Question

On a particular day, the relative humidity is 100% and room temperature is ${30^ \circ }C$ , then the dew point is
A. ${70^ \circ }C$
B. ${30^ \circ }C$
C. ${100^ \circ }C$
D. ${0^ \circ }C$

Answer 1

Hint-The dew Point temperature is the temperature at which the air gets saturated with maximum water vapor. At the dew Point the relative density is the ratio of saturated vapor pressure of water at the dew Point to the saturated vapor pressure of water at atmospheric temperature.
Using this we can find the temperature of dew point

Complete step by step solution:
The dew point is the temperature at which the air will cool down to become saturated with water vapor. If we cool the air below this temperature then the water vapor which is present in the air will condense to form liquid water which is termed as dew. If the air gets cooled to its dew point on a surface that is colder than air then the water will condense on the surface.

The measurement of dew point is directly related to the humidity of the place. If the dew point is higher it means that the moisture in the air is more there.

There are many factors that affect humidity. If all these factors remain constant then the relative humidity will rise when the temperature falls. This happens because only less vapor is needed in that case to saturate the air.

The relative humidity of the atmosphere is defined as the ratio of actual pressure of water vapor in the air to the saturated vapor pressure (S.V.P) of water at the temperature of the atmosphere.
Since the vapor pressure is saturated at the time of dew point, we can write the relative humidity as the ratio of saturated vapor pressure of water at dew point to the saturated vapor pressure of water at the atmospheric temperature.
$ \Rightarrow {\text{Relative}}\,{\text{humidity = }}\dfrac{{{\text{S}}{\text{.V}}{\text{.P}}\,{\text{of}}\,{\text{water}}\,{\text{at}}\,{\text{dew}}\,{\text{point}}}}{{{\text{S}}{\text{.V}}{\text{.P}}\,{\text{of}}\,{\text{water}}\,{\text{at}}\,{\text{air}}\,{\text{temperature}}}}$
It is given that the relative humidity is 100%. The relative density is maximum when dew point temperature is equal to the atmospheric temperature.
Room temperature is given as ${30^ \circ }C$.
$\therefore {\text{dew}}\,{\text{point}} = {30^ \circ }C$

So, the correct answer is option B.

Note:Remember that dew point is the temperature at which air will cool down to become saturated with water vapor. If we cool the air below this temperature then due will start to form. But if air is cooled to a temperature below the freezing point of water then the dew point is called the Frost point because frost starts forming due to the depositions rather than condensation to form dew.