water vapour and clouds - 10/17/07 08:29 PM
I felt that water vapour and clouds needed a topic of its own. In the http://www.scienceagogo.com/forum/ubbthreads.php?ubb=showflat&Number=23878#Post23878 post, Chris said the following:
Ignoring that I was misquoted as suggesting that water and clouds are inseparable instead of water <b>vapour</b> and clouds, there is some discussion worthy ideas in his quote.
"the competition between increased water vapor and increased temperature cause relative humidity to change little as the climate warms" The only way to have a competition like you describe is within a closed system. The earth is not a closed system. As the sun warms us in the morning, the relative humidity goes down. It drops about 4% per degree Celcius of temp increase.
"...it is relative humidity that determines the formation of clouds." This is not true. While you are right when you said that "you need saturation to get a cloud," relative humidity is just a measurement. It determines nothing. Usually the most important factor is pressure. More specifically, it is the weather pressure system that is above the area that will determine weather or not clouds will form.
What affects cloud formation is condensation due to a temperature change. This is most common when we are in a low pressure system that allows the humid surface air to rise and cool. When we are in a high pressure system, then the warm humid surface air is not able to rise. We get clear skies when the pressure rises. This is just the most common. Anyone who has seen storm clouds roll in has witness a warm and a cold front colliding, but that is not as common that I have seen in Ontario.
The discussion about mid-latitudes is wrong since it suggests that temperature determines clouds alone. That is not the case. The key is temperature change. It is the ability for air to rise to the cooler high troposphere. How do I know? I live at a mid latitude (46.6 N). This summer we had back to back low pressure systems in July such that we had a tonne of clouds and near record rainfall.
Finally, for some reason, the "[b]ut Climate models now predict that cloud feedback will be either close to neutral or positive in a warmer climate." line was tagged to the end. When I am walking home at lunch on a mostly cloudy day, I appreciate the few breaks in the clouds. It is those few sunny breaks that warm my face nicely until the next cloud gets in the way. While cloud cover at night will keep some heat in such that dew or frost will not form, clouds during the day repel a lot of nice heat on an otherwise cold and cloudy day. If climate models are predicting the opposite, then their output does not make sense.
Originally Posted By: Chris
Just as a response to some points above- on the feedbacks, I think I already went over this, but the inseparable water and clouds is a falsehood. First, the cooling due to evaporation is how the surface comes back to equilibrium after being perturbed by the increased radiative heating; that is automatically accounted for in models, and is necessary for them to reach equilibrium, which is a prerequisite for an estimate of the "equilibrium climate sensitivity" to a doubling of CO2 that is always cited from the IPCC reports (i.e., 1.5-4.5 deg. C until the most recent report, now 2-4.5 is more like it). Then, it is not true that increased evaporation forms clouds and is an example of a negative feedback. Increased evaporation increases water vapor; the competition between increased water vapor and increased temperature cause relative humidity to change little as the climate warms (you need saturation to get a cloud), and it is relative humidity that determines the formation of clouds. As proof: In midlatitudes it's a lot warmer in summer than winter, and there's lots more evaporation in summer, yet if anything there are fewer clouds because relative humidity is on average lower. But Climate models now predict that cloud feedback will be either close to neutral or positive in a warmer climate.
Ignoring that I was misquoted as suggesting that water and clouds are inseparable instead of water <b>vapour</b> and clouds, there is some discussion worthy ideas in his quote.
"the competition between increased water vapor and increased temperature cause relative humidity to change little as the climate warms" The only way to have a competition like you describe is within a closed system. The earth is not a closed system. As the sun warms us in the morning, the relative humidity goes down. It drops about 4% per degree Celcius of temp increase.
"...it is relative humidity that determines the formation of clouds." This is not true. While you are right when you said that "you need saturation to get a cloud," relative humidity is just a measurement. It determines nothing. Usually the most important factor is pressure. More specifically, it is the weather pressure system that is above the area that will determine weather or not clouds will form.
What affects cloud formation is condensation due to a temperature change. This is most common when we are in a low pressure system that allows the humid surface air to rise and cool. When we are in a high pressure system, then the warm humid surface air is not able to rise. We get clear skies when the pressure rises. This is just the most common. Anyone who has seen storm clouds roll in has witness a warm and a cold front colliding, but that is not as common that I have seen in Ontario.
The discussion about mid-latitudes is wrong since it suggests that temperature determines clouds alone. That is not the case. The key is temperature change. It is the ability for air to rise to the cooler high troposphere. How do I know? I live at a mid latitude (46.6 N). This summer we had back to back low pressure systems in July such that we had a tonne of clouds and near record rainfall.
Finally, for some reason, the "[b]ut Climate models now predict that cloud feedback will be either close to neutral or positive in a warmer climate." line was tagged to the end. When I am walking home at lunch on a mostly cloudy day, I appreciate the few breaks in the clouds. It is those few sunny breaks that warm my face nicely until the next cloud gets in the way. While cloud cover at night will keep some heat in such that dew or frost will not form, clouds during the day repel a lot of nice heat on an otherwise cold and cloudy day. If climate models are predicting the opposite, then their output does not make sense.