The atmosphere s water
Atmospheric Moisture and Precipitation Properties of Water The Hydrosphere and the Hydrologic Cycle Humidity The Adiabatic Process Clouds Precipitation Air Quality
Main points for today Water s properties are due to its polar nature Water has a very high heat capacity and high heat of fusion Warm air can contain more water vapor than cold air Relative Humidity and Dew Point Temperature are the most common ways to express how much water is in the air The Environmental Lapse rate is the observed change in air temperature with change in elevation An Adiabatic Process is the expansion or contraction of an air mass without heat loss or gain
Main points for today (cont) Clouds are classified by altitude and shape Precipitation can occur because of 1) convection, 2) orographic lifting, and/or 3) frontal processes Air Quality
The Hydrosphere and the Hydrologic Cycle majority of water occurs as ocean saltwater remaining water (only 2.8%) is accounted for by glaciers and ice sheets, and groundwater only 0.001% occurs in the atmosphere Figure 4.2, p. 121
The Hydrosphere and the Hydrologic Cycle the global water balance constantly cycles between these reservoirs while the atmosphere contains relatively little water, it is responsible for the largest flow Figure 4.3, p. 122
Properties of Water Physical States only natural substance that occurs naturally in three states on the earth s surface Heat Capacity Highest of all common solids and liquids Surface Tension Highest of all common liquids Latent Heat of Fusion Highest of all common substances Compressibility Virtually incompressible as a liquid Density Density of seawater is controlled by temperature, salinity and pressure Liquid has maximum density at +4 o C; solid phase has lower density!
Properties of Water (cont ) Radiative Properties transparent to visible wavelengths virtually opaque to many infrared wavelengths large range of albedo possible water 10 % (daily average) Ice 30 to 40% Snow 20 to 95% Cloud 30 to 90%
Molecular Structure of Water water molecule ice Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.
Molecular Structure of Water ice Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.
Three States of Water Water can exist in three states - solid (ice), liquid (water), and gas (water vapor) Water exists in the air in the form of water vapor, clouds, fog, and precipitation Figure 4.1, p. 121
Energy associated with phase change 80 calories 100 calories 540 calories!
Why does it take so much energy to evaporate water? In the liquid state, adjacent water molecules attract one another - charge on O attracted to + charge on H we call this hydrogen bonding This same hydrogen bond accounts for surface tension on a free water surface column of water sticks together
Water vapor pressure Molecules in an air parcel all contribute to pressure Each subset of molecules (e.g., N 2, O 2, H 2 O) exerts a partial pressure The VAPOR PRESSURE, e, is the pressure exerted by water vapor molecules in the air similar to atmospheric pressure, but due only to the water vapor molecules often expressed in mbar (2-30 mbar common at surface)
Water vapor saturation Water molecules move between the liquid and gas phases When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium The air is said to be saturated with water vapor at this point Equilibrium does not mean no exchange occurs
How do we express the amount of water vapor in an air parcel? Absolute humidity mass of water vapor/volume of air (g/m 3 ) changes when air parcel volume changes Specific humidity mass of water vapor/mass of air (g/kg) Mixing ratio mass of water vapor/mass of dry air (g/kg) Specific humidity and mixing ratio remain constant as long as water vapor is not added/removed to/from air parcel Dew point temperature the temperature at which the air parcel would be saturated
Expressing the water vapor pressure Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure 100 * e/e S Range: 0-100% (+) Air with RH > 100% is supersaturated RH can be changed by Changes in water vapor content, e Changes in temperature, which alter e S
Humidity Facts Humidity is the amount of water vapor in the atmosphere Warm air can hold much more than cold air Cold dry air can have close to 0% Warm tropical air may have 4-5% Two ways to describe humidity (specific humidity and relative humidity)
Specific Humidity the actual quantity of water vapor in the air expressed as grams of water per kilogram of air (g/kg) used to describe the water content of large air masses, and how it varies by latitude Figure 4.5, p. 124
Relative Humidity a measure of the amount of water vapor present in air relative to the maximum amount that the air can hold at a given temperature (%) e.g. if relative humidity is 50%, then it contains 1/2 the amount of water vapour it could hold at a given temperature relative humidity decreases as temperature increases
Relative Humidity and Temperature if no water vapour is added or removed from the air mass, then relative humidity decreases as temperature increases Figure 4.7, p. 125
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Dew Point Temperature as air is cooled it eventually becomes saturated (100% relative humidity) the temperature of saturation is called the dew point temperature if cooling continues, condensation begins and dew forms
Relationship between e S and T The saturation vapor pressure of water increases with temperature At higher T, faster water molecules in liquid escape more frequently causing equilibrium water vapor concentration to rise We sometimes say warmer air can hold more water There is also a vapor pressure of water over an ice surface The saturation vapor pressure above solid ice is less than above liquid water
Water vapor saturation and atmospheric pressure; at what Temperature does water boil
Dewpoint Temperatures Dewpoint temperature is a measure of the water vapor content of the air It is not a measure of temperature!
The Adiabatic Process when a gas expands, its volume increases and its pressure and temperature decrease air temperature change solely as a result of air expansion or contraction is a result of the adiabatic process the adiabatic lapse rate is used to quantify how the temperature of air decreases as it rises or increases as it ascends lapse rates differ for dry (unsaturated) and wet (saturated) air masses
Dry Adiabatic Lapse Rate Dry Adiabatic Lapse Rate (DALR) - decrease in temperature with altitude: 10 C/1000m Figure 4.10, p. 127
Wet (Saturated) Adiabatic Lapse Rate ranges from 4 to 9 degrees C per 1000 meters varies because it depends on temperature, pressure and water vapour content less than the DALR because as water condenses it releases latent heat, so the temperature decrease is less
Wet Adiabatic Lapse Rate as a parcel of air rises, it cools and becomes saturated at the dew point dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises when it reaches its dew point, condensation occurs (lifting condensation level) Figure 4.10, p. 127
Water vapor is distributed throughout the atmosphere Generally largest amounts are found close to the surface, decreasing aloft Closest to the source - evaporation from ground, plants, lakes and ocean Warmer air can hold more water vapor than colder air
Stability & Instability A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed. If the rock instead accelerates in the direction of the push, it was in unstable equilibrium.
Stability in the atmosphere An Initial Perturbation Stable Unstable Neutral If an air parcel is displaced from its original height it can: Return to its original height - Stable Accelerate upward because it is buoyant - Unstable Stay at the place to which it was displaced - Neutral
Vertical Motion and Temperature Rising air expands, using energy to push outward against its environment, adiabatically cooling the air A parcel of air may be forced to rise or sink, and change temperature relative to environmental air
Environmental Lapse rate The lapse rate is the change of temperature with height in the atmosphere There are two kinds of lapse rates: Environmental Lapse Rate (~ 4 /1000 m) What you would measure with a weather balloon Parcel Lapse Rate The change of temperature that an air parcel would experience when it is displaced vertically This is assumed to be an adiabatic process (i.e., no heat exchange occurs across parcel boundary)
The Adiabatic Process when a gas expands, its volume increases and its pressure and temperature decrease air temperature change solely as a result of air expansion or contraction is a result of the adiabatic process the adiabatic lapse rate is used to quantify how the temperature of air decreases as it rises or increases as it ascends lapse rates differ for dry (unsaturated) and wet (saturated) air masses
Dry Adiabatic Lapse Rate Dry Adiabatic Lapse Rate (DALR) - decrease in temperature with altitude: 10 C/1000m Figure 4.10, p. 127
Wet (Saturated) Adiabatic Lapse Rate ranges from 4 to 9 degrees C per 1000 meters varies because it depends on temperature, pressure and water vapour content less than the DALR because as water condenses it releases latent heat, so the temperature decrease is less
Wet Adiabatic Lapse Rate as a parcel of air rises, it cools and becomes saturated at the dew point dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises when it reaches its dew point, condensation occurs (lifting condensation level) Figure 4.10, p. 127
What if the environmental lapse rate falls between the moist and dry adiabatic lapse rates? The atmosphere is unstable for saturated air parcels but stable for unsaturated air parcels This situation is termed conditionally unstable This is the typical situation in the atmosphere Conditionally unstable air
Clouds Made up of water droplets and/or ice particles form when air is saturated AND contains particles (condensation nuclei) e.g. dust, salts water can remain in liquid state below freezing (supercooled) to as low as -12 C (10 F)
Clouds types - classified by altitude high (eg. cirrus) middle (eg. altocumulus) low (eg stratus, cumulus) Figure 4.11, p. 128 thunder cloud (cumulonimbus) extends from low to high
Fog cloud layer at or close to the Earth s surface radiation fog forms at night when air near the ground falls below the dew point temperature advection fog forms when warm moist air moves over a cool surface sea fog forms when cool marine air comes in contact with cold ocean currents
Precipitation Precipitation formation requires: growth of droplets in clouds ice crystal process - ice particles act as freezing nuclei coalescence process - large droplets collide with smaller ones and coalesce
Precipitation produced in clouds well below the dew point temperature usually near cloud tops all precipitation begins as frozen water if it reaches the ground in liquid form - rain, drizzle (small drops)
Types of Precipitation: freezing rain (ice crystals freeze onto a frozen surface) snow (ice crystals have not melted) sleet (ice crystals melt as they fall) hail (melting and refreezing crystals that form in thunder clouds)
Precipitation: three mechanisms Convectional Orographic Frontal (cyclonic)
1.) Convectional warm air rises cools to dew point temperature - clouds form latent heat release adds energy and increases updraft may produce thunderstorms
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2.) Orographic lifting air rising over a highland intercepting slope = windward slope (wetter) change in temperature alters the humidity leeward slope (drier) (rain shadow)
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3.) Frontal (cyclonic) precipitation where air masses with different temperatures come together warm air lifted by cold dense air along a weather front leads to frontal precipitation Cold air Warm air
Thunderstorms are intense convectional storms associated with massive cumulonimbus clouds. They may produce heavy rains, hail, thunder, lightening, and intense downdrafts (microbursts) which may create hazards for humans Figure 4.21, p. 137
Air Quality Air pollutants are undesirable gases, aerosols, and particulates injected into the atmosphere by human and natural causes Figure E4.1, p. 143
Smog over China
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Cloud type summary
High Clouds High clouds White in day; red/orange/yellow at sunrise and sunset Made of ice crystals Cirrus Thin and wispy Move west to east Indicate fair weather Cirrocumulus Less common than cirrus Small, rounded white puffs individually or in long rows (fish scales; mackerel sky) Cirrostratus Thin and sheetlike Sun and moon clearly visible through them Halo common Often precede precipitation
Cirrus
Cirrus Cirrus Display at Dawn
Cirrocumulus
Cirrocumulus Cirrocumulus at Sunset
Cirrostratus Cirrostratus with Halo
Contrails
Middle Clouds Altocumulus <1 km thick mostly water drops Gray, puffy Differences from cirrocumulus Larger puffs More dark/light contrast Altostratus Gray, blue-gray Often covers entire sky Sun or moon may show through dimly Usually no shadows
Altostratus Alto Stratus Castellanus
Altocumulus
Altocumulus Alto Cumulus Radiatus
Alto Cumulus Alto Cumulus Undulatus
Low Clouds Stratus Uniform, gray Resembles fog that does not reach the ground Usually no precipitation, but light mist/drizzle possible Stratocumulus Low lumpy clouds Breaks (usually) between cloud elements Lower base and larger elements than altostratus Nimbostratus Dark gray Continuous light to moderate rain or snow Evaporating rain below can form stratus fractus
Stratus fractus
Looking down on an eastern Atlantic stratus deck
Stratiform cloud layers
Stratocumulus cloud streets Stratus undulatus
Stratus A Layer of Stratocumulus Cloud viewed from above
Vertically developed clouds Cumulus Puffy cotton Flat base, rounded top More space between cloud elements than stratocumulus Cumulonimbus Thunderstorm cloud Very tall, often reaching tropopause Individual or grouped Large energy release from water vapor condensation
Cumulonimbus with Pileaus caps
Cumulonimbus Clouds Spawn Tornadoes
The Earth s Hydrologic Cycle