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Atmospheric Moisture and Precipitation
GY1003 - Principles of Physical Geography
Autumn 2005
Lecture 14
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
Toand why it rains
To understand convection
Objectives
• 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
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric Moisture and Precipitation
Outline
Introduction
• 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
• Evaporation, condensation
• Transpiration, evapotranspiration
• Energy supply for evapotranspiration
• Latent heat
Humidity
• Vapour pressure
• Saturation
• Mixing ratio and absolute humidity
• Dewpoint
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
• Orographic lifting
• Convective lifting
• Frontal wedging
• Convergence
Convection
Required reading
Smithson, D.P., K. Addison, and K. Atkinson, 2002. Chap. 4, p56-61, 65-72, chap 5, p73,
90-94
Module web site
http://www.geog.le.ac.uk/staff/jk61/teaching/GY1003/
Contact
Office: Bennett Building F63b
Office hours: Tuesday, 16.30-17.30, Wednesday 13.30-14.30 and by appointment, Tel.: (0116) 252 3848
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric 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
• 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
• 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
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric Moisture and Precipitation
Bullets
Introduction
• 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
• 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
• 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 evap-
oration 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 tempera-
tures 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
• 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
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric Moisture and Precipitation
• Temperature dependent -> can be changed by changing water content or by chang-
ing 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
• 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
o• If air is lifted: dry adiabatic rate: 10 C 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
o• Wet adiabatic rate depends on the amount of moisture in the air: 5 C for air with high
oto 9 C 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
• Lifting processes are important as they can force air up to a level where the temper-
ate 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
• http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml
• http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter4/deluxe.html
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric Moisture and Precipitation
Water’s changes of state
Most important
for climate:
Heat energy absorbed
Sublimation
•Energy transfer
•Air density
•Air stability
Evaporation
Melting
•Circulation
Condensation
Freezing Liquid
Ice
•Transpiration
Vapour
•Precipitation
Deposition
Heat energy released
Source: Ahrens, C.D., 1994. Meteorology Today
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric 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
o
(held at 0 C)
Source:
Lutgens, F.K. and E.J. Tarbuck, 1998. The Atmos-
phere
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric Moisture and Precipitation
Air density and water vapour concentration
Wet air is less dense than dry air
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 is independent of the type of gas.
Same temperature and pressure!
(T & P do change distance between
molecules!)
The molecular weights of O (red), N (blue) and H O are about 32, 28,
2 2 2
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.
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric 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
GY1003 - Principles of Physical Geography, Lecture 14, Jörg KadukAtmospheric 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 con-
tainer an equilibrium between evaporation and condensation will form
Dew Point Temperature (Dew Point)
Temperature at which air would be saturated (dew formation would start)
GY1003 - Principles of Physical Geography, Lecture 14, Jörg Kaduk

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