Water in the Atmosphere Understanding Weather and Climate Climate 2 1
Cloud Development and Forms Understanding Weather and Climate Climate 2 2
Learning Objectives 1. The various atmospheric lifting mechanisms and how these influence cloud formation. 2. Static stability and how it influences cloud formation. 3. The major types of clouds that are produced by these processes. Climate 2 3
Poem Climate 2 4
Clouds Crucial Role in the Hydrologic Cycle Most Prominent Feature of Daily Weather Global Importance Climate 2 5
Producing Clouds 1. Adding Water Vapor into the Air 2. Mixing Warm Moist Air with Cold Air 3. Lowering the Air Temperature Most Important for Clouds Lifting the Air Will Cool It Adiabatically Forming Clouds Requires Mechanisms to Lift Air Climate 2 6
Cloud Development When an unsaturated parcel of air rises until it becomes saturated, a cloud forms. The mechanisms responsible for the development of the majority of clouds are: Convection Topography Ascent due to convergence of surface air Uplift along weather fronts Climate 2 7
1. Orographic Lifting: Forcing of Air Above a Mountain (Land) Barrier. Mechanisms That Lift Air 2. Frontal Lifting: Displacement of One Air Mass (Warmer) Over Another Air Mass (Cooler). 3. Convergence: Horizontal Movement of Air Into a area at Low Levels. 4. Localized Convective Lifting: Buoyancy (Heating). Climate 2 8
Cloud Development (Convection and Clouds) When the surface is unevenly heated, a thermal breaks away from the warm surface and rises, expanding and cooling as it ascends. As it rises it mixes with the cooler, drier air around it and gradually loses its identity. At this point its upward movement slows, however other rising air parcels penetrate it and help the air to rise a little higher. Only if the parcel rises to its saturation point, the moisture will condense, and the thermal becomes visible as a cumulus cloud. The level where lifted surface air becomes saturated is called the condensation level. Climate 2 9
Cloud Development (Convection and Clouds) Climate 2 10
Orographic Lifting Forcing of Air Above a Mountain (Land) Barrier. Climate 2 11
Height of Clouds Can Be Much Higher Than Barrier Varies From Day to Day As Characteristics of Air Change Climate 2 12
Rainshadow Effect Windward Side (Upwind): Precipitation Greatly Enhanced Leeward(Downwind): Low Precipitation Climate 2 13
Death Valley Climate 2 14
Frontal Lifting Displacement of One Air Mass (Warmer) Over Another Air Mass (Cooler) Warmer Air Approaches Colder Air Warmer Air Wedges Over the Colder Air Warm Front Smooth Slope Colder Air Approaches Warmer Air Colder Air Wedges Under the Warmer Air Cold Front Blunt Slope Climate 2 15
Horizontal Convergence Horizontal Movement of Air Into an Area at Low Levels Mass of Air Not Evenly Distributed Causes Areas of Higher and Lower Pressure Pressure Difference Cause Wind Horizontal Movement Climate of Air 2 Into a Low Pressure Zone 16 Causes Convergence Lifting
Localized Convection Localized as Opposed to Global Free Convection Heating of Earth s Surface in Localized Areas Buoyancy: Lighter, Warmer Air Rises Can Speed Up or Slow Down Other Lifting Mechanisms Climate 2 17
Static Stability Static Stability Air s Susceptibility to Uplift. Statically Unstable Air Continues to Rise If Given an Initial Upward Push Statically Stable Air Resists Upward Displacement and Sinks Back to Original Level Statically Neutral Air Will not Rise or Sink After Its Initial Upward Push Comes to Rest Where It was Displaced Climate 2 18
Buoyancy Static Stability is Related to Buoyancy Parcel of Air Less dense Than Surrounding Air: Positive Buoyancy Tends to Rise (Warmer) More dense Than Surrounding Air: Negative Buoyancy Tends to Sink if Not Lifted(Colder) A Rising Parcel of Air Stops Rising When It Cools to Surrounding Air Sinks When It Becomes Colder Than Surrounding Air This Suppresses Uplift Climate 2 19
Lapse Rates Lifted Parcel of Air Cools at One of the Adiabatic Lapse Rates Air Around it Maintains Its Original Temperature Profile Relative Density 1. Depends on Saturated or Unsaturated DALR or SALR 2. Environmental Lapse Rate (ELR) Three Types of Static Stability 1. Absolutely Unstable Air 2. Absolutely Stable Air 3. Conditionally Stable Air Climate 2 20
Absolutely Unstable Air -1.0 C -1.5 C ELR = -1.5 C / 100 m Unsaturated DALR = -1.0 C / 100 m Will Keep Rising - Cooling Slower than Surrounding Air Climate 2 21
Absolutely Unstable Air -0.5 C -1.5 C ELR = -1.5 C / 100 m Saturated SALR = -0.5 C / 100 m Will Keep Rising - Cooling More Slowly than Surrounding Air Climate 2 22
Absolutely Stable Air -1.0 C -0.2 C ELR = -0.2 C / 100 m Unsaturated DALR = -1.0 C / 100 m Will Not Rise - Cooling Faster than Surrounding Air Climate 2 23
Absolutely Stable Air -0.5 C -0.2 C ELR = -0.2 C / 100 m Saturated SALR = -0.5 C / 100 m Will Not Rise - Cooling Faster than Surrounding Air Climate 2 24
Stability Rule #2 1. Absolutely Unstable Air Whenever the ELR Exceeds the DALR or SALR (Positive Buoyancy) 2. Absolutely Stable Air Whenever the ELR Is Less Than the DALR or SALR (Negative Buoyancy) Climate 2 25
Making Conditionally Unstable Air ELR = -0.7 C / 100 m Unsaturated DALR = -1.0 C / 100 m Will Keep Rising - Cooling Climate Faster 2 than Surrounding Air 26
Conditionally Unstable Air ELR = -0.7 C / 100 m Unsaturated DALR = -1.0 C / 100 m Saturated SALR = -0.5 C / 100 m Climate 2 27
Level of Free Convection A Conditionally Unstable Air Mass Rises Above the Level of Free Convection Must be Lifted Then Can Rise on Its Own LFC Clouds Increase in Depth Yield Precipitation Climate 2 28
Cloud Development (Convection and Clouds) Climate 2 29
Factors Influencing the Environmental Lapse Rate The Average Environmental Lapse Rate (ELR) -0.65 C / 100 meters Highly Variable in Space and Time Surface Air Temperature Vertical Temperature Profile Influences 1. Heating or Cooling of the Lower Atmosphere. 2. Advection of Cold or Warm Air at Different Levels. 3. Advection of a Different Air Mass with a Different ELR. Climate 2 30
Advection of Cold or Warm Air at Different Levels Advection or Wind Can Be Different at the Surface From That Aloft ELR Can Be Different If Winds Are Different Example Same Direction: -0.5 c/100m Perpendicular : -1.0 c/100m Idealized Example As Winds Gradually Change With Height Seen by Cloud Movement Climate 2 31
Advection of a Different Air Mass with a Different ELR Air Mass Large Area Distinguished From Its Neighbors by Differences in 1. Temperature 2. Water Content (Humidity) Maintain Their Identity (1 & 2) Air Masses Can Migrate Into Other Large Areas Changing ELR Climate 2 32
Cloud Types By Form 1. Cirrus Thin, Wispy Clouds of Ice 2. Stratus Layered Clouds 3. Cumulus Clouds Having Vertical Development 4. Nimbus Rain-producing Clouds Climate 2 33
Cloud Types By Height 1. High Clouds Cirrus Thin, Wispy Clouds of Ice Cirrostratus Layered, Thin, Wispy Clouds of Ice Cirrocumulus Thin, Wispy Clouds of Ice with Vertical Development 2. Middle Clouds Altostratus Higher, Layered Clouds Altocumulus Higher Clouds with Vertical Development 3. Low Clouds Stratus Layered Clouds Stratocumulus Layered Clouds with Vertical Development Nimbostratus Rain-producing, Layered Clouds 4. Extensive Vertical Development Cumulus Clouds Having Vertical Development Cumulonimbus Rain-producing Clouds with Vertical Development Climate 2 34
High Clouds Above 6000 meters Height a Little Dependent on Temperature Composed of Ice (Ave. Temp. -35 C) 1. Cirrus (Ci) 2. Cirrostratus (Cs) 3. Cirrocumulus (Cc) Climate 2 35
Above 6000 meters High Clouds - Cirrus Cirrus (Ci) Thin, Wispy Clouds of Ice Simplest 1.5 km Thick Little Water Vapor - 0.025 g/m 3 Individual Ice Crystals - 8mm (0.3in) Fall at speeds of 0.5 m/s (1 mi /hr) Making Streaks Falling Ice Sublimates Climate 2 36
Cirrus Climate 2 37
Cirrus Climate 2 38
High Clouds - Cirrostratus Above 6000 meters Cirrostratus (Cs) Layered, Thin Clouds of Ice More extensive horizontally Lower Concentration of Ice Surface Objects cast shadows Halo (22 ) around Sun and moon Whitish, milky disk with sharp outline Climate 2 39
Cirrostratus Climate 2 40
High Clouds - Cirrocumulus Above 6000 meters Cirrocumulus (Cc) Individual, Puffy Rows of Clouds of Ice Form When a Wind Shear Exists Wind Speed or Direction Changes with Height Precursor of Precipitation - Warm Front Resemble Fish Scales - Mackerel Sky Climate 2 41
Cirrocumulus Climate 2 42
Middle Clouds 2000 to 6000 meters Composed of Liquid Droplets Alto means Middle 1. Altostratus (As) 2. Altocumulus (Ac) Climate 2 43
Middle Clouds - Altostratus 2000 to 6000 meters Altostratus (As) Middle-level Counterparts to Cirrostratus Liquid Water Droplets Scatter a Lot of Insolation Back to Space Diffused Light Absence of Shadows Sun and Moon (If Seen) Are Bright Spots With No Outline Climate 2 44
Altostratus Climate 2 45
Middle Clouds - Altocumulus 2000 to 6000 meters Altocumulus (Ac) Liquid Water Droplets Layered Clouds Forming Long Bands, or Contains a Series of Puffy Clouds in Rows Gray in Color With Possibly One Part Darker Climate 2 46
Altocumulus Climate 2 47
Low Clouds Bases below 2000 meters Composed of Liquid Droplets 1. Stratus (St) 2. Stratocumulus (Sc) 3. Nimbostratus (Ns) Climate 2 48
Low Clouds - Stratus Bases below 2000 meters Composed of Liquid Droplets Stratus (St) Layered Clouds Formed From Large Areas of Stable Air Slow Uplift (Few10s Cm/s), or Turbulence From Strong Winds Forced Convection Low Water Content (0.1 G/m 3 ) 0.5 to 1.0 Km Thick 100s of Km Horizontally Climate 2 49
Stratus Climate 2 50
Low Clouds - Nimbostratus Bases below 2000 meters Composed of Liquid Droplets Nimbostratus (Ns) Much Like Stratus, Except for Presence of Precipitation Low Moisture Content Produces Light Precipitation Climate 2 51
Nimbostratus Climate 2 52
Low Clouds - Stratocumulus Bases below 2000 meters Composed of Liquid Droplets Stratocumulus (Sc) Layered Clouds With Vertical Development Darkness Varies Because of Vertical Thickness Darker Is Thicker Climate 2 53
Stratocumulus Climate 2 54
Clouds with Vertical Development Bases Below 2000 Meters but Extend Into Middle-level Composed of Liquid Droplets Cumuliform Clouds Those With Substantial Vertical Development Vertical Velocities Exceed 50 M/s (100 Mi/hr) Updrafts Have Speeds Greater Than Weak Hurricanes Water Content ~1 G/m 3 (Much Larger Than Stratiform Clouds) 1. Cumulus (Cu) a. Cumulus humilis b. Cumulus congestus 2. Cumulonimbus (Cb) Climate 2 55
Clouds with Vertical Development Cumulus Cumulus (Cu) a. Cumulus humilis Fair Weather Cumulus Does Not Yield Precipitation Single Raising Plume - Zone of Raising Air Clear Sky - Zone of Sinking Air b. Cumulus congestus Multi-Towers with Several Cells of Uplift Strong Vertical Development from Unstable Air Individual Towers Last Only Tens of Minutes Constantly Replaced by New Ones Large Temperatures from Bottom to Top Supercooled Droplets then Ice Ice Visible Where No Distinct Edges of Clouds Washed Out Appearance Climate 2 56
Clouds with Vertical Development Cumulus humilis Climate 2 57
Clouds with Vertical Development Cumulus congestus Climate 2 58
Clouds with Vertical Development Cumulonimbus Cumulonimbus (Cn) Most Violent Warm, Humid, and Unstable Air Produce Thunderstorms Tops Can Extend Into Stratosphere Anvil Top (Blacksmith) Composed of Ice in High Winds of Stratosphere) Anvil Pushed out from Column Hailstones Fall from End Strong Updrafts are Not Uniform Highest Speeds in Top Third Climate 2 59
Cumulonimbus Climate 2 60
Unusual Clouds 1. Lenticular: Lens-like From Downwind of Mountain Barriers From Disruption of Air Flow Series of Waves Adiabatically Droplets evaporate on Downwind Side, Form on Upwind Usually Only Two or Three Form, but Six Have Been Observed 2. Banner Similar, but Are Located Above Isolated Peaks 3. Mammatus Cumulus Clouds That Seem to Have Sack-like Hangings Places that are Heavy with Water Climate 2 61
Lenticular Climate 2 62
Banner Climate 2 63
Mammatus Climate 2 64
Unusual Clouds Above the Troposphere 1. Nacreous Seen in the Winter at Twilight in the Polar Regions Supercooled Water or Ice Crystals Height: 30 Km (20 miles) in Stratosphere 2. Noctilucent In Mesosphere Illuminated After Sunset or Before Sunrise Ice Crystals Climate 2 65
Nacreous Climate 2 66
Noctilucent Climate 2 67
Cloud Coverage Other Characteristic of Clouds Is Coverage Overcast More Than Nine-tenths (9/10) Broken Six-tenths to Nine-tenths (6/10 to 9/10) Scattered One-tenth to Five-tenths (1/10 to 5/10) Clear-sky Less Than one-tenths (1/10) Climate 2 68
Summary 1. Lifting Mechanisms: frontal uplift, convergence, orographic uplift, and convection. 2. Frontal uplift, convergence, orographic uplift are enhanced or hindered by the static stability of the atmosphere, whereas free convection necessarily occurs only when the air is unstable. 3. Instability implies that if a parcel is given an initial boost upward, it will become buoyant and continue to rise. On the other hand, if the air is stable, a parcel displaced vertically will tend to return to its original position. Climate 2 69
Summary 4. Static stability or instability is determined by the air column's rate of temperature decrease with altitude. When the temperature lapse rate is less than the saturated adiabatic rate, the air is statically stable; when it exceeds the dry adiabatic lapse rate, it is unstable. Conditional instability arises when the lapse rate is between the two adiabatic rates. When the air is conditionally unstable, a lifted parcel will rise on its own accord only if it is lifted above a certain critical point called the level of free convection. 5. Three processes modify the lapse rate: the inflow of warm and cold air at different altitudes, the advection of a different air mass, and heating or cooling of the surface. Climate 2 70
Summary 6. Environmental lapse rates vary not only through time, but also with elevation. Thus, a column of atmosphere might be unstable at one level but stable aloft. 7. No matter what the condition of the troposphere, the stratosphere is always statically stable and thereby limits the maximum height of updrafts. 8. Inversions are a special case in which the temperature increases with altitude. Because of their strong static stability, inversions suppress the vertical motions necessary for cloud formation and for the dispersion of air pollution. Inversions are formed by subsidence (sinking air), the emission of longwave radiation from the surface, and the presence of fronts. Climate 2 71
Summary 9. Clouds have been categorized into ten distinct types grouped according to their height and form. Height 1. Cirrus 2. Stratus 3. Cumulus 4. Nimbus Form 1. High Clouds Cirrus, Cirrostratus, Cirrocumulus 2. Middle Clouds Altostratus, Altocumulus 3. Low Clouds Stratus, Stratocumulus, Nimbostratus 4. Extensive Vertical Development Climate 2 72 Cumulus, Cumulonimbus
Adiabatic Lapse Rate The First Law of Thermodynamics can be expressed as: du = dq + dw where du is the change in internal energy, dq is the heat supplied to the system, and dw is the is the work done on the system. dh, the change in enthalpy, can be written as dh = du + pdv + Vdp When we raise a parcel of air there is no heat input, hence dq=0 (adiabatic) and dw=pdv Therefore dh = -Vdp Climate 2 73
Adiabatic Lapse Rate The heat capacity of a gas at constant pressure, C p, is defined as (dh/dt) so that C p dt= Vdp From the hydrostatic equation we get dp = -g σ dz Hence C p dt = -V g σ dz For a unit mass of gas V=1/σ and we get dt dz The dry adiabatic lapse rate plays an important role in atmospheric stability. g C p d Climate 2 74
Fig. 3.17 Climate 2 75
Lapse Rates and Stability Lapse rate is the rate at which the real atmosphere falls off with altitude the environmental lapse rate An average value is 6.5 ºC per kilometer This should be compared with the adiabatic lapse rate of 10 ºC. If the environmental lapse rate is less than 10 ºC, then the atmosphere is absolutely stable If greater than 10 ºC, it is absolutely unstable Climate 2 76
Wet adiabatic lapse rate The presence of condensable vapors, such as water vapor, complicates the process. As the parcel of air ascends it cools at the dry adiabatic lapse rate until the water vapor reaches saturation then condensation takes place. This releases latent heat which can raise the temperature of the air parcel. Now the lapse rate depends on the amount of water vapor wet adiabatic lapse rate. Climate 2 77
Role of atmospheric stability Temperature inversions produce very stable atmospheric conditions in which mixing is greatly reduced. There are two general types of inversions: surface inversions and inversions aloft. Surface inversions are the result of differential radiative properties of the Earth s surface and the air above. The Earth is a much better absorber and radiator of energy than air; thus, in the late morning and afternoon hours the lower atmosphere is unstable. The opposite is true in the evening; a stable atmosphere with little vertical mixing prevails. Climate 2 78
The Nocturnal Inversion On clear nights, a temperature inversion develops near the surface. - Air temperature usually decreases with height. An inversion is a layer of air where temperature increases with height. - Because the layer of air in the inversion is warmer than the air below it, the cooler air below the inversion cannot rise above it. Pollutants near the surface are therefore trapped below the inversion in the overnight hours. Climate 2 79
Fig. 3.18 Climate 2 80
Role of Atmospheric Stability Inversions aloft are associated with prolonged, severe pollution episodes. These types of inversions are caused by the sinking air associated with the center of high pressure systems (subsidence). As the air sinks it is warmed adiabatically. Turbulence at the very lowest part of the atmosphere prevents subsidence from warming that portion of the atmosphere. Los Angles pollution episodes as well as those over the Mid-Atlantic region are the result of inversions aloft associated with strong high pressure systems. Climate 2 81
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(Unstable) (Near neutral stability) Γ (dashed) DALR Solid - ELR Climate 2 85
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