Atmospheric Moisture and Precipitation
Principles of Physical Geography
Atmospheric Moisture and Precipitation
Aims
To understand the relationship between temperature and humidity
To define evapotranspiration
To understand the concept of stability in the atmosphere
To understand why it rains
To understand convection
Objectives
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To identify the utilization of energy during the various phase change processes of
water.
To describe the saturation process and how the amount of atmospheric water vapour
at saturation depends upon temperature.
To understand the processes of evaporation and transpiration
To explain how the water vapour content of air is quantified and define the following
moisture parameters: dewpoint temperature, wet bulb temperature, vapour pressure,
mixing ratio, relative humidity
To explain the temperature dependency of relative humidity.
To define dry and wet adiabatic temperature changes.
To explain the concept of stability and its application to the atmosphere
To explain absolute stability and absolute and conditional instability of an air column
To explain convection, orographic lifting, frontal wedging and convergence
Atmospheric Moisture and Precipitation
Outline
Introduction
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Moisture and atmospheric stability
Air water vapour content
Relevance of moisture for the hydrologic cycle
Importance for local surface weather and climate, role in tropical storms
Water’s changes of state
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Evaporation, condensation
Transpiration, evapotranspiration
Energy supply for evapotranspiration
Latent heat
Humidity
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Vapour pressure
Saturation
Mixing ratio and absolute humidity
Dewpoint
Relative humidity
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Relative humidity
Changing water content and temperature
Adiabatic temperature changes
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Ideal Gas Law
Dry adiabatic rate
Wet adiabatic rate
Lifting processes and precipitation
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Orographic lifting
Convective lifting
Frontal wedging
Convergence
Convection
GY
Atmospheric Moisture and Precipitation
Main topics
Interactions of water vapour with atmospheric stability
Evapotranspiration
Relationship between temperature and humidity
Concept of stability in the atmosphere
Lifting and precipitation
Convection
Outline
Introduction
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Moisture and atmospheric stability
Air water vapour content
Relevance of moisture for the hydrologic cycle
Importance for local weather and climate, role in tropical storms
Water’s changes of state
• Evaporation, condensation, transpiration, evapotranspiration
• Energy supply for evapotranspiration
• Latent heat
Humidity
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Dewpoint
Saturation
Mixing ratio and absolute humidity
Vapour pressure
Relative humidity
• Relative humidity
• Changing water content and temperature
Adiabatic temperature changes
• Ideal Gas Law
• Dry adiabatic rate
• Wet adiabatic rate
Lifting processes and precipitation
Convection
GY
Atmospheric Moisture and Precipitation
Bullets
Introduction
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Moisture <-> atmospheric stability?
What is stability anyway?
Water vapour - only small fraction of the atmosphere 0<v<5% !
Takes energy to evaporate water - as water vapour condensates, what happens with
the heat being released in the atmosphere?
Why care?
Very important: precipitation, groundwater recharge, cooling, moisturizing
Latent/sensible heat -> cooling of surface
Released energy - used to drive other processes? Warm some areas? One way how
the energy from the sun drives the processes one earth and allows for warming the
higher latitudes
Example for moisture/stability/energy content: tropical convection and Hurricanes
Thus: for several reasons important to understand the water cycle and to measure
water vapour content of air, that is air moisture, humidity
Importance of energy transformations and temperature measurements
Water’s changes of state
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Evaporation - requires energy - latent heat of vaporization, calorie
Change of state
During evaporation some fast molecules escape to the air
Transpiration: water loss from plants
Evapotranspiration: sum of evaporation and transpiration
Evaporation is a cooling process since energy is used to evaporate the water and not
to heat the water or the air.
Heat/energy used for evaporation is now contained in the water vapour
Latent/sensible heat -> forest/open
Latent heat is released again during condensation
Melting, requires energy, too, the latent heat of fusion, released again during freezing
Frost protection by sprinkling water onto fruit crops: heat content of water, latent heat
of fusion is released as the water freezes -> as long as there is liquid water on the
fruit, their temperature will not fall below 0oC as the freezing water releases heat
Humidity
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Humidity - general term for the amount of water vapour in the air
Water vapour pressure - partial pressure of the water vapour in the atmosphere
Saturation of air with water vapour <-> after some time an equilibrium between evaporation and condensation over a water surface in closed container
Saturation vapour pressure - pressure exerted by air saturated with water
Saturation water vapour pressure is temperature dependent as at higher temperatures more water evaporates <-> more molecules have higher energy
Absolute humidity - mass of water vapour in a given volume of air
Mixing ratio - mass of water vapour in a unit mass of dry air
Difficult to determine
Relative humidity
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Relative humidity - ratio of the air’s actual water vapour content compared with the
amount of water vapour required for saturation at the same temperature
Atmospheric Moisture and Precipitation
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Temperature dependent -> can be changed by changing water content or by changing temperature
Increases in water vapour content lead to increases to relative humidity (at the same
temperature) until condensation occurs - at saturation.
Increases in temperature decrease relative humidity because at higher temperatures
air can hold more water
Decreases in temperature increase relative humidity because at higher temperatures
air can hold less water - at some temperature condensation might occur removing
water from the air and relative humidity remains at 100%
1. diurnal cycle due to temperature cycle, 2. air movement horizontally or, 3. vertically
Relative humidity <-> mixing ratio
Dew point: temperature at which a parcel of air would be saturated
Human discomfort: sweating dissipates heat by evaporation of perspiration (latent
heat of vaporization!) - not efficient if surrounding air is very moist and can thus not
take up easily more moisture -> heat stress -> eventually fatal due to failing protein
functions. Very dangerous in enclosed spaces - cars!
Adiabatic temperature changes
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Ideal gas law
Dry adiabatic changes: when air is allowed to expand it cools, when it is compressed
it warms - no heat is added nor subtracted
If air is lifted: dry adiabatic rate: 10oC per 1000m
Applies only to vertically moving unsaturated air
If air cools sufficiently condensation will occur and release heat, thus slowing the rate
of cooling
Wet adiabatic rate
Wet adiabatic rate depends on the amount of moisture in the air: 5oC for air with high
to 9oC for air with low moisture content
Wet adiabatic rate applies above the level, where condensation occurs, the lifting
condensation level -> rain
Cooling is faster at the dry rate than at the wet rate
Importance of moisture and temperature measurements to estimate lifting and thus
precipitation and energy release
Linkage of air movement and energy exchange
Lifting processes and precipitation
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Lifting processes are important as they can force air up to a level where the temperate is below it’s dew point and thus producing clouds and rain
Convective lifting
Orographic lifting: air forced upward by air flow over elevated terrain ->rain shadow
deserts
Frontal wedging: air forced upward by air flow over denser air; often warmer over
colder air.
Convergence
Links
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http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml
http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter4/deluxe.html
Atmospheric Moisture and Precipitation
Water’s changes of state
Heat energy absorbed Sublimation
Most important
for climate:
•Energy transfer
•Air density
Melting
Ice
Freezing
Deposition
Evaporation
•Circulation
Liquid Condensation
•Precipitation
Vapour
•Transpiration
Heat energy released
Source: Ahrens, C.D., 1994. Meteorology Today
GYk
•Air stability
Atmospheric Moisture and Precipitation
Spraying with water used
for frost protection
Freezing releases the energy
used for melting
As long as the water releases
enough heat while it freezes
the fruit will not be damaged
(held at 0oC)
Source:
Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
GYk
Atmospheric Moisture and Precipitation
Air density and water vapour concentration
Wet air is less dense than dry air
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There is a lot of empty space between gas molecules
They neither attract nor repel each other much
Distances between molecules are not affected by the type of gas
The number of molecules in container at same temperature and pressure independent of the type of gas.
Same temperature, T, and pressure,
P! (T, P do change distance between
molecules! And thus the density)
The molecular weights of O2 (red), N2
heavier
lighter
(blue) and H2O are about 32, 28, and
18, respectively. Substituting a water molecule for either an oxygen or a
nitrogen molecule, will decrease the total mass of air in a volume and
thus the density of the wet air is less than the density of dry air.
GYk
Atmospheric Moisture and Precipitation
Water’s changes of state
Effects:
• Change energy content or temperature of air (latent heat)
• Change density (and so the weight!) of air
Impacts on temperature, pressure and circulation
Sources of water vapour?
GY
Atmospheric Moisture and Precipitation
Evapotranspiration
Transpiration: loss of water vapour from
plants by evaporation through stomata
Transpiration
Evaporation
from leaf
surface
Evaporation
from water
surface
Soil
evaporation
Evapotranspiration is the sum of all evaporation and transpiration
• water flux from earth’s surface to the atmosphere
• driving the upward water transport in plants
GYk
Atmospheric Moisture and Precipitation
Humidity
Humidity - general term for the amount of water vapour in the air
• Absolute humidity - mass of water vapour in a given volume of air
• Mixing ratio - mass of water vapour in a unit mass of dry air
• Relative humidity
• Water vapour pressure
Saturation of air with water vapour <-> Over water surface in closed container an equilibrium between evaporation and condensation will form
Dew Point Temperature (Dew Point)
Temperature at which air would be saturated (dew formation would start)
Gk
Atmospheric Moisture and Precipitation
Very strongly
temperature
dependent
T
[oC]
H2O vapor
[g/kg]
5
3.5
10
7
20
14
Source: Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmosphere
Water vapor (g/kg)
Saturation water vapour content of air
Temperature (oC)
Crucial relationship
for understanding the power of convection, e.g. in thunderstorms,
in particular in the tropics -> hurricans
GY
Atmospheric Moisture and Precipitation
Relative humidity
Ratio of the air’s actual water vapour content relative to the amount of
water vapour at saturation at the same temperature
Varies with:
• the amount of water vapour in the air
• temperature, as temperature changes saturation water vapour content
GYk
Atmospheric Moisture and Precipitation
Relative humidity at constant temperature
Source: Lutgens, F.K.
and E.J. Tarbuck, 1998.
The Atmosphere
GY
Atmospheric Moisture and Precipitation
Relative humidity at different temperatures
Dew
point!
Source: Lutgens, F.K.
and E.J. Tarbuck, 1998.
The Atmosphere
Gk
Atmospheric Moisture and Precipitation
GY1
Atmospheric Moisture and Precipitation
Water vapour pressure
• Water vapour pressure - partial pressure exerted by the water vapour
in the atmosphere
Pressure?
GY
Atmospheric Moisture and Precipitation
The Ideal Gas Law
Relationship between pressure, volume and temperature
Air molecules
Density
Mass per volume [kg/m3]
Amount of matter per volume
<-> heavy/light
in low atmosphere high density
further up lower density
G
Air density
Air
pressure
Low
High
Atmospheric Moisture and Precipitation
Pressure
the lower in the atmosphere,
the more air above,
the larger the weight of air above,
the larger the air pressure
GYk
Atmospheric Moisture and Precipitation
The Ideal Gas Law
pV
-------- = K
T
GY1
p - pressure
V - volume
T - temperature
K - a constant
Atmospheric Moisture and Precipitation
Water vapor pressure and
saturation
• Water vapour pressure partial pressure exerted by
the water vapour in the
atmosphere
• Saturation vapour pressure
- pressure exerted by air
saturated with water
20oC
Pressure
gauge
H2O vapour
Dry
air
20oC
• Saturation water vapour
pressure is temperature
Saturated
dependent as at higher temperatures more water evaporates <-> more molecules Evaporation and condensation
have higher energy
occurring at the same rate
GY
20oC
Moist
air
30oC
Atmospheric Moisture and Precipitation
Lifting processes and precipitation
condensation level
(b) Orographic lifting
(a) Convective lifting
Warm air
Cold air
(c) Fontral wedging
GY1003 - Principles of Physical Geography, Lecture 14, Jörg Kaduk
Converging winds
(d) Convergence
Converging winds
Atmospheric Moisture and Precipitation
Convective lifting & cloud formation
• Air at surface becomes less dense
(Warming from surface (Ideal Gas Law))
or increasing water vapour content)
• If air is less dense than air above, air
tends to rise (instability)
• Air rises
• Air expands due to decreasing pressure
(Ideal Gas Law)
• Air cools (Ideal Gas Law) - Cooling rate
(no condensation): Dry adiabatic lapse rate
• Air can rise further - depending on surroundings
• Condensation might occur due to cooling, if dew point was reached
• Condensation releases heat
• air warms, expands, density drops, air rises, cools, further water vapour
condenses,... Cooling rate with condensation: Wet adiabatic lapse rate
GY1k
Atmospheric Moisture and Precipitation
Summary
• Changes of state of water vapour interact with many processes
• Wet air less dense than dry air
• Evapotranspiration - total water flux from surface to atmosphere
• Drives plant water uptake, source for all precipitation
• Saturation water vapour content depends strongly on temperature
• Ideal gas law relating temperature, pressure and volume
• Atmospheric stability modified through reducing the density of near
surface air due to warming or increasing water vapour concentration
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