What is the relative humidity if the dry-bulb temperature is 18°C and the wet-bulb temperature is 11°C?

The amount of moisture air can hold is dependent on its temperature and pressure. The warmer the air the greater the quantity of water vapour it can contain. The air temperature is measured with a normal thermometer this is the Dry-Bulb reading. The actual amount of moisture known as the mixing ratio is measured in grams of water per kilogram of dry air. When air at a certain temperature is saturated it cannot hold any more moisture. The relative humidity of the air is the ratio of the actual amount of moisture in the air to the fully saturated amount.

We can use evaporation to measure the amount of moisture in the air. A wet cloth is placed over the bulb of a thermometer and then air is blown over the cloth causing the water to evaporate. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. The depression in Wet-Bulb temperature allows the humidity to be calculated. If the air is fully saturated (100% relative humidity) the water cannot evaporate, so both the wet and dry bulb temperatures are the same. If the dry and wet bulb temperatures are set to the same value the calculator will show the saturation mixing ratio the amount of water in saturated air at that temperature.

If partly saturated air is cooled without changing its pressure or amount of water vapour, a point is reached when it becomes saturated. The moisture will be given up as dew or ice crystals. This temperature is the Dew Point. This is why condensation will form on a bottle of cold beer, as the air in the vicinity of the bottle is cooled below its dew point. Meteorological reports usually quote the temperature and dew point as well as the station pressure. Moisture content and relative humidity can be calculated from these figures.

The Psychrometer is the name of a device containing both a wet and dry bulb thermometer. This can be a fixed device for meteorology or a handheld sling psychrometer that is often used in air conditioning applications.

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