Calculating the change in relative humidity

[pondwire]

Homework Statement

the air temperature is -15 degrees Celsius and the relative humidity is 70 per cent. If the air is brought indoors and heated to 20 degrees Celsius with no added moisture, what is the new relative humidity?

Homework Equations

RH=e/es x 100% or ws=epsilon*es/p

The Attempt at a Solution

could not do it mathematically -- looked up on a chart and found that sat. vp is about 1.9 mb at 70 per cent RH, so multiplied 1.9 x .70 and got 13.3 mb. Then, divided that by 23.4 mb, from the 20 degree value, and got 57 per cent RH. Book says it's 6 per cent. Any ideas?

 

[rock.freak667]

Which chart did you use? If you used the psychometric chart, then you can easily find the new relative humidity. No moisture added means that the humidity ratio is the same.

 

[kerimek]

As a scientist, it is important to use accurate and reliable methods to calculate changes in relative humidity. While using a chart may provide a rough estimate, it is not the most precise method. In this case, it would be more accurate to use the formula RH=e/es x 100%, where e is the vapor pressure at the new temperature (20 degrees Celsius) and es is the saturation vapor pressure at that temperature. This would give a result of about 6%, which aligns with the book's answer. Additionally, using the equation ws=epsilon*es/p, where ws is the mixing ratio and p is the pressure, may also provide a more accurate result. It is important to use the appropriate equations and values when calculating changes in relative humidity to ensure accurate and reliable results.