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What is the difference between surface specific humidity and surface relative humidity (it's wetter and dryer respectively)? The trends, according to the latest IPCC report, are opposite for the two, and I'd like to know what it entails.

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    $\begingroup$ Can you link the specific reference in the IPCC? $\endgroup$
    – LShaver
    Oct 12 at 22:34
  • $\begingroup$ @LShaver Physical Science Basis, p. 2-185 $\endgroup$ Oct 12 at 22:43
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Let's put it a different way:

Specific humidity is how much actual water (vapor) is in the air.

  • More warmth tends to mean more evaporation.
  • Warming air also raises the limit of how much water it can hold in total. So a second reason specific humidity can go up is that it raises the cap level.

On the other hand, relative humidity is just a measure of how full the air is. It's really not that representative of how much water is truly in the air.

  • If the cap level goes way up, but the amount of actual water only goes up a little... it's actually less near full than before. Think a mostly full bowl compared to a quarter-full bathtub... there's more water in the bathtub, but it's less relatively full.

Hopefully this next graph, showing the weather from Duluth, MN the past couple weeks, can help show the point:

enter image description here

The green and pink lines fluctuate in an almost perfectly opposite pattern, almost like heartbeat consistency, just about touching most days, then separating far apart. That's the daily cycle, and similar over most of the world much of the year... and it's mainly just all caused by temperature change; in the daytime it gets hot, so the air can hold more water, but the amount of actual water (in blue) really doesn't change much each day usually... meaning it's less full. At night the air cools down, "shrinking the container", until it's just about saturated again around dawn, despite not losing much of any water.

RH is honestly a lame moisture measure, meteorologists really don't use it too much. Because it doesn't mean much, it really just reflects that temperature fluctuates each day. And that's not ground breaking information! When forecasting severe weather or precipitation or snow or tropical cyclones... it tells us almost nothing of use.

Dew point, which is probably the most used absolute measure of water... is much more informative to us. It really focuses in on how much water is in the air (which is important for forecasting almost everything weather-wise)

And notice if anything, that dewpoint is going the same way as temperature instead of the opposite.

So this graph shows relative and absolute humidity typically fluctuate in two opposite ways to the temperature.

And so hopefully it makes sense that if it's getting warmer, those opposing trends would be a reasonable result. The overall water goes up... the "fullness percentage" may well still go down.

If total moisture goes up a bit, but the atmosphere's moisture cap goes up much more, it would mean RH and AH change in opposite directions. Since evaporation requires more energy, meaning for weather the dew point is much slower to change than temperature... it lines up that this would be the trend.

Absolute and relative humidity do both "measure" water vapor in the air. But because one, RH, really is more a (inverted) temperature measurement masquerading as a moisture measurement, rather than truly tracking water changes... they still can and very often do move in opposite ways.


(Just a side note: absolute humidity can often be a bit more stable and independent from temperature, more so than this graph suggests. Wind changes like fronts and sea breezes can mean more connected movement, because different airmasses are coming in. It's fall in MN, so there's some of that going on.
But hopefully graphs for other areas can help you explore the trend more and be convinced of the main ideas: (Orlando, San Antonio, Seattle, Phoenix, Nairobi Kenya, Tehran Iran, Singapore, Sydney Australia, Guam)

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Specific humidity is a simple percentage measurement of the total water in the atmosphere, the other term for this is the absolute humidity. Relative humidity on the other hand is the amount of water in the atmosphere as a percentage of the maximum carrying capacity of the air. The carrying capacity of air is temperature dependent, warmer air can hold more water vapour than cooler air so the absolute humidity can rise while the relative humidity actually drops as a body of air warms. The IPCC are probably looking at the world average and the warming trend should be increasing evaporation but carrying capacity is going up faster than atmospheric moisture levels.

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