The Interface between Atmosphere and Hydrosphere
First, Let’s Recall the
5+ Basic Elements of the Atmosphere
– the main ingredients of weather and climate
-- Also called Elements of Weather and Climate
• Solar Energy
-- Insolation and Heat Energy Transfer
• Temperature
• Pressure
• Wind
In this segment, we’ll discuss the fifth element:
• Precipitation
+ Air Masses (and Fronts)
Atmospheric Water – Moisture → the Hydrosphere
 Atmospheric water or moisture is the source of condensation,
cloud formation, and ultimately precipitation, which
is one of the most important elements of weather and
climate, being the principal regulator and modifier of
Earth’s temperature
 These processes are also integral parts of the hydrosphere,
and The Hydrologic Cycle.
 Hence, we’ll first discuss the significance of water in the
context of the hydrosphere and the hydrologic cycle, and
then specifically examine the precipitation process as a
component of weather and climate systems
The Hydrosphere
 Water in its various forms
sustains life; without
water, living things
could not survive.
 73% of the earth surface is
covered with water
(1.33 billion cubic kilometers or 326 million cubic miles)
 97.1% of earth’s waters are in oceans (salt water)
 2.24% are in glaciers, including polar ice sheets (fresh water)
 0.61% are deep and shallow groundwater (fresh water)
 0.05% -- the remaining, are in fresh and salt water lakes,
rivers, streams, soil root zones, and the atmosphere!
 These bodies of water provide reservoirs of heat in the winter, and
cooling effects in the summer
Properties of Water
• A water molecule is composed of two atoms of hydrogen and one atom of
oxygen, and it can exist in all three states or phases of matter.
Phases of Water
• Universal solvent – dissolves
enormous numbers of
substances
• Transports many minerals
and solid particles,
even in suspension
• Capillary action – ability of
water to pull itself
upward through small
openings, against
gravity
http://www.youtube.com/watch?v=dabPFNxo844&feature=related
What percent of the Earth’s total volume of
water is stored in the atmosphere?
0.001%
 Water Vapor
 Clouds (water vapor
condensed on
particulates)
Transfer of Water Into and Out of the Atmosphere
Evaporation  [Contributes 90% of atmospheric water]
The principal way in which water enters the air is through
evaporation. Approximately 600 calories of heat must be added
to a gram of water for it to evaporate into the air. This energy is
called "the latent heat of vaporization." Energy is transferred
between the various spheres of the Earth system via phase
changes of water. [See previous Figure]
Evaporation, thus, is the phase
change from liquid water to a gas
form (vapor). Three important
factors determine the Rate of
Evaporation : 1) temperature of
Available water; 2) relative humidity
of surrounding air; and 3) wind.
Transpiration  [Contributes 10% of atmospheric water]
Plants are another important source of atmospheric moisture.
Plants contribute water to the atmosphere by transpiration.
Transpiration is the transfer of water into the air via leaf pores.
Together, evaporation and transpiration, or Evapotranspiration,
accounts for virtually all the water vapor in the air [See: Potential Evapotranspiration]
Moisture and Humidity
Humidity is a measure of the water vapor content of the air.
There are several ways in which a meteorologist can express
the humidity of the air. Each humidity measure is controlled to
some degree by air temperature.
Absolute humidity -- measure of the mass of water vapor per
unit volume of air e.g., grams per cubic meter). [Often, specific
humidity is used, to express mass of water vapor in grams per
kilogram of air]
Relative humidity -- the ratio between the amount of water
vapor in the air and the water vapor capacity of the air, at a
given temperature; reported as a percentage that expresses
how close the air is to saturation.
Saturation – when air of a given temperature holds all the
water vapor it possibly can, it is said to be saturated, having
reached its capacity.
Dew Point – is the critical temperature at which the air is fully
saturated – the cooling of air to below its dew point
temperature that brings about condensation that must
precede any precipitation.
This completes a cycle –
Evapotranspiration  Water Vapors  Condensation 
Cloud Formation  Precipitation  Evapotranspiration
-- the cycle known as the Hydrologic Cycle!
The Hydrologic Cycle
The Condensation Process
Condensation is the process that changes water vapor
into a liquid form.
• It occurs when air saturated with water vapor is cooled;
So, condensation depends on:
 the relative humidity of the air, and
 the degree of cooling
• During the process, water molecules lose the 600 cal/gm
of latent heat that were added during the process of
evaporation  Latent Heat of Condensation
• Another factor necessary for the process – the presence
of condensation nuclei* – minute particulates in
the air, like sea-salt, dust or pollen
* Or a condensation surface, as in the case of dew formation
• Dew is formed by the condensation of water vapor on a surface
• White Frost forms in below freezing temperatures when water
vapor changes directly to the frozen state through the
sublimation process
• Rime – ice crystal formation on tree branches or airplane wings
when supercooled droplets come in contact with the surface
• Smog is formed when condensation takes place on chemical nuclei,
making it corrosive and dangerous to human health
• Fog and Cloud appear when water vapor condenses on nuclei and
a large number of these droplets form a mass – these
masses of condensed water droplets, fog or cloud, assume any
number of shapes and forms, usually in shades of white or gray;
Fogs are essentially low level clouds that touch the ground
CLOUDS classification based on height and shape
http://www.youtube.com/watch?v=ULvujTon5tU
Cloud Forms
Based on Height or Altitude:
• Strato (low)
• Alto (mid)
• Cirro (high)
Based on Shape or Form:
• Cirrus (wispy)
• Stratus (layers)
• Cumulus (heap or pile)
• Nimbo / Nimbus (rain is falling)
-- rain bearing clouds, e.g., Cumulonimbus
• The cooling process that leads to Cloud Formation is quite different
from that associated with fog, frost or dew (radiation or advection)
• It is known as Adiabatic Cooling  Adiabatic Lapse Rate (5.6oF/1000 ft)
– Dry Adiabatic Lapse Rate vs. Wet Adiabatic Lapse Rate
Also, Adiabatic Heating relative to descending air
Precipitation Processes
Precipitation from a meteorological stand point is water in some form,
falling out of the air, and settling on the surface of the earth.
This distinguishes between forms of condensation in the atmosphere
and condensation that occurs at the surface. Dew is condensation at
the surface and thus is not a form of precipitation.
Forms of Precipitation
There are a variety of different types of precipitation. Rain, snow, hail,
sleet, freezing rain are all forms of precipitation. The kind of precipitation
received depends on the variation of temperature above the surface.
Rain is precipitation in liquid form.
Snow is precipitation in solid form as (typically) a hexagonal crystal
shape. If the air temperatures are below freezing on its journey toward the
surface, precipitation will be in the form of snow.
Sleet occurs when snow falls through a warm layer of air and melts.
Freezing rain or glaze occurs when snow melts upon passing through
a warm layer of air and then freezes on the surface whose temperature
is at or below freezing.
Hail falls as rounded pellets or balls of ice from severe thunderstorms.
In sum, Precipitation occurs when the droplets develop masses too great
to be held aloft.
And, three conditions are necessary for precipitation to occur:
1) Moist air, 2) Condensation nuclei, and 3) Uplift mechanism.
Precipitation Types by Uplift Mechanism:
1. Convectional Precipitation
http://www.youtube.com/watch?v=RkgThul2El8&feature=related
2. Relief /Orographic Precipitation
http://www.youtube.com/watch?v=BVykQfRC_aI&feature=related
3. Frontal Precipitation
http://www.youtube.com/watch?v=D88dYNFyBq8&feature=related
Fig. 6-18, p. 164
4. Cyclonic Precipitation
Occurs when air is lifted up into a low pressure system.
http://www.youtube.com/watch?v=75qAgSuMbzA&eurl=http%3A%2F%2F
www.uwsp.edu%2Fgeo%2Ffaculty%2Fritter%2Fgeog101%2Ftextbook%2
Fweather_systems%2Fhurricanes.html&feature=player_embedded
DISTRIBUTION OF PRECIPITATION – See Figure 6.23 in Textbook
 Isohyet Map – Look for a) general pattern, b) evidence of orographic rain?
VARIABILITY OF PRECIPITATION – See Figure 6.25 in Textbook
 Do the areas of greater variability generally correspond to the dryer areas of
the world, as shown on the other map? (Compare this map with Figure 6.23)