Transcript Understanding Weather and Climate Ch 5

Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 2. Water in the
Atmosphere
Chapter 5.
Atmospheric Moisture
Introduction
Over 70% of the planet is covered by water
Water is unique in that it can simultaneously exist in all
three states (solid, liquid, gas) at the same temperature
Water is able to shift between states very easily
The hydrologic cycle refers to the regular cycle of water
through the earth-atmosphere system
Liquification of water occurs frequently at normal Earth
temperatures
Occurs when air is saturated with respect to water vapor
The addition of water vapor, or the lowering of temperature, in
saturated air will lead to condensation
Evaporation
Occurs if energy is available to a water surface
Water vapor increases in air as surface water evaporates
Upon saturation, condensation will begin and water will return to
the surface
Saturation marks an equilibrium between evaporation and
condensation
Saturation may occur in the presence or not in the presence of dry
air, so that the statement that air “holds” water is erroneous
Changes of state may also occur with regard to water vapor
changing directly to ice
• Deposition
Or, the inverse situation
• Sublimation
Indices of Water Vapor Content
Humidity indicates the amount of water vapor in air
Humidity expressed through a variety of ways
Each has advantages and disadvantages
All indices refer solely to water vapor and exclude liquid and
frozen states
Vapor Pressure
Simply the amount of pressure exerted on the atmosphere by water
vapor
Dependent upon both temperature and density of the vapor with
density most important
The movement of water vapor molecules
exerts vapor pressure on surfaces
The maximum water vapor pressure which can occur is termed
saturation vapor pressure
Saturation vapor pressure is solely temperature dependent
It exponentially increases with temperature such that high
temperatures may have extremely high saturation vapor pressures
compared to lower temperatures
Exponential increase in saturation
vapor pressure with increase in
temperature
Absolute Humidity
Indicates the density of water vapor expressed in g/m3
Changes as air volume changes
Specific Humidity
Represents a given mass of water vapor per mass of air in g/kg
Term does not vary with air volume fluxes
Does not change with temperature changes
Saturated air has the highest specific humidity for a given
temperature and pressure = saturation specific humidity
Mixing Ratio
Very similar to specific humidity in that it expresses the mass of
water vapor relative to air mass
However, mixing ratio expresses the amount of water vapor
relative only to a mass of dry air
Maximum mixing ratio = saturation mixing ratio
Relative Humidity
Most commonly used expression of water vapor content
Indicates the amount of water vapor in the air relative to the
possible maximum
Given as a percentage
Does not indicate the amount of air which is water vapor but
instead describes the amount present relative to a saturation point
The saturation point, thus the relative humidity term, is relative to
air temperature and total water vapor present
More water vapor can exist in warm air than cold, the term is
sometimes misleading
An example involves the diurnal distribution of RH in which the
highest RH occurs in the morning during the coolest time of the
day
The lowest RH values will be recorded in late afternoon, the time
of greatest air temperature
• This makes high temperature/high relative humidities (90oF, 90% RH,
or so) impossible
Because of temperature dependency the term cannot be used to
compare moisture content at different locations having different
temperatures
The relationship between RH and temperature
Dew Point
The dew point temperature is the temperature at which saturation
occurs in air
Reached either by increasing water vapor content or by chilling air
(while holding moisture content constant)
Good indicator of moisture content in air
• Relatively high dew points indicate abundant atmospheric moisture
Dew points can be only equal or less than air temperatures
If saturation is reached and air temperatures cool further, water
vapor is removed from the air through condensation
When air reaches saturation at temperatures below freezing the
term frost point is used
Dew point/temperature relationships in a) unsaturated air
b) and c) saturated air
Methods of Achieving Saturation
Air may become saturated through the addition of water vapor to
air at a constant temperature
• Example: light fogs formed beneath clouds as vapor is added through
falling raindrops
Or by mixing cold air with warm, moist air
• Example: Contrails and steam fogs which develop as cold air passes
over warm water bodies
Or by cooling air to the dew point
• The most common way
Effects of Curvature and Solution
Condensed water suspended in the atmosphere is typically curved
Impurities also exist
Both factor into phase shifts
Effect of Curvature
Small drops exhibit greater curvature than larger ones
Curvature influences saturation vapor pressure with highly curved
drops requiring RHs in excess of 100% to remain liquid
For very small drops, supersaturation may approach 300%
Hygroscopic aerosols acting as condensation nuclei help keep RHs
below these extremes
Condensation onto such particles, called heterogeneous
nucleation, causes dissolution of the aerosol
Larger drops have less
curvature than smaller
ones
Small droplets require
higher RHs to remain
liquid
Effect of Solution
Evaporation from solutions is less than from pure water
This directly opposes curvature influences such that condensation
typically occurs at RHs near 100%
Hygroscopic nuclei abound in the atmosphere from many natural
(salt, dust, ash, etc.) sources and anthropogenic (combustion
derivative) sources
Very small condensation nuclei lead to very tiny water drops =
haze
Ice Nuclei
Atmospheric water does not freeze at 0oC (32oF)
Leads to the presence of supercooled water
Ice crystal formation requires ice nuclei
• A rare temperature dependent substance similar in shape to ice
• Examples: clay, ice fragments, bacteria, volcanics, etc.)
Ice nuclei become active at temperatures below -4oC
Between -10o and -30oC (14-22oF), saturation may lead to ice
crystals, supercooled drops, or both
Below -30oC, clouds are composed solely of ice crystals
At or below -40oC (-40oF) spontaneous nucleation, the direct
deposition of ice with no nuclei present, occurs
Measuring Humidity
The easiest way to measure humidity is through use of a sling
psychrometer
• A pair of thermometers one of which has a wetted cotton wick
attached to the bulb
The two thermometers measure the wet and dry bulb temperature
Swinging the psychrometer causes air to circulate about the bulbs
When air is unsaturated, evaporation occurs from the wet bulb
which cools the bulb
Once evaporation occurs, the wet bulb temperature stabilizes
allowing for comparison with the dry bulb temperature
The wet bulb depression is found with a greater depression
indicative of a dry atmosphere
Charts gauge the amount of atmospheric humidity
Aspirated and hair hygrometers are alternatives
High Humidities and Human Discomfort
Temperature extremes account for more fatalities than severe
storms, of all types, combined
High temperature extremes are compounded by humidity (and
other factors such as wind and intensity of sunlight)
The effect of humidity and high temperatures can be expressed in a
heat index
Humans are cooled by the release of perspiration which cools the
body by evaporating into air
When the atmosphere has a high moisture content, the rate of
evaporation is effectively reduced
This leads to a reduction in the cooling power of perspiration
This increases the apparent temperature of the air leading to heat
related health risks
• Muscle cramps, heat exhaustion, heat stroke (potentially fatal)
Cooling Air to the Dew or Frost Point
Most condensation processes occur as air is chilled to the dew
point
Air temperature changes either from direct energy exchanges
(diabatic processes) or from those involving no net energy
exchange (adiabatic processes)
Diabatic Processes
Involve the direct addition or removal of heat energy
• Example: Air passing over a cool surface loses energy through
conduction
Energy is always transferred from areas of high temperature
toward those of lower temperatures
• The Second Law of Thermodynamics
Diabatic processes are typically associated with fog development
Adiabatic Processes
Cloud formation typically involves temperature changes with no
net exchange of energy
Such processes occur according to the First Law of
Thermodynamics
Rising air expands through an increasingly less dense atmosphere
causing a decrease in internal energy and a corresponding
temperature decrease
Parcels expand and cool at the dry adiabatic lapse rate
• 1oC/100 m (5.5oF/1000 ft)
Sinking parcels experience exactly proportional compression
warming
Parcels may eventually reach the lifting condensation level, the
height at which saturation occurs
Parcels then cool at the saturated adiabatic lapse rate
• ~0.5oC/100 m (3.3oF/1000 ft)
Dry adiabatic
cooling
The Environmental Lapse Rate
The environmental (ambient) lapse rate (ELR) refers to an overall
decrease in air temperature with height
This rate, which changes diurnally from place to place, stems from
the fact that air located farther from surface heating is typically
cooler than that nearer the surface
A comparison of adiabatic and
environmental cooling rates
Forms of Condensation
Many forms of either liquid or solid condensation can occur
depending on particular process characteristics
Dew
Liquid condensation on surface objects
Diabatic cooling of surface air typically takes place through
terrestrial radiation loss on calm, cool, clear nights
Surface air becomes saturated and condensation forms on objects
acting as condensation nuclei
Frost
Similar to dew except that it forms when surface temperatures are
below freezing
Deposition occurs instead of condensation
May be referred to as white or hoar frost
Frozen Dew
Occurs when normal dew formation processes occur followed by a
drop in temperature to below freezing
Ensures a tight bond between ice and the surface
Causes “black ice” on roadways
Fog
Simply a surface cloud when air either cools to the dew point, has
moisture added, or when cooler air is mixed with warmer moister
air
Radiation Fog
Occurs when near surface air chills diabatically to saturation
through terrestrial radiation loss on clear cool nights
Require a slight breeze to vertically mix air through a shallow
column
Dew and Frost
If winds exceed about 5km/hr (3 mph) warmer air from aloft will
mix with the near surface air and evaporate the fog
After sunrise, the fog evaporates from below due to surface heating
Radiation fog in
the Central Valley
of California
Advection Fog
Occurs when warm moist air moves across a cooler surface
Air is chilled diabatically to saturation
Common on the U.S. west coast as warm, moist air from the
central Pacific advects over the cold California ocean current
Frequently develop near boundaries of opposing ocean
temperatures
• Example: Off the northeast coast of the U.S.
Upslope Fog
The only fog developed through adiabatic cooling
Occur when air is advected over land surfaces which increase in
elevation
A common occurrence in the Great Plains of the U.S. where warm,
moist air advects from the Miss. River Valley towards the Rocky
Mountains
Different types of fog found throughout the U.S.
Formation and Dissipation of Cloud Droplets
Clouds are mainly associated with adiabatic cooling of rising air
Dew points decrease as air rises at the shallow dew point lapse rate
• 0.2oC/100 m (1.1oF/1000 ft)
Approximately 50 m above the lifting condensation level, all
condensation nuclei have condensed water attached
Leads to additional growth of those drops over the creation of new
drops
Process soon stops leaving drops to slowly evaporate or sublimate
End of Chapter 5
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt