3. +
Movement of Water Through The
Atmosphere
97% of all water is salt water
Remaining 3% is stored in ice sheets in
Antarctica and Greenland
Only 0.0001% is found in the Atmosphere in
the form of water vapor
4. +
Movement of Water Through The
Atmosphere
Hydrologic
Cycle:
Thecontinuous
exchange of
water among
oceans,
atmosphere and
continents
6. +
Water: A Unique Substance
Water:
1. Water is the only liquid on the surface of the
Earth in large quantities.
2. It exists in all forms on Earth.
3. Ice (solid state) is less dense than liquid.
4. Water has a high heat capacity.
7. +
Water: A Unique Substance
Ithas a unique ability to form hydrogen bonds
(H2O)
When ice forms on a body of water it insulates
the underlying layer of water and slows the
rate of freezing
Thisis how fish can survive in a lake during the
winter :)
10. +
Water Changes States
Ice
Tightorderly network of molecules vibrating in
a fixed state
Water
Molecules can slide past one another
Gas
Very random erratic motion
11. +
Latent Heat
Whenever water changes state heat is lost to its
surroundings.
Latent heat is hidden heat
Adding heat to melt ice does not result in a
temperature change.
Melting 1 gram of ice requires 80 calories.
Latent heat of melting
Freezing 1 gram of water releases 80 calories.
Latent heat of fusion
12. +
Latent Heat
Evaporation
The process of converting a liquid to a gas.
The latent heat of vaporization is the energy
absorbed by water during evaporation.
~ 600 calories/gram for water
Evaporation is a cooling process.
13. +
Latent Heat
Condensation
Is the reverse process, converting a gas to a
liquid.
The process when water vapor changes to the
liquid state is the latent heat of condensation.
Energy is released, which warms the
surrounding air.
Forms clouds and fog
14. +
Latent Heat
Sublimation:
Sublimationis the process that turns a solid to a gas.
Disappearing ice cubes in freezer are a result of
sublimation.
Deposition:
Deposition is the reverse process of changing a vapor
to a solid.
Frost accumulating in a freezer is a result of
deposition.
16. +
Humidity: Water Vapor in the Air
Humidity is amount of water vapor in the air.
1. Absolute humidity is the mass of water vapor
in a given volume of air.
2. The mixing ratio is the mass of water vapor in
a unit of air compared to the remaining mass
of dry air.
Meteorologists like to use this one because it
is not affected by changes in pressure or
temperature
17. +
Humidity: Water Vapor in the Air
3. Vapor pressure is defined as the part of the
total atmospheric pressure attributable to its
water-vapor content.
4. Relative humidity indicates how close air is to
saturation rather than the actual quantity of
water vapor in the air.
5. Dew point is the temperature to which air
needs to be cooled to reach saturation.
**We will talk more about these 3
19. +
Vapor Pressure and Saturation
Vapor pressure is that
part of the total air
pressure attributable to
water vapor content.
Morewater vapor equals
more vapor pressure
20. +
Vapor Pressure and Saturation
Saturation:
Saturationis the equilibrium point between
evaporation and condensation.
Temperature dependent
For every 10°C (18 °C) increases in temperature, the
amount of water for vapor needed for saturation
doubles
Humid air equals a high vapor pressure.
22. +
Relative Humidity
Relative humidity:
Relative humidity is the ratio of the air’s actual
water vapor content and amount of water vapor
required for saturation at a certain temperature
and pressure.
Relative humidity indicates how near the air is
to saturation rather than the actual quantity of
water vapor in the air.
23. +
Relative Humidity
At25°C, air is
saturated at 20 g/kg
Soif we had 10g/kg at
25°C then the relative
humidity is 10/20 or
50%
24. +
Relative Humidity
How relative humidity changes:
100% relative humidity equals saturation.
If water vapor is added, relative humidity goes
up.
If water vapor is removed, relative humidity
goes down.
A decrease in temperature equals an increase in
relative humidity.
25. +
Relative Humidity
A decreasein temperature equals an increase in
relative humidity.
When air aloft is cooled below its saturation
level, some of the water vapor condenses to
form clouds. Clouds are made of liquid droplets,
so the moisture is no longer apart of the water-
vapor content of the air.
27. +
Relative Humidity
Natural changes:
Daily temperature changes affect relative humidity.
Temperature changes are caused by advection, the
primarily horizontal component of convective flow
(wind).
Temperature changes are also caused through
convection, where some of the air in the lowest layer
of the atmosphere, heated by radiation and
conduction, is transported by convection to higher
layers of the atmosphere.
29. +
Dew-Point Temperature
Dew point:
The dew point is the temperature air is cooled to
reach saturation.
Dew-point temperature is a measure of actual
moisture content.
30. +
Dew-Point Temperature
In nature, cooling below the dew point cause
water vapor to condense, typically as dew, fog
or clouds
32. +
How is Humidity Measured?
Hygrometer:
A hygrometer measures moisture the content of
air.
A hair hygrometer operates on the principle
that hair changes length in proportion to
changes in relative humidity.
These are harder to use and need to be
calibrated often
33. +
How is Humidity Measured?
A psychrometer consists of two identical
thermometers; one (dry thermometer)
measures air temperature, the other called the
“wet bulb,” measures the dryness of air.
Thelarger the difference in temperatures
between the wet and dry, the lower the relative
humidity (with a table).
35. +
Adiabatic Temperature Changes
Remember condensation occurs when
sufficient water vapor is added to the air or
more commonly when the air is cooled to its
dew point temperature.
An adiabatic process is one in which no heat is
transferred.
When air is compressed, it warms.
When air expands, it cools.
36. +
Adiabatic Temperature Changes
Adiabatic cooling:
Cooling occurs when air moves up and it
expands and cools.
Unsaturated air cools at a rate of 10° C/1000m;
this is called the dry adiabatic rate.
37. +
Adiabatic Temperature Changes
Adiabatic condensation:
Condensation is triggered when air rises high
enough to reach its saturation point and clouds form,
called its lifting condensation level.
When air ascends above the lifting condensation
level, the rate at which it cools is reduced. The
slower rate of cooling is called the wet adiabatic
rate (because the air is saturated), which varies from
5° C/1000m.
41. +
Processes That Lift Air
4 mechanisms that cause to air to rise
1. Orographic lifting in which air is forced to rise
over a mountainous barrier
2. Frontal wedging in which warmer less dense air is
forced over cooler dense air
3. Convergence is a pileup of horizontal air flow that
results in upward motion
4. Localized convective lifting is unequal surface
heating causes localized pockets of air to rise
because of their buoyancy.
42. +
Processes That Lift Air
Orographic lifting occurs when elevated
terrains, such as mountains, act as barriers to
the flow of air.
Adiabatic cooling can generate clouds and
copious precipitation. Many of the wettest
places in the world are located on windward
mountain slopes.
When air reaches the leeward side, much of its
moisture has been lost.
43. +
Processes That Lift Air
Rain shadow desert- happens on leeward mountain
side, adiabatic warming making ran less likely
44. +
Processes That Lift Air
Frontal wedging:
Masses of warm and cold air collide, producing
fronts. Cooler, denser air acts as a barrier over
which the warmer, less dense air rises. This
process is called frontal wedging.
45. +
Processes That
Lift Air
Convergence:
Converging
horizontal air flow
results in upward
movement.
Can also happen
when an obstacle
slows or restricts
horizontal air flow
46. +
Processes That Lift Air
Localized convective lifting:
Unequal surface heating causes localized pockets of
air to rise.
Can rise high enough, condense and form clouds
48. +
The Critical Weathermaker:
Atmospheric Stability
Stable air resists vertical movement.
Unstable air rises due to buoyancy till it reaches
a temperature that is equal to its surroundings.
Environmental lapse rate is the actual
temperature of the atmosphere.
Airtemperature is measured at various heights in
the atmosphere.
49. +
The Critical Weathermaker:
Atmospheric Stability
Absolute Stability
The environmental lapse rate is less than the
adiabatic rate
The rising air continues to cool
Temperature inversion
Type of environmental lapse rate when the temp
in a layer of air increases with altitude rather
than decreasing
51. +
The Critical Weathermaker:
Atmospheric Stability
Absolute Instability
The environmental lapse rate is greater than the
adiabatic rate
The ascending parcel of air is always warmer
than its environment and will continue to rise
because of its buoyancy
Occurs most often during the warmest months
on clear days when solar heating is intense
53. +
The Critical Weathermaker:
Atmospheric Stability
Conditional instability is the most common
type of atmospheric instability.
This situation prevails when moist air has an
environmental lapse rate between the dry and
wet adiabatic rates.
55. +
In Summary: Atmospheric Stability
The stability of air is determined by measuring
temperature of the atmosphere at various
temperatures (environmental lapse rate)
A column of air is unstable when the air near
the bottom of this layer is significantly
warmer than the air aloft
56. +
In Summary: Atmospheric Stability
Air is considered stable when the temperature
decreases gradually with an increase in
altitude
Most stable condition occurs during a
temperature inversion when the temperature
actually increases with height.
Under
these conditions there is little vertical air
movement
58. +
Stability and Daily Weather
On a dreary overcast day with light drizzle,
stable air is forced aloft
On a day with towering clouds forming the
atmosphere is unstable
59. +
Stability and Daily Weather
How stability changes:
Instability
is enhanced by the following:
Intense warming of the lowest layer of the
atmosphere
Heating of an air mass from below
General upward movement of air caused by
orographic lifting, frontal wedging, and
convergence
Radiation cooling from cloud tops
60. +
Stability and Daily Weather
How stability changes:
Stabilityis enhanced by the following:
Radiation cooling of Earth’s surface after
sunset
Cooling of an air mass from below as it
traverses cold surface
General subsidence within an air column
61. +
Stability and Daily Weather
Temperature changes and stability:
As air moves horizontally over the surface with
different temperatures changes in stability occur
Ex: When warm air from the Gulf of Mexico
moves north over the snow covered Midwest,
the air is cooled from below, it becomes more
stable, often producing widespread fog.
62. +
Stability and Daily Weather
Ex:In winter, polar air move south over over
the Great Lakes. Polar air is then rendered
sufficiently unstable when the cold, dry air
passes over a warm, wet surface, which can
often produce lake effect snow over the Great
Lakes.
63. +Vertical Air Movement and
Stability
Subsidence is a general, downward air flow.
Usually, surface air is not involved.
When there is a general downward airflow, the
upper portion of the subsiding layer is heated by
compression more than the lower. So, the net effect
is to stabilize the air because the air aloft is warmed
more than the surface air.
Thewarming effect of a few 100m of subsidence is
enough to evaporate the clouds. Resulting in clear blue
skies :)
64. +
Final Summary
The air’s stability or, lack of it, determines to a
large degree whether clouds develop and
produce precipitation and whether that
precipitation will come as a gentle shower or a
violent down pour.
65. +
Final Summary
In general, when stable air is forced up, the
associated clouds have very little vertical
thickness and precipitation, if any, is light.
In contrast, clouds associated with unstable air
are towering and are frequently accompanied
by heavy precipitation.
