The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature); at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling. The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel. A wet-bulb thermometer indicates a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only. Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of , equivalent to a heat index of . The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of – equivalent to a heat index of . Intuition If a thermometer is wrapped in a water-moistened cloth, it will behave differently. The drier and less humid the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer's temperature will be relative to air temperature. Water can evaporate only if the air around it can absorb more water. This is measured by comparing how much water is in the air to the maximum that could be in the air—the relative humidity. 0% means the air is completely dry, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source). This is part of the cause of apparent temperature in humans. The drier the air, the more moisture it can take up beyond what is already in it, and the easier it is for extra water to evaporate. The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible. When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer. General The wet-bulb temperature is the lowest temperature that may