agbagb":7qma5dcb said:
I think my head has been clouded by relative and absolute as well.
Atmospheric humidity refers in a non-specific way to water in the form of water vapour in air. Absolute humidity (AH) describes the actual amount of moisture vapour in the air-- another name for absolute humidity is vapour pressure. When air is fully saturated with the maximum possible water vapour it can hold at any given temperature, it is at saturation vapour pressure. Air that has a vapour pressure below the saturation vapour pressure is able to carry more moisture.
Relative humidity therefore is the ratio of the amount of moisture in the air to the maximum amount of moisture that the air can carry at a given temperature X 100, which describes RH as a percentage of AH.
Warm air can carry more water vapour than cool air. For example, 1 cubic metre of air at 35ºC can carry a maximum of 39.54 grammes of water vapour-- the air is fully saturated, ie, it is at saturation vapour pressure: the relative humidity (RH) is 100% and the vapour pressure or absolute humidity (AH) is 39.54 grammes of water vapour. For comparison one cubic metre of air at 15°C can only hold 12.82 grammes of water vapour.
Dew point is the temperature at which water vapour condenses out of air. For example, if fully saturated air cools by even 0.1ºC the gaseous water in it starts to condense into liquid water. This is essentially what happens when the bathroom mirror ‘steams up’ as you take a shower. In this case warm, saturated air hits colder glass, it cools, and this causes the water vapour in it to condense on to the glass.
In the UK house interiors experience lower RH values in the winter than they do in the summer. This is because we heat our houses in the winter. At this time of year fresh air that enters the building through vents, and natural cracks and crevices, is comparatively cold.
Cold air cannot carry as much water vapour (WV) as warm air. Air at 5ºC can carry a maximum of 6.79 grammes of WV per m³. Outside air at 5ºC fully saturated with 6.79 grammes of WV/m³ brought into a house and then heated to 20ºC without adding any more water vapour to that package of air results in an RH of 39.29%. At 20ºC air can hold a maximum of 17.28 grammes/m³ WV. To work out the RH value of the heated air calculate, Absolute Humidity of the air / Fully Saturated level of the heated air X 100. For the parcel of air just described therefore we ascertain the RH percentage as follows: 6.79/17.28 X 100 = 39.29% RH.
In British summers internal RH values are higher than winter RH values. Temperatures are warmer and the outside air is able to carry quite large amounts of water vapour (WV) leading to raised AH values. Open windows and doors let a breeze through the house which brings in water vapour and causes relative humidity within buildings to more closely match external relative humidity.
The above isn't a full explanation, but hopefully you can get the general gist of the subject. Slainte.