How can one determine the relative humidity on a particular day at a given place?

The correct answer is C, if both temperature and absolute humidity are given. Relative humidity compares the water vapour actually present in the air with the most the air could hold at that temperature, so both figures are needed. Absolute humidity is the actual mass of water vapour in a given volume of air, usually stated in grams per cubic metre. The capacity of air to hold vapour depends on temperature: warm air can hold much more than cold air. Relative humidity is therefore absolute humidity divided by the saturation capacity at that temperature, multiplied by 100. When it reaches 100 percent the air is saturated, and further cooling leads to condensation as dew, fog or cloud. In practice it is measured with a hygrometer, such as a wet and dry bulb thermometer. Option A is wrong because temperature alone gives only the air's capacity, not the vapour actually present. Option B is wrong because absolute humidity alone gives no idea of how close the air is to saturation. Option D is wrong because a percentage of absolute humidity is not a measure that fixes the saturation point. Exam tip: relative humidity = actual vapour divided by capacity at that temperature, times 100.