In essence, specific humidity is the ratio of the mass of water vapour to the total mass of moist air. Specific humidity is rarely affected by changes in air pressure or air temperature because it is measured in terms of mass (grams). It is directly proportional to vapour pressure, which refers to the partial pressure exerted by water vapour in the air, and is independent of the presence of other gases. Conversely, it is inversely proportional to total air pressure. Specific humidity decreases from the equator towards the poles. For instance, extremely cold and dry air over the Arctic region during winter generally has a specific humidity of 0.2 grams per kilogram of air, whereas it can be as high as 18 grams per kilogram in extremely warm and moist air over equatorial regions. In a broader sense, specific humidity serves as a geographer’s yardstick for evaluating a basic natural resource—water—across different climatic zones, from equatorial to polar regions. It is also a measure of the amount of water that can potentially be extracted from the atmosphere as precipitation. Relative Humidity is defined as the ratio of the amount of water vapour actually present in a unit volume of air at a given temperature (i.e., absolute humidity) to the maximum amount of water vapour the air can hold at that temperature (i.e., humidity capacity). In other words, it indicates how close the air is to being saturated. Relative humidity is generally expressed as a percentage. For example, if the humidity capacity of air at 20°C is 8 grains per cubic foot, and the absolute humidity is 4 grains per cubic foot, then the relative humidity is 50%. Relative humidity is also closely related to human health and comfort. Very high (above 60%) or very low humidity levels can be harmful to health. This explains why equatorial regions, which have high temperatures and high relative humidity, and tropical hot deserts, which have very low relative humidity, are considered unfavourable for human habitation.