Relation b/w Relative Humidity & Humidity Ratio

I have included these definitions from WIKI. Humidity ratio is a relatively simple concept, but Relative Humidity is not as you will see. Study them. Spend some time and follow the links. Then come back with specific questions (as I am sure you will have). Oh, and if you manage to find your way back here, it would be a good idea to register. Registered members are automatically sent an email when someone answers their post.

Mixing ratio, humidity ratio, or mass-basis absolute humidity:

Mixing or humidity ratio is expressed as a ratio of water vapor mass, m_w, per kilogram of dry air, m_d, in any given parcel of air. The colloquial term moisture content is also used instead of mixing/humidity ratio. Humidity ratio is a standard axis on psychrometric charts, and is a useful parameter in psychrometrics calculations because it does not change with temperature except when the air cools below dewpoint (at which time some water vapor condenses out of the air/vapor mixture).

That ratio can be given as:

Relative humidity:

Main article: Relative humidity

Relative humidity is defined as the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner:

where

is the partial pressure of water vapor in the gas mixture; is the saturation vapor pressure of water at the temperature of the gas mixture; and is the relative humidity of the gas mixture being considered.

Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. For example, at 27°C (80°F), a relative humidity of approximately 75% would feel like about 29°C (84°F).

Hello Mr.Mikerho, Please tell me my understanding on the diff between these two is correct or not?

Relative humidity is a amount of water vapour present in the moist air at the given temperature & pressure, so generally during cooling and dehumdification process the air enters the coil should have more humidty but the percentage of water vaour present in the moist air is less and hence the rh at entry of cooling coil is less, on the other hand air leaves the coil should have maximum amount of water vapour in the mosit air, since during during this process water vapour in the air gets condensed and loose its humidity content at the same time the air could hold more amount of water vapour in the moist air at that temp & pressure so the relative humidity at that point is more....

It is actually the opposite of that. Since the coil condenses or freezes moisture out of the air, the exit RH is less than the entering RH.