I see on weather apps that for a given temperature and humidity, the dew point is x. I looked into it and I learned that the the dew point is the temperature that air must be cooled to in order for dew to form. I also saw that the formula for the dew point (Td) is Td = T-((100-RH)/5) there T is the observed temperature and RH is the relative humidity.
If this formula is accurate, then in my mind, it means that Td would be recalculated whenever T drops, thus making the dew point impossible to reach.
For example: T = 26 and RH = 60% then Td = 18. If the temperature then drops to 18, we have T = 18 and RH = 60%, so Td = 10, and so on, meaning that the dew point for a given moment is never reached…
Where am I going wrong?
In: 3
Hotter air can hold more water, and colder air holds less water. If the amount of water in the air doesn’t change, then as the temperature goes down, Relative Humidity actually goes up. That’s why it’s called relative humidity. As opposed to something like “oz of water per cubic foot” or something which is absolute number.
The dew point is the point where RH would exceed 100%. That’s not possible, and the water in the air turns back into water.
This is why a cold beverage (beer, soda, etc) from a refrigerator gets wet on its own by magic. The beverage is colder than the dew point, so the humidity in the air turns into water on the can/bottle itself.
The dewpoint is a measure of absolute humidity. It is not directly dependent on the temperature. In the night, when the temperature falls, the dewpoint may stay the same, which is why when you wake up there may be dew on the grass. You have relative humidity of 100% when this happens… at least in the air right over the blades of grass.
I agree it’s a confusing scale.
Relative humidity is also temperature dependent! If you cool down the same cube of air, RH goes up! Convert it to absolute humidity (you need temperature, RH and pressure), then convert it back to RH at the lower temperature. You’ll see that the result is at/above 100% – water is gonna drop out of the air.
>For example: T = 26 and RH = 60% then Td = 18. If the temperature then drops to 18, we have T = 18 and RH = 60%, so Td = 10, and so on, meaning that the dew point for a given moment is never reached…
>
>Where am I going wrong?
The problem is when T drops to 18 RH will not be 60% that is if not water is removed.
Use a calculator like [this](https://www.omnicalculator.com/physics/absolute-humidity) and you can see that 60% humidity at 26C is an absolute humidity of 14.62g/m^3. But 60% at 18C is only 9.22g/m^3
If you calculate the relative humidity at 18C and an absolute humidity of 14.62g/m^3 you get a relative humidity of 95.15%.
If you use the Td = T-((100-RH)/5) formula you get 26-((100-60)/5)= 18C That is good approximation of the correct value. If you use [this calculater](https://www.calculator.net/dew-point-calculator.html?airtemperature=26&airtemperatureunit=celsius&humidity=60&dewpoint=&dewpointunit=celsius&x=40&y=18) you get 17.6°C.
The formula Td = T-((100-RH)/5) is an approximation that results in a dew point of +-1C if the relative humidity so start with is above 50% there are more complex and more accurate formulas
https://en.wikipedia.org/wiki/Dew_point
Try stirring some salt into a glass of water. The salt will completely dissolve, up to a point. But, if you hear the water up, you can dissolve more salt into the water. Then, when the water cools down, some of the dissolved salt will “fall out” of the mixture and fall to the bottom of the glass.
Now, the water is the air and moisture is the salt. Warmer air can hold more water. So (I’m using very round numbers here), at 80 degrees the air is holding 70% of the water it can hold, but at 60 degrees it’s holding 99% of the water it can hold. Below 60, some of the water in the air has to come out, so you get dew.
More info: this is how global warming creates more snow in the winter. Air over the ocean that’s just a few degrees hotter can hold exponentially more moisture. When that moisture travels East and hits air below freezing, all that moisture has to come out, hence, more snow even though the average climate is warmer.
Latest Answers