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
>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
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