Atmospheric Moisture

Phase change from liquid to gas Atmospheric Moisture The majority of the water present in the atmosphere is as water vapor. Water vapor enters the atmosphere through three processes: evaporation, transpiration, sublimation. Evaporation Phase change from liquid to gas Sublimation Phase change from solid to gas. Transpiration The release of water vapor from plants. Evapotranspiration: The collective processes of evaporation and transpiration.

Energy of Evaporation “Energetic” water molecules (high kinetic energy) leave the pond through evaporation. The average kinetic energy of the pond is low due to this (temperature of the pond decreases). Most of the energy needed for evaporation comes from energy from the Sun (insolation). Over time, dynamic equilibrium is established between the rate of evaporation and the rate of condensation over the pond. = Evaporation = Condensation

Factors Affecting Evaporation Evaporation Rate Evaporation Rate Wind Speed Air Temperature Evaporation Rate Evaporation Rate Surface Area of the Water Water Vapor in the Air

Humidity Humidity is the water vapor content of the atmosphere Absolute Humidity: The amount of water vapor present in a specific volume of air. Absolute Humidity Relative Humidity: The ratio of the amount of water vapor present in a specific volume of air to the maximum amount of water vapor the volume of air can hold. Relative Humidity is represented as a percentage. Air Temperature If the amount of water vapor stays the same If the air temperature is constant Air Temperature Water vapor Relative Humidity Relative and Absolute Humidity

Dew point Dew point is the temperature at which air is filled with water vapor. It is the temperature at which relative humidity is 100% Sling Psychrometer: An instrument used to measure dew point and relative humidity The dew point is dependent upon the absolute humidity of a parcel of air. If there is an increase in the amount of water vapor in the air, the dew point also rises Wet bulb Dry bulb Dry Bulb: Thermometer designed to measure air temperature. Wet Bulb: Thermometer with a damp cloth attached to the end of it. This thermometer is designed to measure the air temperature as water is evaporating from the psychrometer as it is being slung. The slinging psychrometer increases the evaporation rate (cooling process) of the wet bulb – indicating a drop in temperature.

Determining Dew Point Temperature A student using a sling psychrometer determined the dry-bulb temperature to be 16˚C and the wet-bulb temperature to be 10˚C. What is the dew point temperature? Locate the dry-bulb temperature on the left side of the chart. Determine the difference between the wet-bulb and dry-bulb temperatures and find this value on the top of the chart. Follow the dry-bulb temperature horizontally across the chart. Follow the difference between wet-bulb and dry-bulb temperatures vertically down the chart. Where these two values meet would be the dew point temperature. The dew point temperature would be 4˚C

Measuring Relative Humidity A student using a sling psychrometer determined the dry-bulb temperature to be 24˚C and the wet-bulb temperature to be 26˚C. What is the relative humidity? Locate the dry-bulb temperature on the left side of the chart. Determine the difference between the wet-bulb and dry-bulb temperatures and find this value on the top of the chart. Follow the dry-bulb temperature horizontally across the chart. Follow the difference between wet-bulb and dry-bulb temperatures vertically down the chart. Where these two values meet would be the relative humidity. The relative humidity is 84%

Cloud Formation A cloud is a collection of liquid and/or solid water suspended in the atmosphere. A cloud forms when the air temperature cools below the dew point temperature. At this point, the water vapor will begin to condense into water droplets or sublimate into ice crystals. Water vapor will condense/sublimate onto a surface. Aerosols (such as volcanic ash, bacteria, pollen, dust) serve as the surface for condensation/sublimation to occur. This condensation/sublimation event in cloud formation releases a tremendous amount of latent heat (potential energy) into the atmosphere. The bottom of this cloud indicates the place in the troposphere where the air temperature falls below the dew point temperature and the condensation and/or sublimation of water vapor takes place.

Precipitation Precipitation can take the form of rain, drizzle, snow, sleet, freezing rain or hail. The falling of liquid or solid water from clouds due to Earth’s gravity is precipitation. Precipitation can take the form of rain, drizzle, snow, sleet, freezing rain or hail. Precipitation can occur in a number of ways. One way is convectional precipitation. As Earth’s surface is heated, air will rise (convergence) and a low pressure system will develop. As the air rises in the troposphere, it expands and cools until the air temperature is below the dew point. This will cause condensation of water vapor and clouds will form. The collision of small water droplets within the cloud create larger droplets. The larger droplets are more difficult to keep suspended in the troposphere and will fall to Earth as precipitation.

Visibility and Atmospheric Transparency Atmospheric transparency is a measure of the transparency of the atmosphere to insolation. Increased pollutants in the atmosphere reduce the transparency of the atmosphere and allow for more insolation to be reflected or absorbed in the atmosphere. Haze is a condition where the atmosphere has a larger than normal aersol amount reducing atmospheric transparency. Visibility is a measure of the distance of the Earth’s surface (in miles) that can be seen clearly. A reduction in atmospheric transparency is also a reduction in visibility.

Weather Station Model Cloud cover is a percentage of the current sky covered by clouds. The location has a 75% cloud cover. The visibility of the location is measured in miles (mi). The visibility of the location is ½ of a mile. The Dew point temperature is recorded in (˚F). The location has a dew point temperature of 27˚F The present weather is an indication of current conditions of atmospheric transparency or precipitation The location is currently receiving snow.