Evaporation, dew point, and why it feels so much hotter on humid days.
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00:00 It's not the heat that will get you, it's the humidity.
00:04 It's not the heat that gets you, it's the humidity!
00:10 I think relax, it's boiling, it's humid.
00:13 Man, it's hot. But it's a dry heat.
00:20 You've probably heard people saying that a lot lately.
00:22 But what does that actually mean?
00:24 Well, at its most basic, what it means is that the temperature forecast is not always the best measure of how hot it will feel outside.
00:33 Your body likes to keep itself at around 37 degrees Celsius.
00:37 That's when it's working at its best.
00:39 So when your internal temperature starts to rise above that, it gives your brain the signal to start cooling off by opening your blood vessels and extending blood flow to your arms and legs.
00:51 That's why you might notice your veins are a little more pronounced on hot days.
00:55 You'll also start to produce more sweat, which is your body's way of cooling through evaporation.
01:02 Water needs heat energy to evaporate, and this is called latent heat.
01:08 As sweat evaporates from your skin, it takes latent heat away from your body.
01:12 When there's already a lot of moisture in the air, your sweat won't evaporate as quickly and you'll end up holding on to that extra heat.
01:22 It's why you feel particularly gross on hot and humid days.
01:26 I want to cry so bad. I don't think I can spare the moisture.
01:32 The relative humidity forecast will indicate how much water is likely to be floating around on any given day.
01:39 When the relative humidity is at 100%, it means the air is at saturation.
01:46 There is as much moisture in the air as is possible right now.
01:52 Humidity is measured using a wet bulb thermometer, which is basically just a thermometer swaddled in a wet cloth.
01:59 As the cloth dries through evaporation, the thermometer measures how dry the air around it is staying.
02:05 Let me explain using this half full bottle of water.
02:09 The liquid inside this bottle is slowly evaporating, leaking tiny water molecules around into the air around it.
02:17 At the same time, there are water vapor molecules returning back into the liquid from the air that surrounds it.
02:24 If I heat the water bottle so that its temperature is higher than the air around it, the molecules will start to move faster.
02:32 Evaporation will increase while condensation remains the same.
02:37 That's why on a humid day, there tends to be more cloud cover or fog or dew.
02:42 The water droplets are condensing onto any surface, including specks of dust in the air.
02:48 The amount of humidity you feel is somewhat governed by the temperature of the day, too.
02:53 That's because the saturation point increases with the temperature.
02:57 The hotter the temperature, the more energy water molecules have to move around with.
03:02 So say it's 30 degrees today with a 50% relative humidity reading.
03:08 Meanwhile, tomorrow it will be 35 degrees, but still with a 50% relative humidity reading.
03:15 There will be way more moisture in the air tomorrow than there is today because there's a much higher saturation point to contend with.
03:23 This is what the forecast refers to when it talks about the dew point.
03:27 That tells you how much moisture is in the air in absolute terms.
03:33 The dew point measures the temperature at which the amount of moisture in the air would then condense into dew or fog.
03:41 So if today is 30 degrees with a 50% relative humidity, then the dew point is 18 degrees.
03:50 And tomorrow, when it's 35 degrees with a 50% relative humidity, the dew point will be 23 degrees.
03:59 And as a general rule, if the dew point is around 18 degrees or higher, that's when it's going to start to feel pretty gross.
04:07 That's when you're going to want to be inside a movie theater or a library or really anywhere else but in the great outdoors.
04:16 [MUSIC]