Chapter 6: Atmospheric Moisture – p. 1 of 13

Chapter 6: Atmospheric Moisture

  1. The Impact of Atmospheric Moisture on the Landscape
  1. 3 physical states of water in the atmosphere:

1.solid: snow, hail, sleet, etc

2.liquid: rain, water droplets

3.gas: water vapor (most abundant state of atmospheric moisture)

  1. gas state

1.most important in atmospheric dynamics

2.water vapor stores energy that can incite atmosphere into action

  1. The Hydrologic Cycle

A.hydrologic cycle: unending circulation of our planet’s water supply

B.liquid water (primarily from oceans) evaporates into the air  condenses to liquid or solid state  returns to Earth as precipitation

C.important determinant of climate

  1. The Nature of Water: Commonplace but Unique
  1. water
  1. most widespread substance on Earth’s surface; covers more than 70% of Earth’s surface
  2. properties:
  3. no color, no taste, no smell
  4. turns solid at 0oC/32oF; boils at 100oC/212oF
  1. The Water Molecule
  1. covalent bonds: holds 2 hydrogen atoms and 1 oxygen atom together to form water molecule (H2O)
  2. electrical polarity of water gives it many of its interesting properties, including cohesion
  1. Properties of Water
  1. Liquidity:the fact that water is liquid at the temperatures found most places on Earth enhances its versatility as active agent in the atmosphere, lithosphere and biosphere
  2. Ice Expansion: water expands as it cools from 39oF to freezing point 32oF
  3. important component of weathering
  4. ice floats: ice is less dense than water
  5. lakes freeze from the top down
  6. Surface Tension: because of electrical polarity water molecules stick together
  7. Capillarity
  8. action by which water can climb upward in restricted confinement as a result of its surface tension and adhesion
  9. enables water to circulate upward through rock, soil and the roots and stems of plants
  10. Solvent Ability

a.“universal solvent” – water can dissolve almost any substance

b.water in nature is always impure

c.moving water carries dissolved minerals and nutrients and tiny solid particles

  1. Specific Heat
  2. water has very high heat capacity: when water is warmed it can absorb an enormous amount of energy with small increase in temperature
  3. bodies of water are slow to warm up during the day or in summer and slow to cool off at night or in winter →water has moderating effect on surrounding temperature
  1. Phase Changes of Water
  1. water found naturally in 3 states on Earth: liquid, solid, gas
  1. great majority of world’s moisture in form of liquid water
  2. phase changes (Fig 6-4):

a.evaporation:

1)change of state from liquid to gas (water vapor)

2)540-600 calories absorbed

b.freezing:

1)change of state from liquid to solid

2)80 calories released

c.condensation:

1)change of state from water vapor (gas) to liquid water

2)540-600 calories released

d.melting:

1)change of state from solid to liquid

2)80 calories absorbed

e.sublimation: gas to solid (~680 calories released) or solid to gas directly (~680 calories absorbed)

  1. in each phase change, there is an exchange of latent heat energy
  1. Latent Heat

1.latent heat: energy exchanged in a phase change – the temperature of water does not change while it is undergoing a phase change

2.latent heat of melting: energy required to melt ice (80 cal/g/oC)

3.latent heat of fusion: energy released as water freezes (80 cal/g/oC)

4.latent heat of vaporization: energy required to change water from liquid to gas (540 cal/g/oC)

5.latent heat of condensation: energy released when water vapor changes back to liquid (540 cal/g)

6.latent heat of evaporation: energy required for evaporation (540-600 cal/g/oC, depending on the temperature of the water)

7.about 7 times more heat is needed to evaporate 1 gram of liquid water than is needed to melt 1 gram of ice

  1. Importance of Latent Heat in the Atmosphere

1.especially of significance between liquid and gas phase

2.evaporation

a.energy is removed from the liquid

b.temperature of remaining liquid is reduced

c.cooling process

3.condensation

a.latent heat is released

b.warming process

c.water vapor represents a “reservoir” of heat

  1. Water Vapor and Evaporation
  1. water vapor
  1. water’s gaseous state
  2. minor constituent in atmosphere
  1. variable from place to place and time to time
  2. varies from 0-4% of total atmospheric volume
  3. restricted to lower troposphere
  1. Evaporation and Rates of Evaporation

1.amount and rate of evaporation from a water surface depend on 3 factors:

  1. temperature of both the air and water
  2. amount of water vapor in the air
  3. whether the air is still or moving

2.Temperature

a.high temperature increases energy of molecules accelerates the rate of evaporation

3.Water Vapor Content of Air

  1. vapor pressure: pressure exerted by water vapor
  2. at any given air temperature there is a maximum vapor pressure that water vapor molecules can exert
  3. the higher the temperature, the higher the maximum vapor pressure
  4. saturated air

1)results when there are enough water molecules in the air to exert the maximum vapor pressure at any given temperature

2)when maximum water vapor pressure is exceeded condensation

3)evaporation takes place more rapidly when there is relatively little water vapor in air

4.Windiness

  1. dispersing of water vapor  air further from saturation  rate of evaporation increases
  2. higher temperature, greater windiness, and drier air  greater net evaporation
  1. Evapotranspiration
  1. evapotranspiration: combined process of water vapor entering the air from land sources (soil and other inanimate objects, and plants)
  1. evaporation: process by which liquid water is converted to gaseous water vapor
  2. transpiration: process by which plants give up moisture through their leaves
  1. potential evapotranspiration:
  2. amount of evapotranspiration that would occur if the ground at the location in question were sopping wet all the time
  3. maximum evapotranspiration that could result under local environmental conditions if the moisture were available
  1. water surplus: locations where precipitation exceeds potential evapotranspiration
  2. water deficit: locations where potential evapotranspiration exceeds precipitation
  1. Focus: GOES Weather Satellites
  1. GOES
  1. Geostationary Operational Environmental Satellites
  2. operated by the National Oceanic and Atmospheric Administration (NOAA)
  3. geostationary: orbits in fixed position relative to the surface of the Earth
  1. instrumentation
  1. sounder – detects vertical temperature and moisture variations within atmosphere and ozone distribution
  2. imager: sensors detect radiant and reflected electromagnetic radiation in different wavelengths
  1. products
  1. visible light images measure albedo
  2. infrared images
  1. longwave – thermal IR radiation basically shows differences in temperature
  2. warm objects (dark) emit more longwave radiation than cold objects (whiter)
  1. Measures of Humidity
  1. Absolute Humidity
  1. humidity: amount of water vapor in the air
  2. absolute humidity: amount of water vapor in a given volume of air (g/m3)
  3. the maximum possible absolute humidity (water vapor capacity) for a parcel of air is limited by the temperature
  4. warm air capable of much higher absolute humidity than cold air(Fig 6-7)
  5. if the volume of air changes, the value of absolute humidity changes
  1. Specific Humidity
  1. specific humidity: mass of water vapor in a given mass of air (g/kg)
  2. changes only if quantity of water vapor changes
  3. both absolute and specific humidity are indications of potential for precipitation in a parcel of air
  1. Vapor Pressure
  1. vapor pressure: contribution of water vapor to the total pressure of the atmosphere
  2. saturation vapor pressure: maximum possible vapor pressure at a given temperature
  3. warm air has potential to contain much more water vapor than cold air
  1. Relative Humidity
  1. relative humidity: ratio (expressed as a percentage) that compares the actual amount of water vapor in the air (absolute or specific humidity) to the water vapor capacity of the air (where capacity is the maximum amount of water vapor that can be in the air at a given temperature)
  2. describes how close the air is to saturation with water vapor
  3. relative humidity = (actual water vapor in air/capacity) x100
  4. relative humidity changes
  1. if water vapor content changes
  2. if temperature changes (water vapor capacity changes):

1)air can be brought to saturation (100% relative humidity) through a decrease in temperature with no water vapor added

2)cooling is most common way air is brought to saturation point and condensation

  1. Temperature-Relative Humidity Relationship
  1. the relationship between temperature and relative humidity is one of the most important in all of meteorology:

d.as temperature increases, relative humidity decreases

e.as temperature decreases, relative humidity increases

  1. on a typical day, relative humidity tends to be lowest in mid-afternoon and highest in the early morning (Fig 6-9)
  1. Related Humidity Concepts
  1. Dew Point Temperature
  2. dew point (temperature): temperature at which saturation is reached
  3. varies with the moisture content of the air
  4. Sensible Temperature
  1. sensible temperature: temperature as it feels to a person’s body
  2. involves

1)actual temperature

2)relative humidity

a)on warm, humid days, the air is near saturation so little evaporative cooling takes place; sensible temperature is high

b)on cold, humid days, body heat is conducted away more rapidly because of damp air; sensible temperature is low

3)wind: influences evaporation and the convecting away of body heat

  1. Focus: Water Vapor Satellite Images
  2. satellite water vapor images show regions of dry air and moist air in the atmosphere

1.longwave radiation of wavelengths 6.7 µm – 7. 3 µm (micrometers) is readily absorbed and reradiated by water vapor – high emission of radiation in this band indicates relatively large amounts of water vapor

2.darker gray shades indicate relatively dry air; lighter gray indicates relatively moist air

  1. Condensation
  2. condensation: change in state from gas to liquid

1.opposite of evaporation

2.for condensation to take place, air must be saturated

3.usually result of air being cooled to below dew point temperature

  1. hygroscopic particles/condensation nuclei: tiny atmospheric particles of dust, smoke and salt that serve as collection centers for water molecules during condensation

1.required for condensation

2.most concentrated over cities, seacoasts and volcanoes

  1. supercooled water: water that persists in liquid form at temperatures below freezing; relative humidity > 100%

1.clouds often composed of supercooled water

2.promote growth of ice particles in cold clouds

  1. Adiabatic Processes

A.only way in which large masses of air can be cooled to the dew point is by expansion as the air masses rise

B.only prominent mechanism for the development of clouds and the production of rain is adiabatic cooling

C.as air rises its pressure decreases; it expands and cools adiabatically

D.dry adiabatic rate

1.constant

2.non-saturated air

E.lifting condensation level: altitude at which air cools to the dew point, condensation begins, and clouds form

1.visible as the base of clouds

2.latent heat is released as soon as condensation begins

F.saturated (wet) adiabatic lapse rate: diminished rate of cooling (because of release of latent heat during condensation) of rising air above the lifting condensation level

1.varies but is less than the dry rate

2.averages 3.3oF/1,000 ft (6oC/km)

G.descending air warms at the dry adiabatic lapse rate(5.5oF/1,000 ft -- 10oC/km)

1.causes saturated air to become unsaturated

2.reason descending air does not make clouds

H.adiabatic temperature change only applies to air in vertical motion, air that either rising or descending (environmental or average lapse rate pertains to still air)

I.air moving up and over a mountain range: (Fig 6-14)

1.cools adiabatically as it rises on windward side

2.warms adiabatically as it descends on leeward side

3.arrives on leeward side significantly warmer and drier than it started

4.one way in which deserts are formed

  1. Clouds
  2. intro

1.clouds: collections of minute droplets of water or tiny crystals of ice

2.visible expression of condensation

3.at any given time 50% of Earth is covered by clouds

4.not all clouds precipitate, but all precipitation comes from clouds

5.important influence on radiant energy

  1. Classifying Clouds

3.classified on 2 factors

a.form

b.altitude

4.Cloud Form

a.cirriform clouds: thin wispy clouds formed of ice crystals rather than water droplets

b.stratiform clouds: grayish sheets that cover most or all of the sky

c.cumuliform clouds: massive, rounded clouds with a flat base and limited horizontal extent but often billowing upward to great heights

d.3 cloud forms are subclassified into 10 types based on shape

1)cirrus, stratus, and cumulus clouds are purely 1 form

2)the other 7 types are combinations

e.nimbus: indicates precipitation clouds

5.Cloud Families

a.cloud families based on altitude

b.high clouds

1)above 20,000’

2)thin, white, composed of ice crystals

3)cirrus, cirrocumulus, and cirrostratus

4)harbingers of approaching weather system or storm

c.middle clouds

1)6,500 – 20,000’

2)either stratiform or cumuliform

3)composed of liquid water

4)altocumulus – puffy; indicate settled weather

5)altostratus – associated with changing weather

d.low clouds

1)below 6,500’

2)often general overcast

3)stratus, stratocumulus, and nimbostratus

4)associated with drizzly rain

e.clouds of vertical development

1)grow from low bases to heights of 60,000’

2)restricted horizontal spread

3)indicate very active vertical air movement

4)cumulus – indicate fair weather

5)cumulonimbus – storm clouds

  1. Fog
  1. fog: a cloud on the ground
  2. formation:
  1. when air at Earth’s surface cools to below its dew point
  2. when enough water vapor is added to air to saturate it
  1. types of fog
  1. radiation fog: produced by condensation near the ground, where air is cooled to the dew point by contact with the colder ground; usually at night
  2. advection fog: produced by condensation that results when warm, moist air moves horizontally over a cold surface

1) most common source: air moving from sea to land

  1. upslope/orographic fog: condensation that occurs when humid air is caused to ascend a topographic slope and consequently cools adiabatically
  2. evaporation fog: condensation that results from the addition of water vapor to cold air that is already near saturation
  1. fog distribution in North America
  1. areas of heavy fog are mostly coastal
  2. radiation fog: western mountains and Appalachians
  3. areas of minimal fog

1)Southwest, Mexico, and Great Plains

2)limited available atmospheric moisture and strong winds

  1. Dew
  1. originates from terrestrial radiation
  2. nighttime radiation cools objects  adjacent air cooled by conduction  air cooled enough to reach saturation
  3. if temperature is below freezing, frost is formed
  1. People and the Environment: Global Dimming
  2. atmospheric aerosols
  3. influence weather and climate
  4. release of sulfate aerosols during 1991 Mount Pinatubo eruption lowered global temperatures by ~ 0.9o F for a year
  5. global dimming
  6. anthropogenic (human-released) aerosols
  7. global dimming: cooling of Earth’s surface and lower troposphere as a result of anthropogenic aerosols directly blocking sunlight, absorbing solar energy high in the atmosphere, and creating more reflective clouds
  8. contrails
  1. condensation trails from jet airliners can reflect radiation
  2. absence of contrails during three day grounding of all commercial airline traffic over the US after Sept 11 increased daily temperature range by > 2oF; days were hotter, nights were cooler
  3. paradoxical concern for atmospheric scientists: if human-produced global dimming has been masking some of the effects of global warming, if we continue to reduce “global pollutants” we will face higher temperatures from global warming
  1. The Buoyancy of Air
  1. Atmospheric Stability
  1. buoyancy: tendency of an object to rise in a fluid
  1. parcel of air moves vertically until it reaches a level at which the surrounding air is of equal density
  2. warm air is more buoyant than cool air

1)warm, less dense air tends to rise

2)cool, more dense air tends to sink

  1. stable air: a parcel of air that resists vertical movement
  1. stable air is non-buoyant and thus provides little opportunity for adiabatic cooling unless there is forced uplift
  2. high stability is promoted when there is a temperature inversion – when cold air is beneath warm air
  3. cold winter night is a typical, highly stable situation
  4. highly stable air is normally not associated with cloud formation and precipitation
  1. unstable air: air that either rises without any external force other than the buoyant force or continues to rise after such an external force has ceased to function
  1. unstable air is buoyant
  2. a mass of air that heated enough so it is warmer than the surrounding air is unstable
  3. typical condition on a warm summer afternoon
  4. unstable air rises until it reaches the equilibrium level, the level at which the surrounding air has the same temperature and density
  1. conditional instability: when the environmental lapse rate is between the dry and saturated adiabatic rates
  1. left alone the parcel acts like stable air
  2. forced to rise the parcel might become unstable at the condensation level
  1. Determination of Stability via Temperature and Lapse Rate
  1. stable: if the environmental lapse rate of the surrounding air is less than the dry adiabatic lapse rate of the rising air (Fig 6-23)

1)the rising air is cooler than surrounding air at every elevation

2)air rises only when forced to do so

  1. instability: if the environmental lapse rate of the surrounding air is greater than the dry adiabatic lapse rate of the rising air (Fig 6-24)

1)the rising air is warmer than the surrounding air at every elevation

2)it rises until it reaches an elevation where the surrounding air is of similar temperature and density

  1. if rising air is cooled to its dew point(Fig 6-25)

1)condensation begins

2)latent heat is released so rising air cools at a slower rate – the saturated adiabatic lapse rate

3)increases tendency toward instability

4)reinforces rising

  1. Visual Determination of Stability
  1. cumulus clouds suggest instability
  2. towering cumulonimbus clouds indicate pronouncedinstability
  3. stratiform clouds characterize stable air forced to rise
  4. cloudless skies indicate stable, immobile air

1)although no clouds form unless air is cooled to the dew point regardless of stability

  1. Precipitation
  1. The Processes
  1. intro
  1. all precipitation originates in clouds, but most clouds do not yield precipitation
  2. many droplets must join together to form a drop large enough to overcome turbulence and evaporation and fall to ground under influence of gravity
  1. Collision/Coalescence
  1. in warmer clouds
  2. rain is produced by the collision and coalescing (merging) of water droplets
  3. condensation allows water molecules to grow large enough to coalesce then fall and coalesce more as they collide with other droplets
  4. accounts for most precipitation in the tropics and some in midlatitudes
  1. Ice-Crystal Formation
  1. in high, cold clouds
  2. Bergeron process

1)ice crystals grow by attracting water vapor supplied by evaporating water droplets