Dew. Condensation. AT350 Introduction to Weather and Climate. Scott Denning CSU Atmospheric Science Spring Condensation: Dew, Fog and Clouds

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Condensation: Dew, Please read Chapter 5 in Ahrens Condensation Condensation is the phase transformation of water vapor to liquid water Water does not easily condense without a surface present

1 Condensation: Dew, Please read Chapter 5 in Ahrens Condensation Condensation is the phase transformation of water vapor to liquid water Water does not easily condense without a surface present Vegetation, soil, buildings provide surface for dew and frost formation Particles act as sites for cloud and fog drop formation Surfaces cool strongly at night by radiative cooling Strongest on clear, calm nights The dew point is the temperature at which the air is saturated with water vapor If a surface cools below the dew point, water condenses on the surface and dew drops are formed Dew does not fall Dew Scott Denning CSU Atmospheric Science Spring

If the temperature is below freezing, the dew point is called the frost point If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals deposition The

form cloud/fog drops Drop formation occurs on particles known as cloud condensation nuclei (CCN) The most effective CCN are water soluble.

2 If the temperature is below freezing, the dew point is called the frost point If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals deposition The resulting crystals are known as frost, hoarfrost, or white frost Frost Cloud and fog drop formation If the air temperature cools below the dew point (RH > 100%), water vapor will tend to condense and form cloud/fog drops Drop formation occurs on particles known as cloud condensation nuclei (CCN) The most effective CCN are water soluble. Without particles clouds would not form in the atmosphere RH of several hundred percent required for pure water drop formation Typical Sizes Very Small Drops Tend to Evaporate! Surface of small drops are strongly curved Stronger curvature produces a higher e sat Very high RH required for equilibrium with small drops ~300% RH for a 0.1 µm pure water drop If small drops evaporate, how can we ever get large drops?! Scott Denning CSU Atmospheric Science Spring

3 Homogeneous Nucleation Formation of a pure water drop without a condensation nucleus is termed homogeneous nucleation Random collision of water vapor molecules can form a small drop embryo Collision likelihood limits maximum embryo size to < 0.01 µm e sat for embryo is several hundred percent Embryo evaporates since environmental RH < 100.5% The Solute Effect Condensation of water on soluble CCN dissolves particle Water actually condenses on many atmospheric salt particles at RH ~70% Some solute particles will be present at drop surface Displace water molecules Reduce likelihood of water molecules escaping to vapor Reduce e sat from value for pure water drop Water molecule Solute molecule Curvature and Solute Effects Compete Steps in Cloud/Fog Formation Relative Humidity % g g g Curves are equilibrium droplet growth curves. Peak of each curve is critical point for cloud drop activation. Can define corresponding critical RH. Numbers in red are the mass of the salt condensation nucleus Air parcel cools causing RH to increase Radiative cooling at surface (fog) Expansion in rising parcel (cloud) CCN (tenths of µm) take up water vapor as RH increases Depends on particle size and composition IF RH exceeds critical value, drops are activated and grow readily into cloud drops (10 s of µm) Droplet Radius (micrometers) Scott Denning CSU Atmospheric Science Spring

Natural CCN Sea salt particles (NaCl) Particles produced from biogenic sulfur emissions Products of vegetation burning CCN from human activity Pollutants from fossil fuel combustion react in the

4 Where do CCN come from? Not all atmospheric particles are cloud condensation nuclei (CCN) Good CCN are hygroscopic ( like water, in a chemical sense) Many hygroscopic salt and acid particles are found in the atmosphere Natural CCN Sea salt particles (NaCl) Particles produced from biogenic sulfur emissions Products of vegetation burning CCN from human activity Pollutants from fossil fuel combustion react in the atmosphere to form acids and salts Sulfur dioxide reacts to form particulate sulfuric acid and ammonium sulfate salts Nitrogen oxides react to form gaseous nitric acid which can combine with ammonia to form ammonium nitrate particles More About CCN Sulfate aerosol (SO 4 2- ) SO 2 H 2 SO 4 SO 2 Clouds even contribute to CCN production Clouds ingest sulfur dioxide (SO 2 ) Chemical reactions in the cloud drops convert dissolved SO 2 to soluble forms of sulfate (SO 4 2- ) When the cloud drops evaporate, soluble sulfate particles are left behind CCN concentrations vary in time and space Typically per cubic centimeter High in polluted environments Higher CCN concentrations give rise to greater cloud drop concentrations Climate impacts of these more reflective clouds? Fogs Radiation Fog Fogs are clouds in contact with the ground Several types of fogs commonly form Radiation fog Advection fog Upslope fog Evaporation (mixing) fog Surface radiation and conduction of heat away from the overlying air cool air temperatures near the ground A layer of air near the ground becomes saturated and fog forms Fog deepens as radiative cooling from the fog top continues overnight Solar heating warms the ground and causes the fog to burn off from the ground up What type of meteorological conditions would favor radiation fog? Scott Denning CSU Atmospheric Science Spring

Advection Fog Other Fog Types Warm air moves (is advected) over cold surface Cold surface cools warm air If saturation is reached, fog forms Common

Warm moist air from Pacific is advected over upwelling cold coastal waters As foggy air moves ashore, solar heating warms the ground and overlying

moist air with colder air produces saturated air parcel Examples Exhale on a cold day Evaporation of water from relatively warm, wet surface and

(Smokestack plume, contrails) Upslope fog Moist air flows up along sloped plain, hill or mountain Expansion of rising air causes cooling and RH

accidents and closures Remedies Fog monitoring and warning (optical sensors) Fog dispersal (expensive and of limited utility) Clouds Clouds result

5 Advection Fog Other Fog Types Warm air moves (is advected) over cold surface Cold surface cools warm air If saturation is reached, fog forms Common on west coast of U.S. Warm moist air from Pacific is advected over upwelling cold coastal waters As foggy air moves ashore, solar heating warms the ground and overlying surface Fog evaporates near ground Coastal advection fogs are key moisture sources for California Redwoods Evaporation (mixing) fog Mixing of warm, moist air with colder air produces saturated air parcel Examples Exhale on a cold day Evaporation of water from relatively warm, wet surface and mixing with colder air above. (Smokestack plume, contrails) Upslope fog Moist air flows up along sloped plain, hill or mountain Expansion of rising air causes cooling and RH increases Fogs and Visibility Light scattering by fog drops (geometric scatterers) degrades visibility, leading to Traffic fatalities Airport accidents and closures Remedies Fog monitoring and warning (optical sensors) Fog dispersal (expensive and of limited utility) Clouds Clouds result when air becomes saturated away from the ground They can be thick or thin, large or small contain water drops and/or ice crystals form high or low in the troposphere even form in the stratosphere (important for the ozone hole!) Clouds impact the environment in many ways Radiative balance, water cycle, pollutant processing, earthatmosphere charge balance, etc. Scott Denning CSU Atmospheric Science Spring

Cloud Classification Cloud type summary Clouds are categorized by their height, appearance and vertical development High Clouds - generally above

Clouds - below 7,000 ft Main types Stratus, stratocumulus, nimbostratus Vertically developed clouds (via convection) Main types Cumulus, Cumulonimbus

Indicate fair weather Cirrocumulus Less common than cirrus Small, rounded white puffs individually or in long rows (fish scales; mackerel sky)

6 Cloud Classification Cloud type summary Clouds are categorized by their height, appearance and vertical development High Clouds - generally above 16,000 ft at middle latitudes Main types - Cirrus, Cirrostratus, Cirrocumulus Middle Clouds 7,000-23,000 feet Main types Altostratus, Altocumulus Low Clouds - below 7,000 ft Main types Stratus, stratocumulus, nimbostratus Vertically developed clouds (via convection) Main types Cumulus, Cumulonimbus High Clouds Cirrus 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 Scott Denning CSU Atmospheric Science Spring

7 Cirrus Cirrocumulus Cirrus Display at Dawn Cirrocumulus Cirrostratus Cirrocumulus at Sunset Cirrostratus with Halo Scott Denning CSU Atmospheric Science Spring

Contrails Middle Clouds Altocumulus <1 km

Differences from cirrocumulus Larger puffs

blue-gray Often covers entire sky Sun or

8 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 Scott Denning CSU Atmospheric Science Spring

Altocumulus Altocumulus Alto Cumulus Radiatus Alto

Uniform, gray Resembles fog that does not reach

mist/drizzle possible Stratocumulus Low lumpy

Lower base and larger elements than altostratus

$form stratus fractus Scott Denning CSU Atmospheric$

9 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 Scott Denning CSU Atmospheric Science Spring

$Stratus fractus$ an eastern

10 Stratus fractus Looking down on an eastern Atlantic stratus deck Stratiform cloud layers Scott Denning CSU Atmospheric Science Spring

Stratocumulus cloud streets Stratus A Layer

Stratus undulatus Vertically developed clouds

from water vapor condensation Cumulonimbus

11 Stratocumulus cloud streets Stratus A Layer of Stratocumulus Cloud viewed from above Stratus undulatus 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 Scott Denning CSU Atmospheric Science Spring

12 Cumulonimbus Clouds Spawn Tornadoes Satellite Observations Satellites can be Geostationary Monitors fixed spot on Earth s surface Polar orbiting Orbit poles with Earth revolving below Satellites observe Clouds Water vapor Precipitation Surface properties (temperature, snow cover, vegetation, etc ) Scott Denning CSU Atmospheric Science Spring

13 Visible and Infrared Satellite Photos Visible IR Scott Denning CSU Atmospheric Science Spring

Water, Phase Changes, Clouds

TUESDAY: air & water & clouds Water, Phase Changes, Clouds How can freezing make something warmer? 'warm air can hold more water' why? How do clouds form? The (extraordinary) properties of Water Physical