When air near the ground becomes saturated the water vapor condenses into a liquid and forms

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Humidity

Humidity is the amount of water vapor in the air in a particular spot. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, it means that the air contains 80% of the total amount of water it can hold at that temperature. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation.

Since warm air can hold more water vapor than cool air, raising or lowering temperature can change air's relative humidity (Figure below). The temperature at which air becomes saturated with water is called the air's dew point. This term makes sense, because water condenses from the air as dew, if the air cools down overnight and reaches 100% humidity.

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Clouds

Clouds have a big influence on weather:

  • by preventing solar radiation from reaching the ground.
  • by absorbing warmth that is re-emitted from the ground.
  • as the source of precipitation.
When there are no clouds, there is less insulation. As a result, cloudless days can be extremely hot, and cloudless nights can be very cold. For this reason, cloudy days tend to have a lower range of temperatures than clear days.Clouds form when air reaches its dew point. This can happen in two ways: (1) Air temperature stays the same but humidity increases. This is common in locations that are warm and humid. (2) Humidity can remain the same, but temperature decreases. When the air cools enough to reach 100% humidity, water droplets form. Air cools when it comes into contact with a cold surface or when it rises.Rising air creates clouds when it has been warmed at or near the ground level and then is pushed up over a mountain or mountain range or is thrust over a mass of cold, dense air.Water vapor is not visible unless it condenses to become a cloud. Water vapor condenses around a nucleus, such as dust, smoke, or a salt crystal. This forms a tiny liquid droplet. Billions of these water droplets together make a cloud.

Clouds are classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure below).

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

High Clouds

​High clouds (Figure below) form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds.

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrus clouds may indicate that a storm is coming.

Middle Clouds​

Middle clouds, including altocumulus and altostratus clouds, may be made of water droplets, ice crystals or both, depending on the air temperatures (Figure below).

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation, is coming.

Low Clouds

​Low clouds (Figure below) are nearly all water droplets. Stratus, stratocumulus and nimbostratus clouds are common low clouds.

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Nimbostratus clouds are thick and dark. They bring steady rain or snow.

Vertical Clouds

Clouds with the prefix 'cumulo-' (Figure below) grow vertically instead of horizontally and have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward.

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Fog

Fog (Figure below) is a cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. The several types of fog that each form in a different way.

  • Radiation fog forms at night when skies are clear and the relative humidity is high. As the ground cools, the bottom layer of air cools below its dew point. Tule fog is an extreme form of radiation fog found in some regions.
  • San Francisco, California, is famous for its summertime advection fog. Warm, moist Pacific Ocean air blows over the cold California current and cools below its dew point. Sea breezes bring the fog onshore.
  • Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface and condenses as it cools, appearing like steam.
  • Warm humid air travels up a hillside and cools below its dew point to create upslope fog

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Precipitation

Precipitation (Figure below) is an extremely important part of weather. Some precipitation forms in place .

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

The most common precipitation comes from clouds. Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets (Figure below). They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. One million cloud droplets will combine to make only one rain drop ! If temperatures are cold, the droplet will hit the ground as a snowflake.

When air near the ground becomes saturated the water vapor condenses into a liquid and forms

Questions:
1. What happens when a batch of air reaches its dew point? What is the temperature?
2. What effect do clouds have on weather?
3. You are standing in a location that is clear in the morning, but in the afternoon there are thunderstorms. There is no wind during the day, so the thunderstorms build directly above you. Describe how this happens.
4. What are the four different cloud groups and how are they classified?
5. How does sleet form? How does glaze form?
6. What circumstances must be present for enormous balls of hail to grow and then fall to the ground?

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When air near the ground becomes saturated the water vapor condenses into a liquid and forms

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MOISTURE, CONDENSATION AND PRECIPITATION

Physical States of Water

  1. Solid state (e.g. ice) occurs at temperature below

      freezing point 00 C or 320 F. (Ice)

  2. Gaseous state (e.g. water vapor) occurs at

      temperatures 1000 C or 2120 F

  3. Liquid state (e.g. sea water) occurs between freezing

      and boiling point temperatures (320 F - 2120 F).

Sources of Moisture in the Atmosphere

        i)    Evaporation from oceans, lakes, rivers & soil

        ii)   Transpiration from plants and other vegetation

        iii)  Perspiration from people and animals. 

        iv)  Sublimation: change from ice to water vapor

HUMIDITY:

Absolute Humidity is the maximum amount of water vapor that a    given column of air can hold. Relative Humidity expressed as a percentage (%) is the amount of water vapor in a column of air compared to the total amount of moisture that the same column of air can hold. When water vapor becomes saturated, it reaches its capacity for holding water so further cooling results in condensation and possibly rainfall.

CONDENSATION AND PRECIPITATION

Condensation is a process by which a gas such as water vapor is   changed into liquid water. When moisture cools and reaches saturation point, the tiny particles of water condenses into larger drops of water.

Forms of Condensation:

Dew: Tiny drops of water formed when condensation of

          water vapor occur at or near the surface of the earth.

Frost: It is a frozen condensation that occurs when air at

            ground level is super cooled below the freezing

            point.

Fog:   A mass of tiny drops of water that form when water

           vapor condenses on a nuclei near the earth's surface.

Clouds: A cloud is a mass of tiny drops of water that

             results from condensation which takes place high

             up in the atmosphere.

Adiabatic Cooling: 

 When air rises, it moves from a zone of dense air on the surface to areas of less dense air in the atmosphere. The rising air thus has less weight above it and the lower pressure allows the air to expand and cool down. The decrease in air temperature that result from expansion of rising air is called Adiabatic Cooling.

Adiabatic Warming

 It is the warming of air that results from the compression of the air as it falls from the sky to the earth's surface. When air falls from higher up in the atmosphere it moves into a region of denser air on the earth's surface. The air is compressed in the process resulting in it being warmed. 

PRECIPITATION:

Precipitation occurs when tiny droplets of water, ice, or frozen water vapor join together into masses too big to be held above the earth. They then fall to the ground as precipitation.

Forms of Precipitation:

a) Snow: When water vapor is frozen directly into a solid

    without first forming a liquid, it forms tiny ice crystals

    called snow

b) Sleet: is a frozen rain that forms when rain droplets

    encounter a cold air and freezes into ice before falling

    from the sky.

c) Hail: Rounded lumps of ice that falls from the sky.

d) Rain: Consists of droplets of liquid water that falls

     from the sky.

Types of Rainfall:

There are three main causes of rainfall. These are: 

1.  Convectional Rainfall:  The rainfall results when a heated air expands, rises and in the process condenses to form drops of rain. Convectional rainfall is more common in the Humid Tropical Regions that receive much of the sun's energy. It usually occurs in the afternoons after surface air has been heated.

2.  Orographic (Relief) Rainfall:  The rainfall results from uplift of air above some highland. Mountain ranges force air blowing over them to rise, cool and then condense to fall as rain. The side of the mountain that faces the wind and receives the rainfall is the Windward side. The opposite side that receives dry winds is the Leeward side.

3.  Frontal (Cyclonic) rainfall:  This type of rainfall occurs along the zone of contact between a warm and cool air mass. When two large air masses of different temperature meet, the warmer and hence lighter air is lifted above the cooler air. The warm air then rises, cools and condenses to form rain. The boundary that separates the cold air and the warm air is called a Front.