what is effect of temperature on dew point?

Question

I read that relative humidity changes with change in temperature though there can be constant humidity. So, if RH is changing then dew point should get increase with increase in temperature?

https://en.wikipedia.org/wiki/Dew_point

Answer 1

Specific humidity is the amount of water in air, normally g/kg (grains of moisture per pound often in the US). This doesn't change with temperature. What changes is air's ability to hold more moisture as it gets hotter (hotter air holds more moisture).

Relative humidity ("RH") is the measure of how much water vapor is in the air relative to the maximum water vapor that could be held in the air at that temperature. As such, it's a hard number to understand, since it will change all day and night as temperature goes up (lower RH) and down (lower RH). People who claim 90% RH in the summer typically don't actually know what the true RH is. A dew point of 77 F (25 C) is a typical south Louisiana (where I live) RH in the summer, and most people will recognize it as oppressively humid, though at 95 F (35 C) the corresponding RH is about 55%.

Dew point is one way to express specific humidity. It is the temperature at which the air will be fully saturated with water vapor - no more can be held (also known as the saturation temperature). For a given specific humidity, the dew point will be the temperature that corresponds to 100% relative humidity. Dew point only changes if more or less water vapor exists in the air. The meteorological phenomenon associated with being at (or below) the dew point is fog. The condensation on your iced drink is because air at the glass's surface is cooled below the dew point, resulting in condensation.

Answer 2

I read that relative humidity changes with change in temperature though there can be constant humidity.

If you are referring to that Wikipedia article you reference, what they mean is that for a given, fixed, partial pressure of water vapor in the air, and for a fixed overall air pressure, the relative humidity will change with temperature.

This is because the thing that relative humidity is relative to is vapor pressure of water at the ambient temperature. As the temperature increases the vapor pressure of water goes up -- this means that the air can hold more water. Basically, the vapor pressure of water is the divisor, and the actual partial pressure of water vapor is the numerator.

So, if RH is changing then dew point should get increase with increase in temperature?

Not if the partial pressure of water vapor isn't changing. The dew point is the temperature at which the partial pressure of water vapor in the air is equal to the actual vapor pressure of water in the air. It's a 1:1 mapping -- one vapor pressure = one dew point.