Water in the Atmosphere Water is introduced into the atmosphere through evaporation Humidity The amount of water vapor in the air varies depending on air temperature and moisture SPECIFIC HUMIDITY: The actual amount of water vapor in the air. Depends on air temp. Warm air holds more vapor than cool air When the air has a maximum amount of water vapor it can hold, it is said to be saturated. Saturation is when actual vapor density=saturation vapor density

Water in the Atmosphere Air with higher temperatures can hold more water vapor than air at lower temperatures. Relative Humidity: How near the air is to saturation. Looks at actual vapor density in relation to saturation vapor density for air at a given temperature. @ 10 C a cubic meter of air holds at most 8 grams of water vapor If there were 8 grams in the air R.H. = 100% If there were 4 grams in the air R.H. = 50%

Water in the Atmosphere Measuring Humidity Air temperature is measured with a dry-bulb thermometer Wet bulb thermometer uses evaporation as a cooling process. Drier air has higher rates of evaporation, which causes a lower wet-bulb temperature. More moist air has lower rates of evaporation, which causes a higher wet-bulb temperature. Difference between dry-bulb temperature and wet-bulb temperature can give relative humidity.

Water in the Atmosphere Humidity and Condensation Condensation occurs when air becomes cooler. Temperature at which condensation occurs for a given humidity is known as dew point. Ex.: Wet grass in the morning

Water in the Atmosphere Measuring Relative Humidity Psychrometer Has a wet-bulb and dry-bulb thermometer The bulb of the wet-bulb thermometer has a cloth covering moistened in water When slung air blows over both thermometers The wet-bulb thermometer is cooled by evaporation and the reading drops below the dry-bulb thermometer reading The relative humidity can be found by comparing the temperatures of the wet-bulb and dry-bulb thermometers

Water in the Atmosphere