Science Behind Foggy Glasses

If you wear glasses and walked outside today, you probably noticed a cloud of fog. This happens often when it is hot and soupy out!

Have you noticed or maybe wondered why? Let me try to help you understand the science behind what is happening.



daphne glasses

Screen grab from my Facebook Live experimentation. If you missed it, check it out here –>

A dew point is the temperature to which a parcel of air must cool in order to reach saturation. If the parcel cools further than that temperature, it will start condensing liquid water out.

As you know, water has three different forms – solid, liquid and gas. Water decides its own state, all dependent on temperature. So in this case, water vapor in the surrounding air will condense onto a surface as liquid water. If there is no surface to condense onto, a liquid drop will form and fall to the ground.


Now, let’s turn our attention back to the foggy lenses.


Let’s say, you’re inside your car on a hot and humid summer day so you crank up the air conditioning. The lenses on your glasses will match the air temperature of its surrounding air temperature in the car. Can we say 68 degrees? I like it a little cooler. Now you’ve reached your destination, turn the car off and step outside. The moment the lenses (which are now at at 68 degrees) encounter the hot and humid air, the water vapor in the air surrounding the lenses will now condense into liquid form creating the ‘fog effect.’

This literally creates a cloud on your lenses so now you’re like a mobile cloud!

Dew points in Middle Tennessee were in the upper 70s today (oppressive on the muggy meter). It is now 9:08 pm and dew points are still very sticky…



I had a first hand experience as I stepped into the station today. My lenses fogged up so bad, I had to take them off in order to see anything at all! The greater the difference between the dew point temperature from the temperature that of the lense, the foggier the turn out and the longer it will take for that fog to lift off. This is the same idea with an ice cold glass of lemonade.

The icy cold beverage on the inside of the glass is far cooler than the dew point temperature just surrounding the glass. This will lead to the liquid drops of condensation you have probably noticed multiple times. Now you can explain why. Hope this helped!

7 Day PM

I’ll be timing out how the remainder of your weekend shapes up plus when your town can expect to see more showers and storms in the week ahead, all on channel 4 at 10 pm.

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