Moisture and CloudsMoisture and Clouds

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Weather Unit

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Water in the AtmosphereWater in the Atmosphere

• Water vapor is the source of all condensation (clouds, fog, dew) and precipitation (rain, snow, sleet)

• Most important atmospheric gas to understand weather processes

• Water vapor is the source of all condensation (clouds, fog, dew) and precipitation (rain, snow, sleet)

• Most important atmospheric gas to understand weather processes

Changes of StateChanges of State

• All changes require a transfer in heat

• Heat is absorbed to break bonds

• Heat is released when bonds form

• Latent heat: “hidden”; major energy source for thunderstorms, tornadoes and hurricanes

• All changes require a transfer in heat

• Heat is absorbed to break bonds

• Heat is released when bonds form

• Latent heat: “hidden”; major energy source for thunderstorms, tornadoes and hurricanes

Some general terms…Some general terms…

• Melting (solid to liquid): energy absorbed to break bonds

• Evaporation (liquid to gas): energy absorbed to break bonds

• Condensation (gas to liquid): energy released when bonds formed

• Sublimation (solid to gas): dry ice, for example

• Deposition (gas to solid): dew, for example

• Melting (solid to liquid): energy absorbed to break bonds

• Evaporation (liquid to gas): energy absorbed to break bonds

• Condensation (gas to liquid): energy released when bonds formed

• Sublimation (solid to gas): dry ice, for example

• Deposition (gas to solid): dew, for example

HumidityHumidity

• General term for water vapor in the air

• Expressed as gm/kg

• Saturated air: – Dependant on air temperature– Warm air can “hold” more water vapor than cold air– Considered saturated (full) when cannot hold any more water

vapor at that temperature

• General term for water vapor in the air

• Expressed as gm/kg

• Saturated air: – Dependant on air temperature– Warm air can “hold” more water vapor than cold air– Considered saturated (full) when cannot hold any more water

vapor at that temperature

Relative Humidity (RH)Relative Humidity (RH)

• A ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure

• Indicates how near the air is to saturation

• A ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure

• Indicates how near the air is to saturation

More on Relative Humidity…More on Relative Humidity…

• Two ways it can change:1) Adding or removing water

vapor2) Lowering or raising air

temperature

• When water vapor content remains constant, a decrease in temperature causes RH to increase. Conversely, an increase in temperature causes RH to decrease.

• Two ways it can change:1) Adding or removing water

vapor2) Lowering or raising air

temperature

• When water vapor content remains constant, a decrease in temperature causes RH to increase. Conversely, an increase in temperature causes RH to decrease.

Dew PointDew Point

• Temperature to which air would need to be cooled to reach saturation

• If cooled further, excess water vapor would condense (dew, fog, clouds)

• Cloud base indicates elevation where dew point is achieved (air is saturated and condensation occurs).

• Temperature to which air would need to be cooled to reach saturation

• If cooled further, excess water vapor would condense (dew, fog, clouds)

• Cloud base indicates elevation where dew point is achieved (air is saturated and condensation occurs).

More on dew point…More on dew point…

• Dew points close to current air temperature indicate humid conditions (air nearly saturated)

• Dew points far from current air temperature indicate dry conditions (air can “hold” much more moisture before saturated)

• Dew points close to current air temperature indicate humid conditions (air nearly saturated)

• Dew points far from current air temperature indicate dry conditions (air can “hold” much more moisture before saturated)

Measuring HumidityMeasuring Humidity

• Psychrometer: measures RH based on the amount of evaporation that takes place

• Dry Bulb (room temperature)

• Wet Bulb (wet wick)

• Water from wet bulb evaporates as fanned, cooling the temperature on thermometer.

• Greater evaporation=dry air

• Less evaporation=moist air

• Psychrometer: measures RH based on the amount of evaporation that takes place

• Dry Bulb (room temperature)

• Wet Bulb (wet wick)

• Water from wet bulb evaporates as fanned, cooling the temperature on thermometer.

• Greater evaporation=dry air

• Less evaporation=moist air

Measuring Relative HumidityMeasuring Relative Humidity

Cloud FormationCloud Formation

• Clouds form from condensation of water vapor (vapor to liquid)

• Air becomes saturated when cools. Remember that excess water vapor condenses when saturation occurs.

• Air must be cooled to its dew point (100% RH) for clouds to form.

• Clouds form from condensation of water vapor (vapor to liquid)

• Air becomes saturated when cools. Remember that excess water vapor condenses when saturation occurs.

• Air must be cooled to its dew point (100% RH) for clouds to form.

Air Compression and ExpansionAir Compression and Expansion

• When air expands, it cools and when compressed, it warms (adiabatic changes).

• Adiabatic changes don’t involve adding or subtracting heat, it simply involves changes in pressure.

• Dry (unsaturated) air cools and warms at a different rate than wet (saturated) air.

• When air expands, it cools and when compressed, it warms (adiabatic changes).

• Adiabatic changes don’t involve adding or subtracting heat, it simply involves changes in pressure.

• Dry (unsaturated) air cools and warms at a different rate than wet (saturated) air.

Forces That Lift AirForces That Lift Air

• In general, air resists vertical movement. (It tends to remain at a constant elevation.)

• However, air that is heated at the surface will become less dense and rise (localized convective lifting).

• Mechanical processes, too, can force air to rise (orographic lifting, frontal wedging and convergence).

• In general, air resists vertical movement. (It tends to remain at a constant elevation.)

• However, air that is heated at the surface will become less dense and rise (localized convective lifting).

• Mechanical processes, too, can force air to rise (orographic lifting, frontal wedging and convergence).

Processes That Lift AirProcesses That Lift Air

Processes That Lift AirProcesses That Lift Air

• Orographic Lifting: When elevated terrains (mountains) force air up

• Air cools adiabatically on windward side generating much rain.

• Air warms adiabatically on leeward side forming warm, dry conditions (deserts).

• Orographic Lifting: When elevated terrains (mountains) force air up

• Air cools adiabatically on windward side generating much rain.

• Air warms adiabatically on leeward side forming warm, dry conditions (deserts).

• Frontal Wedging: When warm and cold air masses collide, producing a front.

• Cooler, denser air acts as a barrier. Warmer, less dense air is forced to rise above.

• Middle-latitude cyclones produced (more on this later).

• Frontal Wedging: When warm and cold air masses collide, producing a front.

• Cooler, denser air acts as a barrier. Warmer, less dense air is forced to rise above.

• Middle-latitude cyclones produced (more on this later).

Processes That Lift AirProcesses That Lift Air

• Convergence: Occurs when air in the lower atmosphere convergences (comes together) and lifting results.

• Converging air cannot go down, so forced up.

• Air cools adiabatically when it rises, forming clouds.

• Convergence: Occurs when air in the lower atmosphere convergences (comes together) and lifting results.

• Converging air cannot go down, so forced up.

• Air cools adiabatically when it rises, forming clouds.

• Localized Convective Lifting: Occurs when unequal heating of the Earth’s surface causes pockets of air (thermals) to be warmed more than surrounding air. Air becomes less dense and rises, cooling adiabatically.

• Forms clouds and precipitation.

• Localized Convective Lifting: Occurs when unequal heating of the Earth’s surface causes pockets of air (thermals) to be warmed more than surrounding air. Air becomes less dense and rises, cooling adiabatically.

• Forms clouds and precipitation.

Processes That Lift AirProcesses That Lift Air

• All processes that force air to rise can create clouds if cooled to its condensation level (dew point)

• Remember that colder air cannot “hold” as much water vapor

• All processes that force air to rise can create clouds if cooled to its condensation level (dew point)

• Remember that colder air cannot “hold” as much water vapor

Air StabilityAir Stability

• Air stability is determined by measuring the temperature of the air at various heights.

• Aircraft and radiosondes• Temperature Inversions

occur when air becomes warmer with height. (Air on ground cools faster than aloft. Warmer air above creates a “cap” on atmosphere below.)

• Little vertical movement

• Air stability is determined by measuring the temperature of the air at various heights.

• Aircraft and radiosondes• Temperature Inversions

occur when air becomes warmer with height. (Air on ground cools faster than aloft. Warmer air above creates a “cap” on atmosphere below.)

• Little vertical movement

• Air that rises cools adiabatically. If rising parcel remains colder than surrounding air, it sinks again (stable air).

• Air continues to rise as long as it is warmer (less dense) than surrounding air (unstable air).

• Stable air tends to remain in its original position, while unstable air tends to rise.

• Air that rises cools adiabatically. If rising parcel remains colder than surrounding air, it sinks again (stable air).

• Air continues to rise as long as it is warmer (less dense) than surrounding air (unstable air).

• Stable air tends to remain in its original position, while unstable air tends to rise.

Stability and Daily WeatherStability and Daily Weather

• When stable air is forced above Earth’s surface, the clouds that form are widespread and have little vertical thickness, thus light precipitation.

• When unstable air is forced up, clouds are often towering and can generate thunderstorms and tornadoes.

• Picture from Mancelona, MI

• When stable air is forced above Earth’s surface, the clouds that form are widespread and have little vertical thickness, thus light precipitation.

• When unstable air is forced up, clouds are often towering and can generate thunderstorms and tornadoes.

• Picture from Mancelona, MI

Condensation NucleiCondensation Nuclei

• Air must be saturated for condensation to occur

• Must be a surface for water to condense on

• Dew (grass, cars, etc)

• Clouds (dust, smoke, salt particles)– Millions of particles

– So small that they remain suspended until too “heavy” (precipitation)

• Air must be saturated for condensation to occur

• Must be a surface for water to condense on

• Dew (grass, cars, etc)

• Clouds (dust, smoke, salt particles)– Millions of particles

– So small that they remain suspended until too “heavy” (precipitation)