Transcript Ch06Pres - UK Ag Weather Center

Weather Studies
Introduction to Atmospheric Science
American Meteorological Society
Chapter 6
Humidity, Saturation, and Stability
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants
Case-in-Point
 Cloud forests are forests that are perpetually
shrouded in clouds or mist
– They are found from 2000-3000 m (6500-9800 ft) in
elevation in the tropics and subtropics
 Onshore and upslope winds that are warm and
humid supply the moisture
– Warm air blowing upslope cools through expansion
– Expansional cooling raises the relative humidity to
saturation and water vapor condenses into low clouds
and fog
– The tree canopy strips moisture from the clouds and this
water drips to the forest floor
 Deforestation reduces available moisture and
raises air temperature → clouds form less readily
and at higher elevations
2
Case-in-Point
 If global warming translates into higher sea
surface temperatures (SSTs) in the tropics,
cloud forests could be affected
– Air flowing onshore would be warmer
– Greater ascents would be required to produce
clouds
– Clouds would be at higher elevations
 Perhaps even lift off the mountains
– Cloud forests are extremely sensitive to climate
variations
 They may prove to be early indicators of effects of
global-scale climate change
3
Driving Question
 How does the cycling of water in the Earthatmosphere system help maintain a
habitable planet?
– This chapter will tell us:
 How the global water cycle functions
– Especially as it relates to transference between
the Earth’s surface and the atmosphere
 How to quantify the water content of air
 How air becomes saturated through uplift and
expansional cooling
 How atmospheric stability affects the ascent of
air
4
Global Water Cycle
 Assumption – the amount of water in the Earthatmosphere system is neither increasing or
decreasing
– Internal processes continually generate and break down
water molecules
 Volcanoes and meteors (minute amount) add water
 Photodissociation of water vapor and chemical reactions break
down water molecules
– Fixed quantity of water in Earth-atmosphere system is
distributed in 3 phases among various reservoirs, mostly
the ocean (97.2%) and ice sheets and glaciers (2.15%)
– The sun powers the global water cycle and gravity
keeps water from escaping to space, causing water to
fall from the sky as precipitation and flow to oceans
5
The Global Water Cycle
6
Where is the Water Stored?
Note the small
percentage of the
total water that is
stored in the
atmosphere.
Even though small
in percentage, this is
vital to weather
processes
7
The Global Water Cycle
 Transfer processes
1. Phase changes
 Evaporation – more molecules enter the atmosphere as vapor
then return as liquid to the water surface
 Condensation – more molecules return to the water surface as
liquid then enter the atmosphere as vapor
 Transpiration – Water that is taken up by plant roots escapes as
vapor from plant pores
– Evapotranspiration is the total of evaporation and
transpiration
 Sublimation – ice or snow become vapor without first becoming
liquid
 Deposition - water vapor becomes solid without first becoming
liquid
 All 3 phases of water exist in the atmosphere
2. Precipitation
 Rain, drizzle, snow, ice pellets, and hail
8
Percent of Precipitation Originating
from Land Sources
Ocean evaporation is the origin of most precipitation.
9
Pathways
Taken by
Precipitation
Falling on Land
10
The Global Water Budget
Via precipitation and evaporation, the ocean has a net
loss of water and the land has a net gain.
11
How Humid is it?
 Humidity describes the amount of water vapor in
the air
– This varies with time of year, from day-to-day, within a
single day, and from place-to-place
– Humid summer air, and dry winter air cause discomfort
 Ways of measuring humidity:
–
–
–
–
–
–
–
Vapor pressure
Mixing ratio
Specific humidity
Absolute humidity
Relative humidity
Dewpoint
Precipitable water
12
How Humid is it?
 Vapor pressure
– Water vapor disperses among the air molecules and
contributes to the total atmospheric pressure
 This pressure component is called the vapor pressure
 Mixing ratio
– Mass of water vapor per mass of the remaining dry air
 Expressed as grams of water vapor per kilograms of dry air
 Specific humidity
– Mass of the water vapor (in grams) per mass of the air
containing the vapor (in kilograms)
 In this case, the mass of the air includes the mass of the water
vapor
 Mixing ratio and specific humidity are so close
they are usually considered equivalent
13
How Humid is it?
 Absolute humidity
– The mass of the water vapor per unit volume of humid
air; normally expressed as grams of water vapor per
cubic meter of air
 Saturated air
– This is the term given to air at its maximum humidity
– A dynamic equilibrium develops where the liquid water
becomes vapor at the same rate as vapor becomes
liquid
– “Saturation” may be added to various humidity terms
 Saturation vapor pressure, saturation mixing ratio, saturation
specific humidity, saturation absolute humidity
– Changing the air temperature disturbs equilibrium
temporarily
 Example: heating water increases kinetic energy of water
molecules and they more readily escape the water surface as
vapor. If the supply of water is sufficient, a new dynamic
equilibrium is established with more vapor at higher temp.
14
Variations with Air Temperature of
Vapor Pressure
Saturation Mixing Ratio
15
How Humid is it?
 Relative humidity
– Probably the most familiar measure
– Compares the amount of water vapor present to the
amount that would be present if the air were saturated
– Relative humidity (RH) can be computed from vapor
pressure or mixing ratio
 RH = [(vapor pressure)/ (saturation vapor pressure)] x 100
 RH = [(mixing ratio)/(saturation mixing ratio)] x 100
– At constant temperature and pressure, RH varies directly
with the vapor pressure (or mixing ratio)
– If the amount of water vapor in the air remains constant,
relative humidity varies inversely with temperature
 See next slide
16
The Relationship of Relative
Humidity to Temperature
17
How Humid is it?
 Dewpoint
– The temperature to which the air must be cooled
at constant pressure to reach saturation
 At the dewpoint, air reaches 100% relative humidity
 Higher with greater concentration of water vapor in air
 With high relative humidity, the dewpoint is closer to the
current temperature than with low relative humidity
– Dew is small drops of water that form on surfaces
by condensation of water vapor
– If the dewpoint is below freezing, frost may form
on the colder surfaces through deposition
 Dewpoints below freezing are sometimes referred to as
frostpoints
18
How Humid is it?
 Precipitable water
– The depth of the water
that would be produced if
all the water vapor in a
vertical column of
condensed into liquid
water
 Condensing all the water
vapor in the atmosphere
would produce a layer of
water covering the entire
Earth’s surface to a depth of
2.5 cm (1.0 in.)
– Highest in the tropics
Map of precipitable water
at various locations
19
Monitoring Water Vapor
 Humidity instruments
– Hygrometer
 Measures the water vapor concentration of air
– Dewpoint hygrometer
 Uses a temperature-controlled mirror and an infrared beam
– When the mirror temperature reaches a point that
condensation forms, the reflectivity of the mirror is changed,
altering the reflection of the beam. The temperature is
recorded as the dewpoint.
 These are common at NWS forecast stations
– Hair hygrometer
 Relates changes in length of a humid hair to humidity – hair
lengthens as relative humidity increases
– Hygrograph
 Provides a record of humidity variations over time
– Electronic hygrometer
 Based on changes in resistance of certain chemicals as they
absorb or release water vapor to the air
20
Monitoring Water Vapor
The temperature/dewpoint sensor (hygrothermometer) used in the
NWS Automated Surface Observing System (ASOS)
21
Monitoring Water Vapor
 Sling psychrometer
– Wick is wetted in distilled water
– Instrument is ventilated by whirling
– Wet-bulb and dry-bulb temperatures are
recorded
– Dry bulb – actual air temperature
– Water vapor vaporizes from the wick as it
is whirled and evaporated cooling lowers
the temp. to the wet-bulb temperature
– Important to remember – use the
depression of the wet bulb on the chart
 This is the difference between the wet
and dry bulb temperatures
 Aspirated psychrometers do the same
thing, but use a fan instead whirling
22
Monitoring Water Vapor
The difference between the dry-bulb temperature and the wet-bulb
temperature, known as the wet bulb depression, is calibrated in
terms of percentage relative humidity on a psychrometric table.
23
Monitoring Water Vapor
The dewpoint can be obtained from measurements of the dry-bulb
temperature and the wet-bulb depression.
24
Monitoring Water Vapor
 Water vapor satellite
imagery
– IR imagery using
infrared wavelengths
that detect water vapor
Water vapor imagery indicates presence
of water vapor above 3000 m (10,000 ft)
The whiter the image, the greater the
moisture content of the air
This image shows moisture plumes
extending from the Pacific Ocean into the
central U.S. and in the southeastern U.S.
from the Gulf of Mexico and Atlantic
Ocean
25
How Air Becomes Saturated
 As relative humidity nears 100%, condensation or
deposition becomes more likely
 Condensation or deposition will form clouds
– Clouds are liquid and/or ice particles
 Humidity increases when:
– Air is cooled; saturation vapor pressure decreases while
actual vapor pressure remains constant
– Water vapor is added at a constant temperature; vapor
pressure increases while saturation vapor pressure
remains constant
 As ascending saturated air (RH about 100%)
expands and cools, saturation mixing ratio and
actual mixing ratio decline and some water vapor
is converted to water droplets or ice crystals
26
How Air Becomes Saturated
 Adiabatic process and lapse rates (review from
Chapter 5)
– During an adiabatic process, no heat is exchanged
between the air parcel and its environment
– Expansional cooling and compressional heating of
unsaturated air are referred to as adiabatic processes if
no heat is exchanged with surroundings
– Air cools adiabatically as it rises
 Lower pressure with altitude allows the air to expand
 Unsaturated ascending air cools at 9.8° C/1000 m (5.5° F/1000 ft)
and it warms at the same rate upon descent.
– This is called the dry adiabatic lapse rate
– Upon saturation, air continues to cool, but at the
moist adiabatic lapse rate of 6° C/1000 m (3.3° F/1000
ft) → rate is lower because latent heat released upon
condensation partially offsets cooling as parcel rises
27
Atmospheric Stability
 Air parcels are subject to buoyant forces caused
by density differences between the surrounding air
and the parcel itself
 Atmospheric stability is the property of ambient air
that either enhances (unstable) or suppresses
(stable) vertical motion of air parcels
– In stable air, an ascending parcel becomes cooler and
more dense than the surrounding air
 This causes the parcel to sink back to its original altitude
– In unstable air, an ascending parcel becomes warmer
and less dense than the surrounding air
 This causes the parcel to continue rising
28
Stable Air
 Note that movement of the
parcel upward means it is
colder than the
surrounding air, so it sinks
back down to its original
altitude
 Also, in movement of the
parcel downward, it
becomes warmer than the
surrounding air, and
returns to its original
altitude
 Stable air inhibits vertical
motion
29
Unstable Air
 Note that movement of the
parcel upward means it is
warmer than the
surrounding air, so it
continues rising.
 Also, in movement of the
parcel downward, it
becomes colder than the
surrounding air, and
continues descending
 Unstable air enhances
vertical motion
30
Atmospheric Stability
 Soundings
– These are the temperature profiles of the ambient air
through which air parcels are moving
– Soundings (and hence stability) can change due to:
 Local radiational heating and cooling
– At night, cold ground cools and stabilizes the overlying air
– During day, warm ground warms and destabilizes the overlying air
 Air mass advection
– Air mass is stabilized as it moves over a colder surface
– Air mass is destabilized as it moves over a warmer surface
 Large-scale ascent or descent of air
– Subsiding air generally becomes more stable
– Rising air generally becomes less stable
31
Atmospheric Stability
 Absolute instability
– Occurs when the air temperature is dropping more
rapidly with altitude than the dry adiabatic lapse rate
(9.8° C/1000 m)
 Conditional instability
– Occurs when the air temperature is dropping with
altitude more rapidly than the moist adiabatic lapse rate
(6° C/1000 m), but less rapidly than the dry adiabatic
lapse rate
– Air layer is stable for unsaturated air parcels and
unstable for saturated air parcels
– Implies that unsaturated air must be forced upwards in
order to reach saturation
32
Atmospheric Stability
 Absolute stability
– Air layer is stable for both unsaturated and saturated air
parcels and occurs when:
 Temperature of ambient air drops more slowly with altitude than
moist adiabatic lapse rate
 Temperature does not change with altitude (isothermal)
 Temperature increase with altitude (inversion)
 Neutral air layer
– Rising or descending parcel always has same
temperature as ambient air
– Neither impedes nor spurs upward or downward motion
of air parcels
33
Atmospheric Stability
34
Stüve Diagrams
Temperature – Horizontal axis, increasing from left to right
Pressure – vertical axis, decreasing upward
35
Lifting Processes - Convective
Lifting
36
Lifting Processes - Frontal Lifting
 Frontal uplift occurs where contrasting air masses
meet – leads to expansional cooling of rising air,
and possible cloud and precipitation development
 Warm front – as a cold and dry air mass retreats,
the warm air advances by riding up and over the
cold air
– The leading edge of advancing warm air at the Earth’s
surface is the warm front
 Cold front – cold and dry air displaces warm and
humid air by sliding under it and forcing the warm
air upwards
– The leading edge of advancing cold air at the Earth’s
surface is the cold front
37
Lifting Processes –
Orographic Lifting
38
Lifting Processes –
Convergent Lifting
 When surface winds converge, associated upward
motion leads to expansional cooling, increasing
relative humidity, and possible cloud and
precipitation formation
 For example, converging winds are largely
responsible for cloudiness and precipitation in a
low-pressure system
 In another example, converging sea breezes
contribute to high frequency of thunderstorms in
central Florida
39