Atmospheric Moisture

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

Most of the energy needed for evaporation comes from energy

from the Sun (insolation).

“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).

Over time, dynamic equilibrium is

established between the rate of

evaporation and the rate of condensation

over the pond. = Evaporation = Condensation

Factors Affecting Evaporation

Evap

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Evap

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Evap

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Evap

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Wind Speed Air Temperature

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.

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.

Abso

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Hum

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Relative Humidity

If the amount of water vapor stays the same

Air

Tem

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If the air temperature is constant

Wat

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apor

Relative and Absolute Humidity

Air Temperature

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%

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

Sling Psychrometer: An instrument used to

measure dew point and relative humidity

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.

Dry bulbWet bulb

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?1. Locate the dry-bulb

temperature on the left side of the chart.

2. Determine the difference between the wet-bulb and dry-bulb temperatures and find this value on the top of the chart.

3. Follow the dry-bulb temperature horizontally across the chart. Follow the difference between wet-bulb and dry-bulb temperatures vertically down the chart.

4. Where these two values meet would be the relative humidity.

The relative humidity is 84%

Determining Dew Point Temperature

1. Locate the dry-bulb temperature on the left side of the chart.

2. Determine the difference between the wet-bulb and dry-bulb temperatures and find this value on the top of the chart.

3. Follow the dry-bulb temperature horizontally across the chart. Follow the difference between wet-bulb and dry-bulb temperatures vertically down the chart.

4. Where these two values meet would be the 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?

The dew point temperature would be 4˚C

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

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 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

The Dew point temperature is recorded in

(˚F).

The location has a dew point temperature of 27˚F

The visibility of the location is measured in

miles (mi).

The visibility of the location is ½ of a mile.

The present weather is an indication of current conditions of

atmospheric transparency or precipitation

The location is currently receiving snow.

Cloud cover is a percentage of the current

sky covered by clouds.

The location has a 75% cloud cover.