Objectives

1. Describe the movement of water through the hydrologic cycle.
2. Discuss the role of condensation nuclei in the formation of fog and clouds.
3. Define saturation.
4. State the relationship between temperature and the amount of water vapor required for saturation.
5. Define relative humidity and describe how it might change.
6. Define dew point temperature.
7. Describe how the relative humidity typically varies on a daily basis.
8. Discuss the formation of dew and frost.
9. List several types of fogs and discuss the details of their formation.
10. Identify major cloud types based on their appearance and height.
11. Explain the terms used to describe sky conditions from cloud observations.
12. Describe the two types of weather satellites used to observe clouds.

The Hydrologic Cycle

• circulation of earth’s water supply
• evaporation = precipitation
  

Humidity describes the amount of water vapor in the air

• absolute humidity - mass of water vapor / volume of air
• specific humidity - mass of water vapor / total mass of air
• mixing ratio - mass of water vapor / mass of dry air
• relative humidity
• dew point temperature

Saturation - Vapor Pressure

• vapor pressure: part of total atmospheric pressure due to water vapor content
• saturation: vapor molecules leaving = returning

Relative Humidity

• ratio of air’s water vapor content to amount of water vapor required for saturation at that temperature
• actual water vapor / water vapor at saturation (x 100%)
• relative humidity can change in either of two ways:

1. add or remove moisture (ex. evaporation from ocean)
2. increase or decrease temp - if T Ý , % relative humidity ß

Dew Point

• Temperature to which air must be cooled in order to reach saturation.
• Temperature-dew point spread (difference between the air temperature and the dew point temperature is a useful measure of the amount of moisture in the air.)
  The smaller the temp-dew point spread, the higher the humidity.
• Humidity usually reaches its peak in the early morning hours
  

Dew and Frost

• air cools to dew point, condensation on surfaces
• dew, frozen dew, frost

Condensation Nuclei

Airborne particles on which water vapor can condense to produce cloud droplets

Fog

• cloud with base at or near ground - formed by cooling, or evaporation and mixing
• radiation fog or ground fog - radiation cooling of ground and adjacent air
• advection fog - warm moist air blown horizontally over cool surface
• evaporation (mixing) fogs cool air moving over warm water (steam fog over lake)
• upslope fog - moist air flows up along an elevated plain, hill, or mountain

Cloud Formation

• For condensation to occur:
  • air must be saturated (cooled to dew point)
  • surface to condense on - condensation nuclei (dust, smoke etc.)
• rising air cools adiabatically due to expansion
• adiabatic process - temperature change without heat transfer
• once the condensation level is reached, clouds form

Naming Clouds

Clouds are classified by appearance and height, using Latin words. Below is a table of how basic cloud names are formed. For example, if you were looking at an altostratus cloud, it would be a layered cloud located at middle altitudes.

Cloud Photos

Cloud Photo Page

Distribution of Ice and Water in a Cumulonimbus Cloud

Sky Observations