A: Relative
humidity gives the ratio of how much moisture the air is holding to how
much moisture it could hold at a given temperature.
This can be expressed in terms of vapor pressure and
saturation vapor pressure:
RH = 100% x (E/Es)
where, according to an approximation of the Clausius-Clapeyron equation:
E = E0 x exp[(L/Rv) x {(1/T0) - (1/Td)}] and
Es = E0 x exp[(L/Rv) x {(1/T0) - (1/T)}]
where E0 = 0.611 kPa, (L/Rv) = 5423 K (in Kelvin, over a flat surface of water), T0 = 273 K (Kelvin)
and T is temperature (in Kelvin), and Td is dew point temperature (also in Kelvin)
So, if you know the temperature, you can solve for Es, and substitute the equation for E into the expression for relative humidity and solve for Td (dewpoint).
--Michael Bell