The most important property for us to grasp it really means when we use the term ‘Relative Humidity’. The best way to describe both terms starting with ‘Humidity’.
Our common experience of humidity is based on climate. Depending on where we live and local climate, or you have been lucky enough to have gone somewhere warm in holiday, you have most probably at some point said or heard somebody say “it’s humid”. If it gets more humid than is comfortable, we hear this described as “muggy”. These terms describing the climate are our perception of a key property of water. Whether we can see it or not, air contain water.
How much water there in the air depends on very important factor that is temperature?  However, our common experience of warm or hot days that are not humid, tells us it is not simply an increase in temperature or amount of water that is the answer. What we need to consider is how much water the air can hold and that is totally depends on temperature.
Now we understand from that, the “Humidity” part of “Relative Humidity” has to do with the amount of water in the air. This can be referred to as moisture or to be more precise “water vapour”. We are all familiar with water vapour in many ways, tow common experience are steam from boiling kettle and clouds, either up they usually are or dawn here to ground level as mist.
The term “Humidity” is used loosely on weather forecasts were often the “Relative” part is dropped. If humidity describesthe amount of moisture in the air then what has the “Reative” bit got to with it and where dose temperature fit in? this is where it gets more tricky but it is absolutely essential to understand why “Relative”.
You will hear and see on weather reports that humidity is given as a percentage (%). This is where the “Relative” part comes from. One scientific part that you must know first, air can only hold a certain amount of water.
Common experience tells us that when clouds have built up, and keep building up, at some point it rains. Because the air cannot hold any more water vapour and droplets are formed are heavier than air and fall to earth.
The maximum amount of water vapour that can be held by air called “Saturation” . to understand how saturation influences humidity, imagine you are in a room 5× 5× 5m in size and the temperature is 21°C. the air in the room can hold about 9kg of water vapour and would be saturated. If that amount of water vapour was percent, you will soon will feel uncomfortable. Let us now half the amount of water vapour 4.5kg and this would give 4.9/9.0 resulting in a relative amount of water equivalent to half, or 50%as a percentage, to what it would be at saturation level. Now you are sitting in this room in 21°C and 50% relative humidity (RH) a much more comfortable environment.
Relative humidity in definition: is a measure of the actual amount of water compared to the amount of water there would be at saturation and expressed as percentage:
A scientific fact is that the amount of water vapour that air can hold at saturation is strictly dependent on the temperature. As temperature increases the amount of water vapour air can hold also increases. This ties nicely with our experience of being in humid climate. It has be warm and there has to be a source of water such as sea, a lake, rain or any combination of this.
The relation between air and humidity
Now there is a further rule of nature that has to be obeyed, warm air holds more water than cold air. Quite a simple rule but it’s explain so much. For example, to get starting on exploring the world from the perspective of humidity, is where you get rain shadow on one side of mountain.
Using your knowledge of water vapour to describe that the air cools cannot hold water because of condensation and rain. The point where no more water can be held by the air called “saturation point”. For example, the Himalayas are so high, the drop in temperature lowers the saturation point so much that little water is retained and dry air of low relative humidity fows over the top of the mountain depriving the leeward side of the rain.
In easy experience laboratory, use closed container with a built-in temperature/humidity prop that displays via laptop the actual temperature and relative humidity inside of the container. By applying a small amount of heat to rise the internal temperature by a few degrees the response is an immediate change in the relative humidity.
Look at this graph:
At the start the temperature is about 20°C and the relative humidity is around 60%. By external heat, the top line in the graph can be seen to rise. The %RH immediately begins to fall. The graph shows what happen when you let the test run until temperature and humidity return to about where they started.
To explain what happened
Warm air holds more water than cold air, as temperature increase water vapour air can hold increases and relative humidity is a measure of the actual amount of water compared the amount of water there would be at the saturation point.
The key to understanding the symmetry of the graph is remembering that the amount of water does not change inside the container. So, as the temperature changes and consequently changes the saturation point, the amount of water relative (percentage or ratio) to what could be held at saturation must also change up or down depending on whether the temperature goes up or down.
As we heat the container the internal air warms up and because warm air can hold more water vapour it has a higher saturation point. Since the amount of water in closed container doesn’t change, then the amount of water vapour relative to the saturation point must decrease as the saturation point increase. Just as with clouds rising up the mountains, as the temperature drops the saturation level decrease and the relative humidity must increase. At the point the temperature start to decrease the opposite happens and the relative humidity increase.
In fact, if we continued to cool the container, the relative humidity can be increased to the saturation point and we would get condensation inside the container.
Now you thing, condensation on windows and walls on a cold night, condensation on kitchen where steams comes in direct contact with a surface, kitchen and bathrooms windows getting steam up. These observation can all be explained using the above properties of water.


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