Water in the Air

1
Water in the Air
2
Humidity

• Humidity measures the total weight of water that
  can be evaporated into a set weight of dry air.
• Use the numbers on the far right of the chart to
  calculate how much water the air can hold.

3
Relative Humidity

• A PSYCHROMETER is used to measure find relative
  humidity.
• It consists of two thermometers, one which is dry
  and one covered with a wet cover.

4
Relative Humidity

• If the wet bulb temperature goes down a long way,
  it is because the air is dry a lot of water
  evaporates.
• Use the difference between wet and dry bulbs on
  the chart to find the percent humidity.

5
Dew Point

• Dew Point is the temperature at which water
  begins to condense into a liquid. The more humid
  the air is, the higher the dew point and the more
  uncomfortable the weather can be.

6
Cloud Formation

• When air rises it cools. If the air cools to its
  dew point, then the water vapor condenses and
  clouds may form.

7
Clouds

• Clouds are named based on their form/shape and
  their height.
• High cirrus
• Medium alto
• Puffy cumulus
• Layer stratus
• Wispy cirrus
• Nimbus precipitation

8
Clouds

• Cirrus high, thin and wispy, made of ice
  crystals

9
Clouds

• Stratus layered

10
Clouds

• Cumulus puffy, cottonball, usually
  flat-bottomed with vertical development

11
Clouds

• Nimbus precipitation

12
Clouds

• Alto middle level in height, above low clouds
  and below cirrus

13
Precipitation

• Any of all of the forms of water particles,
  whether liquid or solid, that fall from the
  atmosphere and reach the ground. The forms of
  precipitation are rain, drizzle, snow, snow
  grains, snow pellets, diamond dust, hail, and ice
  pellets.

14
Precipitation

• rain, rainfall - water falling in drops from
  vapor condensed in the atmosphere
• sleet - partially melted snow (or a mixture of
  rain and snow)
• snow, snowfall - precipitation falling from
  clouds in the form of ice crystals

• virga - light wispy precipitation that evaporates
  before it reaches the ground (especially when the
  lower air is low in humidity)
• diamond dust, frost mist, frost snow, ice
  crystal, ice needle, poudrin, snow mist - small
  crystals of ice

15
Precipitation

• Bergeron Process
• Studies have shown that water in very small
  drops, such as the size of a cloud droplet, can
  exist at temperatures well below freezing (as low
  as -40 C)!
• An interesting and useful fact is that these
  supercooled water droplets will freeze almost
  instantly if they come into contact with a solid
  particle that resembles an ice crystal. This
  solid particle is called a freezing nucleus.
• Also they will freeze if agitated sometimes just
  by coming into contact with one another causes
  ice crystals to form.
• Many of the puffy cumulus clouds that you see in
  the sky may be made up entirely of supercooled
  liquid water droplets!

• Cloud Temperature
• Droplets?
• Above 0 C (32 F)
• Liquid Water
• -10 to 0 C (12-32 F)
• Supercooled Water
• -40 to -10 C (-4-14 F)
• Supercooled Water and Ice Crystals Coexist (mixed
  clouds)
• Below -40 C (-4 F)
• Mainly Ice Crystals (glaciated clouds)

16
Bergeron process

• There are more water molecules surrounding the
  water droplets than there are surrounding the ice
  crystals. This occurs because the saturation
  vapor pressure over a water surface is greater
  than that over an ice surface at the same
  subfreezing temperature. Saturation vapor
  pressure describes how much water vapor is needed
  to make the air saturated at any given
  temperature and in effect, is the pressure that
  the water vapor would exert if the air were
  saturated with respect to a given temperature.
  The supercooled liquid droplets are more readily
  able to evaporate and contribute to the vapor
  pressure in the surrounding air than the ice
  crystals are able to sublimate and contribute to
  the vapor pressure. Therefore, when ice and
  liquid coexist within a cloud, water vapor must
  evaporate from the drop and flow toward the ice
  crystal in order to maintain equilibrium. As
  this water vapor diffuses toward the ice crystal,
  the droplet must evaporate more in order to keep
  the vapor pressure in equilibrium with its
  surroundings. Therefore, what happens, is a
  viscious cycle of water vapor evaporating from
  the drop, collecting on the ice crystal, and
  freezing so that the crystal continuously grows
  at the water droplet's expense.

17
Collision/Coalescence Process

• The Collision and Coalescence Process typically
  occurs within relatively warm clouds with tops
  warmer than -15C.
• 1) There must be a high liquid water content
  within the cloud.
• 2) There must be sufficiently strong and
  consistent updrafts within the cloud.
• 3) A large range of cloud droplet sizes is very
  helpful.
• 4) The cloud must be thick enough so that the
  cloud droplets have enough time to gather
  surrounding smaller droplets.
• 5) The electric charge of the droplets and the
  electric field in the cloud and its effects are
  still being studied.

• Within the warm cloud there is an updraft of air
  caused by air coming together or converging at a
  point beneath the cloud. After the air
  converges, it is forced upward. This process is
  what initially helps to build the cloud and now
  that it has formed, it continues and carries
  smaller cloud droplets up into the cloud while
  larger droplets stay suspended within the cloud
  or even fall downward slowly. As you might
  guess, with billions upon billions of cloud
  droplets hanging out in the cloud, some of them
  are bound to bump into each other!

18
Collision/Coalescence Process

• As the cloud droplets experience millions of
  collisions, they sometimes join together (or
  coalesce) and form larger cloud droplets. The
  larger cloud droplets then fall faster (because
  they have a higher terminal velocity) and collide
  with smaller droplets in their path. Studies
  done in laboratories have shown that not all
  collisions result in coalescence, that is to say,
  that some of the drops break apart after
  colliding. The studies have shown that
  "coalescense appears to be enhanced if colliding
  droplets have opposite (and, hence attractive)
  electrical charges... especially in thunderstorm
  precipitation coalescence where strongly charged
  droplets exist in a strong electrical field.

19
Collision/Coalescence Process
20
Hail Formation

• Hail can be found in the middle and upper
  portions of almost all thunderstorms. However,
  most either melts before hitting the ground, or
  being very soft, disintegrates in the violent
  thunderstorm interior.

• Hailstones generally begin forming on seeds of
  small frozen raindrops or soft ice particles
  known as graupel which are hardened conglomerates
  of snow flakes. Graupel or frozen droplets are
  not the only embryos for hail. Hailstones
  sometimes contain foreign matter such as pebbles,
  leaves, twigs, nuts, and insects that have been
  lofted into the storm cloud by strong updraft
  winds.

21
Hail Formation

• The size of hailstones usually increases with the
  intensity of the storm cell from which they
  spawn. To form hailstones the size of golf balls
  requires over ten billion supercooled droplets be
  accumulated, and thus they must remain in the
  storm cloud for at least 5 to 10 minutes.
  (Compare this to the one million or so droplets
  needed to form the typical raindrop.) Therefore,
  large hail (gt 5 cm / 2 inches) forms mostly in
  supercell thunderstorms which have strong updraft
  winds.

• In order for the frozen raindrops or graupel to
  grow into true hailstones, they must accumulate
  additional ice, a process called accretion. To do
  so, the hail embryo must spend time in cloud
  regions rich in supercooled water, a layer where
  temperatures are below the 0oC (32oF) level.

22
Hail Formation
23
Hail Formation

• Hail swaths can pile hail so deep it must be
  removed with a snow plow. In Orient, Iowa, for
  example, in August 1980, "hail drifts" were
  reported 2 metres (6.5 feet) deep. Large and
  severe hail swaths may devastate one field of
  crops while leaving a neighbor's untouched.