Relative humidity is amount of moisture in the air expressed in percent of how much moisture the air can hold.
Relative humidity, expressed in a percentage value, is just what its name implies - relative. It is relative to the air temperature.
By knowing the relative humidity you know how close the air is to complete saturation
The lower the humidity value the drier the air, and the more potential the air has to hold moisture. Relative humidity shows the degree of saturation, but it gives no clue to the actual amount of water vapor in the air.
Such as if you had a 50% relative humidity, you would know that at that exact temperature at that exact time the air is halfway to being saturated. However that gives no indication whatsoever of how much moisture is present in the air, it just simply tells you that the air is 50% from being completely saturated.
If the air temperature goes up or down at any time (which on most days temperature is always changing)... the relative humidity will also change. Yet, the moisture in the atmosphere has not changed at all. Saturation occurs when the air temperature and dew point are exactly equal, thus making the relative humidity 100%. When this is accomplished, you know at that time the air cannot hold any more moisture. BUT, you still have no idea how much moisture is in the atmosphere!
Relative humidity is amount of moisture in the air expressed in percent of how much moisture the air can hold. If the relative humidity is 90% then the air is 90% saturated. By knowing the relative humidity you know how close the air is to complete saturation. In meteorology relative humidity is the ratio times 100 (for %) of the actual vapor pressure to the saturation vapor pressure. When the air can hold no more gaseous water it has reached the saturation vapor pressure. How much gaseous water is in the air is the actual vapor pressure. Given the flux of temperatures on earth water can exist in all three states: a liquid, a solid, and a gas. Water evaporates from our oceans, rivers, and lakes continuously and is drawn up into our atmosphere in the form of water vapor. In turn water vapor is continuously released back to the surface of the earth through the process of condensation largely in the form of precipitation. This transfer of water a liquid to water a gas back to water a liquid i.e. rain or solid i.e. hail, sleet or snow is possible because warm air rises as it is heated by the sun. The Troposphere, where all our weather occurs, is a vertically mixed layer because the temperature decreases with height. (There are exceptions of course, but generally as you ascend the temperature drops rather dramatically.) As warm air rises it cools and condenses the moisture back to earth in the form of rain if enough warm, moist air was cooled, or condenses the moisture into a cumulus cloud.
The lower the humidity value the drier the air, and the more potential the air has to hold moisture. A common misconception is the belief that if the humidity is 100% it must be precipitating. (i.e. raining or snowing). Actual there are quite a few instances in the morning when it is clear and calm when the humidity is 100%. The moisture had to be produced, but where is it? A dense fog or more likely a heavy dew is the answer. Along that line of reasoning the humidity does not have to be 100% before it precipitates. If there is a big rain or snowstorm usually the humidity is between 90 % and 100 %, but not always 100 %. Many times in the winter when there is a cold, dry airmass already in place over our area the humidity during the day may be only 35 %. As clouds move in and moisture increases the humidity will slowly rise. As precipitation gets closer and closer and then is finally overhead it may evaporate before reaches the ground. Precipitation aloft is known as ' virga ', and may actually show up on radar but not on our lawns. When it does finally begin to snow the relative humidity may only be 75 %. If the storm is intense, and lasts 6 - 12 hours the air will become more and more saturated until the humidity is between 90 % and 100 %.
The word humidity is from the Latin'humiditas' which means dampness and from the Latin 'humidus' which means moisture. Add the word 'relative' which means something considered in its relation to something else.