Why is relative humidity, and not absolute humidity,
measured in weather recordings?
Answers
Answer:
Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3). ... Relative humidity is cited in weather forecasts as it affects how we “feel” temperature.
Explanation:
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Answer:
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Explanation:
The reason for this is that it is relative humidity that matters for comfort, not absolute humidity
Humidity is a measurement as a percentage of the amount of water contained in the air. A volume of air that is 50% water by weight is 50% absolute humidity - 1kg of air that contains .5kg water. The thing is, that’s not all that interesting a measurement, typically, because it doesn’t give any indication of what that means for our subjective sensation of that air; we won’t know what that means for how the air feels.
We care about humidity measurements primarily because we want to know how we have to dress or behave or whatever according to our own comfort. What matters to our sensation of comfort, when it comes to humidity, is what the relative humidity is, not the absolute humidity. We’re not usually planning to try to extract the water from the air to have the water, so knowing how much water there is in the air, that there is .5kg of water per kg of air, is no good. However if we know the relative humidity, we can be much better prepared.
Relative humidity is a measure of how much water there is suspended in the air compared to how much water there could be suspended in the air. That is, if the air at a given temperature is saturated with water, any more water added to the air will not be able to stay suspended and will precipitate out, hence precipitation - water falling out of the air.
The amount of water that the air can hold is dependent on the temperature of the air - hotter air can hold more water than colder air. As the temperature falls, the amount of water that the air can hold decreases, but the amount of water that the air has in it stays the same. This means that the relative humidity is rising as the air cools, but the absolute humidity stays the same, until the relative humidity reaches 100% and the water starts to fall out of the air as rain, fog, dew, condensation. You see this when you have a cold glass of something and it ‘sweats’. It’s not sweating, of course, it’s making the air at the surface drop below the dew point, 100% relative humidity, and the water is condensing out of the air on the cold surface. A really cold glass of drink won’t sweat if the air is sufficiently arid that the dew point is below the temperature of the glass.
How relative humidity affects our comfort is by our ability to cool ourselves, or conversely, keep warm.
If the air is hot and dry, your sweat will dry quickly, removing excess heat from your body by the endothermic phase change of water turning from liquid to gas.
If the air is hot and humid, your sweat doesn’t dry, and you just get wet and may become dangerously overheated if you can’t find other ways of cooling off.
Interestingly, if the air is warm, say 85°F, but very dry, say 10% relative humidity, it can feel chilly with just a light breeze - the water being sucked out of your body by the super low humidity is taking heat along with it which you would rather keep at that moment.
If the air is cool and humidity is high, as in the SF Bay Area a lot of the time, you can feel very cold when the air temperature isn’t otherwise really that low. Midwesterners find this out all the time; because it can be so bitterly cold there in the winter, they don’t think 55°F is cold, but since your sweat doesn’t dry off, you just feel clammy and can lose body heat surprisingly fast when you get sweaty. There’s a double-bind - if you dress warmly for the cool feeling temperature, anything you do that makes you warm up will make you sweaty, like in hot/humid climes, but as soon as you start sweating a little, you’ll feel clammy cold in your warm clothes and the breeze will suck heat out of you uncomfortably fast. If you dress for the activity you might sweat while doing, the heat you generate will keep you warm while you’re doing it, but then you’ll be cold when you stop, because you’ll be all wet. It’s the opposite of what they, and even we who live here, expect. It’s why there’s the old saw “the coldest winter I ever spent was summer in San Francisco”.
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