is super-saturated. Commonly, the humidity is in the 80-95% range in Georgia, and you can wave your hand in the air, and then watch the water drip off it.

I'm not kidding.

Answer 4

Air pressure would have a larger factor on precipitation because it determines the condensation of water vapor in the air. While a higher humidity will cause more water vapor to condense on surfaces, it will not trigger a precipitation event.

Answer 5

The relative humidity is 95% in my town right now, and it isn't raining (but it is foggy and extremely muggy). It just means how much water vapor is in the air.

It can get above 100% under certain circumstances. Check this out to see what I mean: http://en.wikipedia.org/wiki/Relative_humidity