Transcript 06_HumiditySaturationStability

NAS 125: Meteorology
Humidity, Saturation, and Stability
Cloud forests, part 1
• Some tropical and subtropical forests are perpetually
enshrouded in clouds and mist; these are called cloud
forests.
• Cloud forests play an important role in terms of
biodiversity and water supply, but they are among the
most threatened ecosystems in the world as a result of
development pressures as well as possible climate
change.
– There are 605 cloud forests in 41 nations, primarily in
Central and South America.
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Cloud forests, part 2
• Cloud forests are typically found at elevations
ranging from 2,000 m to 3,500 m, but may be found
at lower elevation on humid islands.
• Onshore, upslope flow of warm, moist air helps fuels
the low clouds, fog, and mist characteristic of cloud
forests.
– Air becomes saturated with moisture as it rises and cools.
– Forest canopy traps the moisture, encouraging the
formation of water droplets that drip to the forest floor.
• Trapped moisture is equivalent to 20 percent to 60 percent of the
local precipitation totals.
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Cloud forests, part 3
• Persistent cloud cover reinforces the cool, moist
conditions by reducing solar radiation reaching the
ground and in turn suppressing the evapotranspiration
of water from the surface.
• Climate change could reduce the extent of cloud
forests by increasing the amount of cooling necessary
to produce conditions favorable for condensation of
water, but could conversely increase the amount of
water vapor in the air that feeds the cloud forest
ecosystems.
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Water
• Water occurs in all three states of matter:
– Solid (snow, sleet, hail, ice);
– Liquid (rain, water droplets);
– and Gas (water vapor).
• The gaseous state is the most important driver of the
dynamics of the atmosphere.
– Changes of state of water serve an important role in
transferring energy through the Earth-atmosphere system.
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Reservoirs of water
• The primary reservoir of water is the ocean basins (97
percent).
• Two percent of water is in ice sheets and glaciers.
• About 0.6 percent is in groundwater.
• Very little water is stored in the atmosphere.
– But the atmosphere is the primary conduit for bringing
water from the oceans to the land surfaces.
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Hydrologic cycle, part 1
• Water is distributed very unevenly around Earth.
• Less than 1% of Earth’s total moisture is involved in
the hydrologic cycle.
• The hydrologic cycle is a series of storage areas
interconnected by various transfer processes, in
which there is a ceaseless interchange of moisture in
terms of its geographical location and its physical
state.
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Hydrologic cycle, part 2
• Surface-to-air water movement
– Evaporation is responsible for most of the moisture that
enters the atmosphere from Earth’s surface.
• Of the moisture evaporated, more than 84% comes from ocean
surfaces.
• The water evaporated becomes water vapor, and though it stays in
atmosphere only briefly (hours to days), it can travel a considerable
distance, either vertically or horizontally.
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Hydrologic cycle, part 3
• Air-to-surface water movement
– Water vapor will either condense to liquid water or
sublimate to ice to form cloud particles.
– Clouds drop precipitation (rain, snow, sleet, hail).
– Precipitation and evaporation/transpiration balance in time.
• They do not balance in place.
• Evaporation exceeds precipitation over oceans.
• Precipitation exceeds evaporation over lands.
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Hydrologic cycle, part 4
• Movement on and beneath Earth’s surface
– Runoff is the flow of water from land to oceans by
overland flow, streamflow, and groundwater flow.
• Runoff is why the oceans do not dry up and continents become
flooded despite the imbalance of evaporation and precipitation
through space (oceans and continents).
• Runoff water amounts to 8% of all moisture circulating in global
hydrologic cycle.
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Hydrologic cycle, part 5
• Residence times
– At any given movement, the atmosphere contains only a
few days’ potential precipitation.
– The residence time of a molecule of water can be hundreds
of thousands of years to only a few minutes.
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Evaporation, part 1
• Evaporation is the process by which liquid water is
converted to gaseous water vapor.
– Molecules of water escape the liquid surface into the
surrounding air.
• Temperature is a key factor in evaporation, both in
water and in the air around it.
– Molecules become more agitated the higher the
temperature, and this agitation leads to evaporation.
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Evaporation, part 2
• Temperature works in conjunction with pressure.
– Vapor pressure is the pressure exerted by water vapor in the
air.
• At any given temperature, there is a maximum vapor pressure that
water vapor molecules can exert.
• Saturated air: Air becomes saturated when it reaches the point at
which some water vapor molecules must become liquid because
maximum vapor pressure is exceeded.
• The warmer the air, the more water vapor it can hold before
becoming saturated.
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Evaporation, part 3
• Still Versus Moving Air
– Movement in air through windiness and/or turbulence helps
promote evaporation by removing saturated air.
• Disperses vapor molecules and thus makes air above water surface
less saturated, so rate of evaporation can increase.
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Evapotranspiration, part 1
• Evapotranspiration is the process of water vapor
entering the air from land sources.
– Evapotranspiration occurs through two ways:
• Transpiration is the process by which plant leaves give up their
moisture to the atmosphere;
• Evaporation from soil and plants.
– Most evapotranspiration occurs through plants.
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Evapotranspiration, part 2
• Potential evapotranspiration is the maximum amount
of moisture that could be lost from soil and
vegetation if the ground were sopping wet all the
time.
• Potential evapotranspiration rate and actual rate of
precipitation play a key role in determining a region’s
groundwater supply (or lack of it).
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Condensation, part 1
• Condensation is the process by which water vapor is
converted to liquid water; it is the opposite of
evaporation.
• For condensation to take place, air must be saturated.
– Condensation cannot occur, however, even if the air is
saturated, if there is not a surface on which it can take
place.
• Air becomes supersaturated if surface is not available.
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Condensation, part 2
• Air must be saturated (continued):
– In upper atmosphere, surfaces are available through
hygroscopic particles or condensation nuclei—tiny
atmospheric particles of dust, smoke, and salt that serve as
collection centers for water molecules.
• Most common are bacteria blown off plants or thrown into air by
ocean waves.
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Other state changes
• Freezing is the process by which liquid water is
converted to ice, thus giving off heat.
• Melting is the process by which ice is converted to
liquid water, thus absorbing heat.
• Sublimation is the process by which water vapor is
converted directly to ice, or vice versa.
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Global water budget
• The amount of water falling on land surfaces exceeds
the amount lost from the land surfaces to the air via
evapotranspiration or sublimation.
• The amount of water evaporating from oceans
exceeds that falling over the oceans.
• Excess water from land returns to oceans via runoff.
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Measures of humidity, part 1
• Humidity is the amount of water vapor in the air.
• Dalton’s law: The total pressure of a mixture of gases
equals the sum of the partial pressures contributed by
each gas.
– The partial pressure contributed by water vapor is called
the vapor pressure.
• Air is said to be saturated if it is at its maximum
humidity – when the amount of water molecules
leaving the liquid state to become a gas equals the
number leaving the gaseous state to become liquid.
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Measures of humidity, part 2
• Mixing ratio is the ratio of the mass of water vapor
per mass of the remaining dry air.
• Absolute humidity is a direct measure of the water
vapor content of air.
– It is expressed as the weight of water vapor in a given
volume of air, usually as grams of water per cubic meter of
air.
• The amount is a function of how much volume is being considered.
– If the volume of air doubles, the absolute humidity halves.
• Absolute humidity is limited according to temperature.
– The colder the air, the less vapor it can hold.
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Measures of humidity, part 3
• Specific humidity is a direct measure of water-vapor
content expressed as the mass of water vapor in a
given mass of air (grams of vapor/kilograms of air).
• The saturation vapor pressure of air is a function of
the temperature of air – the amount of water air can
hold increases with increasing temperature.
– Related measures:
• Saturation mixing ratio
• Saturation absolute humidity
• Saturation specific humidity
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Measures of humidity, part 4
• Relative humidity is an expression of the amount of
water vapor in the air in comparison with the total
amount that could be there if the air were saturated.
This is a ratio that is expressed as a percentage.
– Relative Humidity = Actual Water Vapor in Air/Capacity x
100
– Relative humidity changes if either the water vapor content
or the water vapor capacity of the air changes.
– Calculation of relative humidity:
• RH = (vapor pressure/saturation vapor pressure) * 100
• RH = (mixing ratio/saturation mixing ratio) * 100
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Measures of humidity, part 5
• Relative humidity (continued):
– Relative humidity also changes if temperature changes.
• The relationship between temperature and relative humidity is one
of most important in all meteorology.
• Relative humidity and temperature have an inverse relationship – as
one increases, the other decreases.
• Relative humidity can be determined through the use of a
psychrometer.
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Measures of humidity, part 6
• Cooling is the most common way that air is brought
to the point of saturation and condensation.
– Dew point is the temperature to which air must be cooled to
achieve saturation.
• Dew refers to water droplets that form when water vapor condenses
on a cold surface.
– Frost point is the air temperature at which frost forms.
• Frost refers to ice crystals that form when air becomes saturated at
a temperature below freezing.
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Measures of humidity, part 7
• Precipitable water is the depth of liquid water that
would be formed if all water vapor in a column of air
condenses.
– All water in the atmosphere would reach 2.5 cm
– Precipitable water ranges from more than 4.0 cm in the
humid tropics to 0.5 cm in polar regions.
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Measurement instruments, part 1
• A hygrometer measures the water vapor concentration
of air.
– A dewpoint hygrometer air passes over the surface of a
metallic mirror that is cooled electronically; the
temperature at which a condensation film forms on the
mirror is recorded as the dewpoint temperature.
– A hair hygrometer uses human hair, which lengthens as it
absorbs water and shortens as it dries out; hair is connected
to a dial calibrated to read in percent relative humidity.
• A hydrograph uses a hair hydrograph and it traces the trend in
relative humidity on paper pulled across a clock-driven drum.
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Measurement instruments, part 2
• Hygrometer (continued):
– Electronic hygrometers measure the change in electrical
resistance of certain materials as they respond to changing
relative humidity.
• Psychrometers measures temperature with two
identical liquid-in-glass thermometers.
– One has a dry bulb; it measures actual air temperature (drybulb temperature).
– One has a bulb wrapped in muslin, that is in turn
moistened; it measures the wet-bulb temperature.
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Measurement instruments, part 3
• Psychrometers (continued):
– The psychrometer can be either whirled around by hand (a
sling psychrometer) or it can be ventilated by a small fan
(an aspirated psychrometer).
– The drier the air, the greater the evaporation from the wet
bulb thermometer, thus the cooler the wet-bulb
temperature.
• The difference between the air temperature and the wet-bulb
temperature is called the wet-bulb depression.
– The relative humidity is looked up on a chart using the drybulb temperature and wet-bulb depression.
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Measurement instruments, part 4
• Psychrometers (continued):
– It is difficult to get accurate wet-bulb temperatures under
cold or subfreezing conditions as well as under very dry
conditions.
– The dry-bulb temperature and wet-bulb temperature can be
used to look up the dewpoint temperature.
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Water vapor satellites
• Water vapor satellite images use special infrared
sensors to measure water vapor in the atmosphere.
– Water vapor does not appear on visible or on conventional
infrared sensors.
– Such images allow meteorologists to track movement of
plumes of moisture through the atmosphere.
– Current platforms measure clouds and water vapor
concentrations above 3,000 m.
– A gray scale is used, so that little or no water vapor appears
black, while high concentrations appear milky white.
Upper-level clouds appear as bright white blotches.
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Volume and temperature
• The principal way that clouds form in the atmosphere
is by expansional cooling.
– When a parcel of warm air rises, the pressure upon it is
less, so the parcel expands; according to the ideal gas law,
temperature of a gas decreases as the volume increases.
• Conversely, descending air dries out by
compressional warming.
– When a parcel of air descends, the pressure upon it
increases, so the parcel contracts; according to the ideal gas
law, the temperature of a gas increases as the volume
decreases.
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Adiabatic processes, part 1
• Adiabatic cooling or warming of parcels of air occurs
without any heat exchanged between the parcel and
the surrounding environment.
• The dry adiabatic lapse rate is the rate at which a
parcel of unsaturated air cools as it rises; this rate is
relatively steady (10 °C/km).
– Air is not necessarily “dry,” just not saturated.
– Descending air warms, and it does so at the dry adiabatic
lapse rate.
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Adiabatic processes, part 2
• Lifting condensation level (LCL) is the altitude at
which rising air cools. sufficiently to reach 100%
relative humidity at the dew point temperature, and
condensation begins.
• The saturated adiabatic lapse rate is the diminished
rate of cooling, which occurs when air rises above the
lifting condensation level. It depends on temperature
and pressure, but averages about 6 °C/km.
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Buoyancy of air, part 1
• Buoyancy is the tendency of an object to rise in a
fluid.
– A parcel of air moves vertically until it reaches a level at
which the surrounding air is of equal density (equilibrium
level).
• Stability
– Stable air resists vertical movement; it is nonbuoyant, so it
will not move unless force is applied.
– Unstable air is buoyant; it will rise without external force
or will continue to rise after force is removed.
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Buoyancy of air, part 2
• Stability (continued):
– Conditional instability is intermediate condition between
absolute stability and absolute instability; it occurs when an
air parcel’s adiabatic lapse rate is somewhere between the
dry and wet adiabatic rates. Conditionally unstable air acts
like stable air until an external force is applied; when
forced to rise, it may become unstable if condensation
occurs (which releases latent heat that provides buoyancy).
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Buoyancy of air, part 3
• Determining air stability
– Air stability is related to adiabatic temperature changes.
– Accurate determination of stability of any mass of air
depends on temperature measurements, but one can get a
rough indication from looking at cloud patterns.
• Unstable air is associated with distinct updrafts, which are likely to
produce vertical clouds.
• Cumulous clouds suggest instability.
• Towering cumulonimbus clouds suggest pronounced instability.
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Buoyancy of air, part 4
• Determining air stability (continued):
– Accurate determination of stability (continued):
• Horizontally developed clouds, most notably stratiform,
characterize stable air forced to rise.
• Cloudless sky indicative of stable, immobile air.
– Soundings (temperture profiles) of the atmosphere are
useful tools in determination of the stability of air.
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Stüve diagrams, part 1
• Soundings and adiabatic processes can be illustrated
graphically with Stüve diagrams.
• Characteristics of Stüve diagrams
– X-axis: temperature (increasing to right)
– Y-axis: pressure (decreasing upward)
• Also altitude, based on standard atmosphere
– Dry adiabatic lapse rates denoted by straight red lines (dry
adiabats) trending from lower right to upper left
– Wet adiabatic lapse rates denoted by curved dashed lines
(wet adiabats)
– Sauration mixing ratio (slanted black lines; g/kg)
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Stüve diagrams, part 2
• Interpretation of Stüve diagrams
– Unsaturated air follows dry adiabats, or a line that parallels
the dry adiabats.
– Saturated air follows moist adiabats or a curve that more or
less parallels the moist adiabats.
• Moist adiabats become nearly indistinguishable from dry adiabats
at very low pressures and temperatures.
– Plots of soundings on a Stüve diagram enables a
meteorologist to assess atmospheric stability and the
potential for convective activity.
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Stüve diagrams, part 3
• Interpretation of Stüve diagrams (continued):
– Plotted values of the saturation mixing ratio helps
meteorologists determine relative humidity and saturation
levels of rising air parcels.
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Convection currents
• Heated air expands and moves upward in the
direction of lowest pressure. The cooler surrounding
air then moves in to fill the empty space, and the air
from above moves in to replace that cooler air. One
ends up with an updraft of warm air and a downdraft
of cool air.
– Convection currents fuel storm development.
• A similar convective system occurs in each
hemisphere during its summer and throughout the
year in the tropics.
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Lifting and precipitation, part 1
• Significant amounts of precipitation can originate
only by rising air and adiabatic cooling.
• There are four principal types of atmospheric lifting:
–
–
–
–
Convective lifting
Orographic lifting
Frontal lifting
Convergent lifting
• More often than not, the various types operate in
conjunction with one another.
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Lifting and precipitation, part 2
• Convective precipitation is showery precipitation
with large raindrops falling fast and hard; caused by
convective lifting, which occurs when unequal
heating of different air surface areas warms one
parcel of air and not the air around it.
– This is the only spontaneous of the four lifting types; the
other three require an external force.
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Lifting and precipitation, part 3
• Orographic precipitation occurs with orographic
lifting, caused when topographic barriers force air to
ascend upslope; only occurs if the ascending air is
cooled to the dew point.
– A rain shadow is an area of low rainfall on the leeward side
of a topographic barrier; can also apply to the area beyond
the leeward side, for as long as the drying influence
continues.
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Lifting and precipitation, part 4
• Frontal precipitation occurs when air is cooled to the
dew point after unlike air masses meet, creating a
zone of discontinuity (front) that forces the warmer
air to rise over the cooler air (frontal lifting).
• Convergent precipitation is showery precipitation
caused by convergent lifting, the least common form
of lifting, which occurs when air parcels converge
and the crowding forces uplift, which enhances
instability. This precipitation is particularly
characteristic of low latitudes
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