Transcript L12-Moisture-and-Precipitation

Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Course Schedule Overview
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•
GG101 Overview (L01)
Part 1:
– General context/physical geography (L02)
– Global radiation and energy system (L03-L09)
– Temperature regimes and temperature cycles (L10-L11)
– Midterm-01 (Feb-18-05)
•
Part II:
– Focus on the dynamics/processes associated with weather (L12-L23)
• Clouds, winds, fronts, air masses, etc…
• Feb-22 to Mar-25
• 12 Lectures (Myneni=8, Anderson=4)
• Guest Lecture by Tucker=1 and Myneni Review for Midterm-02=1
• Midterm-02 (Apr-01-05)
•
Part III:
– Global climates and Climate Change (L24-L32)
Myneni
Feb-22-05
(1 of 1)
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
(1 of 13)
Further Reading: Chapter 06 of the text book
Outline
- the hydrosphere and the hydrological cycle
- humidity
- consequences
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Introduction
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Focus on dynamics of atmosphere itself (Part II)
Consists of three components:
– Moisture in the atmosphere
• Clouds and precipitation
– Atmospheric motions
• Winds and global circulation
– Weather systems
• Mechanisms behind day to day weather
•
Moisture in the Atmosphere
– Key component of climate and weather systems
• Hydrologic cycle
• Atmospheric humidity
• Cloud formation
• Precipitation
Two important points made earlier
Water can take three forms within the earth system: liquid, solid, gas
Evaporation/condensation involves significant amounts of energy
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Hydrosphere-1
– Note that 97% of all water on Earth is tied up in the oceans
• This water remains in the oceans for long periods of time (~1000yrs)
• However the ocean’s circulation patterns are important for global climate, climate
change, and geographic patterns in weather - we’ll get to this later
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Hydrosphere-2
– Only ~3% of water takes other forms
• Most is trapped in the form of ice and glaciers - again this is an inert form in that it
doesn’t allow for transfer of water between systems
• 0.6% is actually stored in ground water in the bedrock, more than in all the lakes and
streams combined
• This leaves only 0.02% of all water which is “active” in that it actively cycles
between the oceans, land, and atmosphere
• This cycling is called the “hydrologic cycle” and takes place mainly within streams,
the atmosphere, and soil water
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
(5 of 13)
The Hydrological Cycle-1
Represents the constant cycling and movement of water between the oceans,
land and atmosphere
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
(6 of 13)
The Hydrological Cycle-2
– Represents the constant cycling and movement of water between the oceans, land and
atmosphere
– Ocean/land/atmosphere represent “reservoirs” of water
– Fluxes between reservoirs occurs via:
• Evaporation
• Condensation (cloud formation)
• Precipitation
• Runoff (I.e. flow from the land to the ocean)
– Note that the Hydrologic cycle dynamics involves only a very small percentage of water
in the total earth system
• Atmosphere holds 0.001% of all water
• Land holds 0.02% of all water
– But the change of state of water (i.e. its thermodynamics involves large amounts of
energy:
• Latent heat is ~22% of total solar energy
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Phases of Water
– Energy is removed from water when it goes from gas to liquid to solid;
this represents a release of energy by the water
– Energy is put into water when it goes from solid to liquid to gas; this
represents a absorption of energy by the water
Let’s examine the gas phase of moisture first …
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Humidity
– Water is one of the key constituents of the atmosphere
– This is how we define how much water is in the atmosphere (two key
variables)
• How much is actually in the atmosphere
• How much can the atmosphere actually hold
• Specific Humidity:
– How much water vapor is in the atmosphere
– Given as kg(water)/kg(air)
– We measure this
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
(9 of 13)
Saturation Humidity
• Saturation humidity: Maximum amount of water the atmosphere can hold
for a given temperature
– Saturation humidity depends primarily on
temperature of the atmosphere
– Cold temperatures
• Can hold little water
• Large temperature change doesn’t add
much water
– Warm temperatures
• Can hold lots of water
• Small temperature changes produces
large change in how much water
atmosphere can hold
– We estimate this, i.e. we measure temperature then calculate what this
number is
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
(10 of 13)
Relative Humidity
Ratio of Specific Humidity to Saturation Humidity
– Given as percentage
– Highest percentage is 100%, i.e., specific humidity
is equal to saturation humidity
• Atmosphere is saturated
• If water is added, it must go into liquid,
not vapor; i.e. it condenses out
– For unsaturated air, if we fix the temperature
and increase the water vapor, relative humidity goes up
– If we fix the water vapor amount and increase the temperature, relative humidity goes down
– Now if we have a fixed amount of water vapor, and the temperature decreases we find that some of
the water has to condense out as liquid water (clouds and rain) because the atmosphere can’t hold
as much in the form of vapor
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Dew Point Temperature
– Given a fixed amount of water vapor, how much would we have to cool the parcel
to have water begin to condense out
– Always less than or equal to the actual temperature
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
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Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Consequences-1
– We can chart the three types of humidity over their diurnal cycle
– We can look at dew point and see that, as long as there is no evaporation, it stays fairly
constant - this means the specific humidity is fairly constant
– We can also look at the temperature which increases during the day; this means that the
saturation humidity is also increasing
– Because the specific humidity is constant and the saturation humidity is increasing, we
expect the ratio, i.e. the relative humidity to decrease over the course of the day
Natural Environments: The Atmosphere
GG 101 – Spring 2005
Boston University
Myneni
Lecture 12: Moisture-and-Precipitation
Feb-22-05
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Consequences-2
– Specific humidity maximum at the equator and minimum at the poles -> warm air can
hold more water than can the cold air
– Also specific humidity in deserts more than at the poles
– At poles, the air is closer to saturation
– Deserts have lower relative humidity than at the poles -> we consider the deserts “dry”
because relative humidity is low