The temperature rises gradually with high humidity and overcast skies. This most often applies to measurements of temperature, dewpoint temperature, humidity and air pressure. Grey skies, breezy conditions, higher humidity and cooler temperatures prevail. Powdery mildew grows well in environments with high humidity and moderate temperatures. Evaporative cooling performance is variable due to changes in external temperature and humidity level. The relative permittivity of air changes with temperature, humidity, and barometric pressure. Summers are wet with high temperatures, high humidity, and a high heat index. The threshold temperature for snowmaking increases as humidity decreases. Summers can bring periods of high temperatures accompanied with high humidity. Cotton is sensitive to temperature and humidity. Summers are marked by plentiful sunshine and moderate humidity and temperature. This retains a temperature of and high humidity. Chocolate is very sensitive to temperature and humidity. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Moisture absorption occurs when the material is exposed to high humidity or water.