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 Cloud Formation

 

Water Vapors



Precipitation



Condensation



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

– 

Dry Adiabatic Lapse Rate Adiabatic Lapse Rate

vs. (5.6

o F/1000 ft)

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)