Chapter 18: Water, Clouds, and Precipitation

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Chapter 18 Water, Clouds, and Precipitation
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Water in the Atmosphere

• The amount of water vapor in the air can vary
  from 0-4 by volume depending on location.
• However, its importance can't be overstated.
• Water is responsible for clouds, rainfall, snow,
  and for moderating Earth's temperatures.

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Water Vapor in the Air
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Changes of State in Water

• Earth is unique in that water can exist in three
  different states at normal temperatures.
• When water is a solid (ice) and it melts into a
  liquid, it absorbs heat while it is melting.
• The amount of heat that is required to change a
  state of water is called latent heat, because
  while the water is changing a state, it's
  temperature does not change.

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Changes of State in Water

• When liquid water changes into a gas by
  evaporation, it requires latent heat to change
  the liquid into a gas.
• So, to clarify, if you set out a piece of ice at
  room temperature, the ice and the melt water that
  is produced by the ice would all stay at 0
  Celsius until all of the ice melted.
• If you boiled water in a pan, the water would not
  get any hotter than 100 Celsius until all of the
  water evaporated completely.

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Latent Heat and Water
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Other Important Changes of State in Water

• If a gas changes into a liquid, this process is
  referred to as condensation.
• If a solid turns directly into a gas without
  changing into a liquid first, this process is
  referred to as sublimation.
• If a gas turns directly into a solid without
  changing into a liquid first, this process is
  referred to as deposition.

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Changes in State of Water
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Changes in State of Water
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Humidity vs. Relative Humidity

• The word humidity very general and can have many
  different meanings.
• Sometimes we may say the humidity is high, or
  it's really humid today.
• But this is a general term that has no real
  quantitative value associated with it.
• Meteorologists use relative humidity to measure
  how much water vapor is in the air.

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Relative Humidity

• To understand relative humidity, we have to first
  explain the concept of percent water saturation
  in the air.
• First of all, warm air always holds more water
  vapor than cold air.
• If a parcel of air contains the maximum amount of
  water it can hold at a specific temperature, we
  say it is saturated.

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Warm Air holds more moisture than Cold Air
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Relative Humidity

• Relative Humidity is a ratio or percentage of how
  much water vapor is in the air compared to how
  much water vapor could be in the air to make the
  air completely saturated.
• There are basically two different ways to change
  the relative humidity of air.
• The first way is, you could simply add or remove
  moisture from the air.

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Relative Humidity
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Relative Humidity and Dew Point

• The second way you can change the relative
  humidity of a parcel of air is to change the
  temperature of the air.
• If you change the temperature of air, you change
  the capacity of the air to hold moisture.
• For example, If the temperature of air is
  lowered, the relative humidity increases.
• Once air temperature drops to a temperature where
  the air becomes saturated, this temperature is
  referred to as the dew point temperature.

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Dew Point
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Dew Point and Fog
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Relative Humidity and Dew Point

• So for example, if a parcel of air at 20 Celsius
  had a relative humidity of 50, and the air
  temperature dropped to 10 Celsius, the relative
  humidity would become 100.
• So in this example, 10 Celsius would be the dew
  point temperature.
• Relative humidity can be measured with a
  hygrometer or a sling psychrometer.

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Hygrometer and Sling Psychrometer
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Adiabatic Temperature Changes

• Have you ever pumped up a bike tire and felt the
  pump get warmer. Or let air out of an air
  compressor and felt the cold air being released.
• This is because the increase in pressure causes
  the molecules in the air to collide more
  frequently, creating thermal energy or heat.
• As altitude increases, air pressure decreases,
  and temperature decreases.

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Adiabatic Temperature Changes

• These changes in air temperature caused by
  changes in altitude and pressure are called
  adiabatic temperature changes.
• When air is unsaturated, it cools about 10
  Celsius every 1km increase in elevation.
• This is called the dry adiabatic rate.
• When air is saturated, it cools about 5-9
  Celsius, every 1km increase in elevation.
• This is called the wet adiabatic rate.

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Adiabatic Temperature Changes
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The Formation of Clouds

• When air rises and cools, it eventually reaches
  its dew point.
• When air reaches its dew point, condensation
  occurs, and this forms clouds.
• There are four different mechanisms that cause
  air to rise and form clouds.
• They are orographic lifting, frontal wedging,
  convergence, and localized convective lifting.

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Orographic Lifting

• Orographic lifting occurs when air rises as it
  ascends over mountains, forming clouds.

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Orographic Lifting
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Frontal Wedging

• Frontal wedging occurs when a parcel of warm air
  encounters a parcel of cold air.
• When this occurs, the warm air ascends over the
  cold air, creating a wedge-shaped front.
• As the ascending warm air rises, it cools to its
  dew point and forms clouds.

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Frontal Wedging
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Convergence

• When air masses converge and collide, like in the
  case of areas of low pressure, then the resulting
  air masses rise.
• This mechanism of rising air and cloud formation
  is called convergence.

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Convergence
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Localized Convective Lifting

• Because the surface of the ground is not equal,
  some places heat up more quickly than other
  places.
• For example an area with bare rock is more likely
  to heat up than an area with vegetation.
• In places that heat up more quickly, a rising
  pocket of hot air results causing localized
  convective lifting.

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Localized Convective Lifting
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Stability of Air

• When meteorologists refer to stability of air,
  what they are referring to is how rapidly air is
  rising or falling.
• Hot air is more buoyant than cold air. This
  causes hot air to rise.
• If air has very little difference in temperature
  with height, the density will be close to the
  same and the air will be stable.

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Stability of Air
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Stability of Air

• Unstable air that rises quickly, forms clouds.
• Air that gets colder gradually as elevation
  increases is stable.
• The most stable air is when air gets warmer with
  increasing elevation.
• This is referred to as a temperature inversion.

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Temperature Inversion
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Condensation of Water Vapor

• When air becomes saturated with water vapor,
  condensation occurs.
• When condensation occurs, there has to be some
  surface for the water to condense on.
• In the air, the water usually condenses on small
  particles called condensation nuclei.
• Condensation nuclei may include dust, smoke,
  volcanic ash, pollen, ions etc.

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Types of Clouds

• Cirrus the name means a curl of hair. Clouds
  are high, white, and thin. They have a delicate
  wispy or feathery appearance.
• Cumulus the name means a pile. Clouds look
  like rounded cloud masses. They have flat bases
  with the appearance of rising domes or towers.
  Frequently described as having a cauliflower
  appearance.

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Cirrus Clouds
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Cumulus Clouds
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Types of Clouds

• Stratus name means a layer. Clouds appear as
  sheets or layers that cover most or all of the
  sky. No distinct separation.
• Types of clouds can also be subdivided by their
  elevation above the ground.
• High clouds Different types of cirrus clouds
  are found at these elevations. Usually
  associated with good weather.

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Stratus Clouds
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Types of Clouds

• Middle Clouds Clouds that are found about 2-6
  km (about 1-4 miles) above the ground are
  classified as middle clouds. These clouds are
  usually named with the prefix alto- before them.
• Low Clouds Clouds below 2 km (1 mile).
• Nimbus is a Latin word that is used to describe a
  rainy cloud.

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Types of Clouds

• By joining the different names of clouds we can
  classify many different types of clouds.
• For example, for high clouds we can have cirrus,
  cirrostratus, and cirrocumulus.
• For middle clouds we have examples like
  altocumulus and altostratus.
• For low clouds we have examples like
  stratocumulus, stratus, nimbostratus, cumulus and
  cumulonimbus.

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Types of Clouds
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Types of Clouds
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Types of Clouds
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Fog, Hail, and Sleet

• Fog is a cloud that forms close to the ground.
• One way fog can form is when a parcel of air
  reaches its dew point.
• Another way that fog can form is when cool air
  moves over warmer water and evaporation from the
  water forms fog.
• Hail and sleet form when liquid water falls from
  a warm pocket of air and then goes through a
  layer of air at freezing temperatures.

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Fog
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Hail and Sleet