Hygrometer
Both cities said 95°F. You packed for 95°F. You got off the plane in the second one and within about four minutes understood that you had prepared for the wrong thing entirely.
The thermometer was not lying. It was just answering a different question from the one your body was asking.
What the thermometer left out
| Dry climate | Humid climate | |
|---|---|---|
| Temperature | 95°F | 95°F |
| Dew point | 40°F | 76°F |
| Relative humidity | 15% | 54% |
| Water in the air | 5.9 g/m³ | 21.5 g/m³ |
| Sweat | Evaporates as fast as you make it | Runs off without cooling you |
More than three and a half times the water in the second. And notice that the relative humidity — 54% — does not sound alarming at all. That is the percentage understating a hot humid place, because it is measured against an enormous ceiling.
This is exactly why the absolute figure is the one worth comparing between climates.
Why your body cares so much
Above about 35°C, the body's only meaningful route for shedding heat is evaporating sweat. Radiation and convection stop working once the air is near or above skin temperature.
And evaporation depends on the gradient between the water at your skin and the water already in the air. Dry air takes it greedily. Air at a 76°F dew point takes it slowly, so sweat accumulates instead of cooling — you get all the fluid loss and much less of the benefit.
Which is the whole content of "it's a dry heat", stated properly.
What else notices
Your skin. Dry air draws water out of it continuously, so the dry-climate complaint is cracking and tightness. The humid-climate complaint is the opposite — sweat sitting on the surface.
Your sleep. Falling asleep requires your core temperature to drop, largely through evaporation. A high dew point puts a floor under the overnight low, so humid nights do not cool down and sleep is measurably worse.
Everything you brought with you. Leather, wood, paper and anything organic equalises with the new air. Guitars, books and cigars all notice a move between climates, in the same direction you do.
Acclimatising
Real, and quicker than people expect: roughly one to two weeks of regular exposure, with most of the adaptation in the first week. Sweat rate rises and becomes more dilute, and plasma volume increases.
It helps more in dry heat than in humid heat, though, because sweating more only helps if the sweat can evaporate.
Comparing the two ends before you go
The trick is to stop comparing percentages. Compare dew points — under 55°F is comfortable, over 70°F is oppressive, and that scale means the same thing in every city.
Or compare grams per cubic metre directly with the converter, which is the version that makes the difference above visible rather than something you discover on the airport concourse.