How can warm, humid tropical air have a lower relative humidity than cold, dry polar air?
- TQLv 73 months ago
Relative humidity (RH) is a dimensionless ratio ... expressed in percent ... of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.
Since the latter amount is dependent on temperature ... relative humidity is a function of both moisture content and temperature. Relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.
Tropical air with a dry bulb temperature of 90°F and a dew point of 70°F has an RH of 52%.
Polar air with a dry bulb temperature of 30°F and a dew point of 20°F has an RH of 66%.
The difference between the dry bulb air temperature and the dew point temperature of the polar air is smaller that the difference of the tropical air; therefore ... the RH is higher. The polar air is closer to saturation i.e., 100%.
The idea that it is the air which determines the amount of water vapor which can be present through some sort of 'holding' capacity is an eighteenth century idea shown to be false both empirically and theoretically about two hundred years ago!
The air's ability to 'hold' more water vapor at higher temperatures is not merely a simplification ... it is categorically wrong.
Don't take my word for it.
Ask Dalton.Source(s): Meteorologist.
- Atarah DerekLv 73 months ago
Because RELATIVE humidity is basically just a measure of how far away from the dew point you are, and thus how much moisture the air can potentially hold. ABSOLUTE humidity is how much water vapor there actually is in the air at a given point. If there's almost no water in the air, but the temperature is within a degree of the dew point, the relative humidity is still going to be 95% or greater.
- NancyLv 73 months ago
Because relative humidity is "relative," based on how much moisture the air can hold relative to its temperature. At 100 degrees, air can hold a lot more moisture than at 35 degrees, but relative humidity is a percentage of how much moisture the air can possibly hold at whatever temperature it is, so while 50% relative humidity at 100 degrees actually indicates the air is holding far more more moisture in it than 35 degree air at 100% relative humidity, the relative humidity at 35 degrees is 100% and higher than the 50% relative humidity at 100 degrees because the 35 degree air is holding all it can while the 100 degree air is only only half of what it can.
Imagine two glasses, a giant 64 oz Big Gulp cup you got pop in at the gas station and a tiny 8 oz juice glass from your grandma's cupboard. Now imagine that the 64 oz cup is half-full of water and the 8 oz glass is all the way full of water. The 64 oz cup actually has four times as much water in it as the 8 oz glass, but the relative humidity of the 64 oz cup is only 50% since it's only half full but the relative humidity of the 8 oz glass is 100% since it's all the way full, meaning the thing that can hold less water can have a higher relative humidity because relative humidity is a percentage of how much of its capacity to hold water is being used.
- Anonymous3 months ago
The key word is 'relative'.