Humidity is the condition of the atmosphere in relation to its water vapour content. It can be measured in various ways, most commonly as absolute humidity and relative humidity.
In simple terms, when a person experiences heat it is natural for their body to perspire. If the humidity level of the atmosphere is low, making it relatively dry, the perspiration will bring some relief from the heat as it evaporates from the body. However if the humidity of the air is high there will be no such evaporation, therefore no sensation of relief from the perspiration and, in fact, the moistness of the atmosphere will add to the moist affect on the body of the perspiration. Shirts will become damp and sleeves will stick to our arms - very uncomfortable.
The absolute humidity is the total mass of water within a given volume of air. In other words this is the density of water vapour in the air and it is measured in grams per cubic metre. This measurement is not often used in meteorology, and a more common term is the mixing ratio, which is the mass of water vapour in grams per kilogram of dry air. Measurements taken at Portland, Maine between 1946 and 1955 have shown that most of the atmosphere’s water content is contained below about 5,500m of altitude.
One of the best ways to illustrate an extreme humidity difference is to compare conditions in India during the summer monsoon with those of a hot desert. In India a column of air could be expected to hold 5 - 6 cm of precipitable water, whereas in a hot desert the amount is likely to be about 1 cm. Lowest values tend to occur during winter as colder air is able to hold less water before condensation than warm air.
Relative humidity describes the moisture content of a mass of air as a percentage of the moisture held in the same volume of saturated air. In other words relative humidity measures a level of saturation. The relative humidity will vary according to the ambient air temperature for the reason given above, but also according to the mixing ratio.
An example of this can be given using an outdoor line of washing. When the clothes are pegged out they are saturated. If the air is still, i.e. there is a low mixing ratio, the clothes will dry very slowly, but with a breeze blowing the drying process will be accelerated. The higher the wind speed the greater the mixing ratio.
Also, at a low atmospheric temperature, the clothes will dry very slowly as the cold air is not able to hold very much moisture, and therefore the moisture will tend to stay in the clothes. Whereas at a higher temperature, moisture will pass from the clothes to the atmosphere and the clothes will dry. As has been said, with a wind blowing, this process will be accelerated, which explains why a warm windy day is the most ideal for drying washing outdoors.
If these descriptions are related to personal experience during a heat wave, it soon becomes apparent why humidity can seem to heat up summer. A heatwave tends to occur when the air is still and air pressure is high. This high atmospheric pressure causes air to be heavy and sink, thus preventing cloud formation. So the sun rises high, the air is still, so the heat which is generated builds up quickly. If the air is humid in those conditions, with a low mixing ratio and a high temperature, the humidity level will soar, making it feel extremely uncomfortable.
Between March and May in the Indian sub-continent, the air pressure tends to be high and moist air moving up from the south charges the rapidly heating air with moisture. The effect on people not used to these conditions can be such as to drive them mad. This period became known as the monsoon madness season, and relief only arrives when the humidity level rises so high that condensation forms clouds and the monsoon rains begin.
In point of fact humidity itself can never heat up anything, it can merely make the air appear to be hotter by making it much more uncomfortable. So the wording of the title describes an impression rather than a fact. High humidity can make a heat wave seem even hotter, and therefore appear to heat up summer.