🧪 Weather Science Wednesday: Dew Point vs. Relative Humidity
We often talk about heat in summer — but it’s the humidity that really makes things feel heavy and uncomfortable. This week’s demo helps explain why — using just a sponge and a couple of cups of water.
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🧷 What You’ll Need:
• 1 clean sponge
• 2 clear cups of different sizes
• Water
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🧠 The Demo:
1. Fill the two cups with water:
One small, one large — both filled to the brim. These represent cold air and warm air at 100% relative humidity.
Cold air is like the small cup: it’s full, but it simply can’t hold very much water.
Warm air is like the big cup: also full — but with way more water inside.
2. Now bring in the sponge:
When the sponge is dry, it easily soaks up moisture — just like dry air.
Once it’s saturated, it drips — that’s how humid air feels. Your sweat can’t evaporate, so your body can’t cool down. That’s why sticky days feel so much worse.
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📚 The Science:
💧Relative humidity tells us how full the air is compared to what it can hold.
💧Dew point tells us how much moisture is actually there.
So yes — both cups might be “100% full”… but the big cup holds way more water.
That’s why dew point is a better measure of how humid it actually feels — especially in summer.
☀️ As the day heats up, warmer air can hold more water — so relative humidity often drops, even if the moisture hasn’t changed. So when you check the RH, look at it in the afternoon, not the morning.
Or nerd out like me and just go straight to the dew point. 😉
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💨 A Note on Sweat (and Why Humidity Feels So Gross):
When you sweat, your body isn’t just being dramatic — it’s trying to cool you down! As your sweat evaporates, it takes heat with it. This is called evaporative cooling: the water on your skin absorbs energy (heat) from your body to change from liquid to gas. That energy transfer helps lower your body temperature.
But on humid days? The air is already packed with moisture — like a saturated sponge — so your sweat just sits there, not evaporating, and your body can’t cool itself efficiently. That’s why you feel hotter and more drained, even if the air temperature isn’t sky-high.
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🧑🏫 Try This at Home or in the Classroom:
This is a fantastic visual for curious kids, parents, or teachers looking for a simple, hands-on weather demo.
And, watch a video form of this explainer here on my social media!
📩 Want the free printable version of this activity?
Kelsey McEwen 