Chapter 4 Water in the Atmosphere

Figure CO: Chapter 4, Water in the Atmosphere - Spring storm over field © TobagoCays/ShutterStock, Inc.

Humidity measures how much water vapor is in the air

• • Water vapor gets into the atmosphere by

evaporation

(and sublimation) • Water vapor leaves the atmosphere by

condensation

(and deposition) • Evaporation increases humidity and condensation decreases humidity

Saturation

occurs when rate of condensation = rate of evaporation

Figure 01ab: The sequence of events that leads to saturation of air.

Figure 01cd: The sequence of events that leads to saturation of air.

Why measure water vapor content (humidity) in the air?

• Water changing phase is a source of energy for storms • Water vapor is the source of clouds and precipitation • The amount of water vapor (humidity) determines the rate of evaporation • Water vapor is the most abundant greenhouse gas

One way to measure humidity: mixing ratio

• Mixing ratio is the ratio of the mass of water vapor in a given volume of air to the mass of the other molecules in that volume of air • Mixing ratio does not change if the temperature changes • Evaporation and condensation change the mixing ratio

Figure 02: the vertical distribution of the mixing ratio for three different atmospheric conditions

Another way to measure humidity: water vapor pressure

• Water vapor pressure is the amount of the total atmospheric pressure that is due to the water vapor molecules • Water vapor pressure is often called just vapor pressure • Vapor pressure is measured in mb, just like total atmospheric pressure • Saturation vapor pressure depends on temperature

Figure 03: Vapor pressure as a function of temperature Source: Black Rock Forest Consortium.

Another way to measure humidity: the relative humidity

• Relative humidity is expressed as a percentage, where 100% is saturation • Relative humidity can be defined in terms of the vapor pressure • Relative humidity = 100% x vapor pressure ÷ saturation vapor pressure • A low relative humidity allows a higher evaporation rate

Figure B01: Heat index values

Relative humidity has disadvantages

• Relative humidity tells how the air is from saturation • 0% relative humidity: No water vapor • 100% relative humidity is saturated • But air at a high temperature with relative humidity of 50% may have more water vapor than air at a lower temperature with a relative humidity of 90%

Figure 04: A climatology of hourly temperature and relative humidity Data from: Diurnal Cycle. Retrieved December 10, 2010, from http://vista.cira.colostate.edu/improve/data/graphicviewer/diurnal.htm

Condensation in air (not on a surface like frost or dew)

• In a lab with perfectly clean air, saturation requires a relative humidity of more than 200%. RH > 100% is supersaturation • Condensation is inhibited by the curvature effect • Small, very curved droplets have molecules with few neighbors, and are quick to evaporate

Figure 07: The smaller the drop, the more curved the surface

Condensation in the atmosphere (continued)

• Supersaturation (RH > 100%) does not occur in the atmosphere. The solute effect cancels the curvature effect.

• When the relative humidity reaches 100%, cloud particles form.

• Cloud at the surface is called fog.

• Fog reduces visibility to less than 1 km or 0.6 miles • Heavy fog is a travel hazard

Another way to measure humidity: dew point

• Dew point is also called the dewpoint temperature, abbreviated as T d or T D • Dew point is defined as the temperature to which air must be cooled (without changing the pressure) to become saturated • Dew point does not exceed the temperature • The dew point depression is the difference between the temperature and the dew point

Courtesy of Steven Ackerman Figure 05: Dew in spider web

Figure T01: Various humidity quantities for two air temperatures and two relative humidities for an atmospheric pressure of 1000 mb

More on dewpoint

• When the dewpoint is below 0°C (32°F), it is called the frost point, because deposition (water vapor to ice) in the form of frost will occur when the air becomes saturated • When air cools to the dewpoint, condensation occurs • On surfaces, this condensation is called dew (or frost)

Courtesy of Steven Ackerman Figure 06: Frost

Frozen Dew

• Occurs in two steps • First, condensation occurs and the temperature is above freezing (32ºF); that is, the dew point is above freezing • The condensation is dew • Second, the temperature falls below freezing. The dew freezes to frozen drops • This ice is called frozen dew or black ice

Frozen Dew (continued)

• Frozen dew is also called “black ice” • It is a major traffic hazard • It also causes slips and falls for people on foot • Frozen dew is hard to see • Frozen dew frequently forms on roads where there is a significant slope, as well as bridges and overpasses

Condensation

• In the lab with perfectly clean air (no aerosol) takes a relative humidity of more than 200% – Relative humidity > 100% is supersaturation • Condensation is inhibited by the curvature effect • Small, very curved droplets have molecules with few neighbors, and are quick to evaporate

Condensation in the atmosphere

• Is inhibited by the curvature effect • Is enhanced by the solute effect • Some aerosol, salt particles for example, dissolve and have the ability to hold on to water molecules and suppress evaporation • Other aerosol particles form nuclei, or small surfaces for condensation

Condensation in the atmosphere (continued)

• A cloud nucleus gives water molecules more neighbors, by acting like a small flat surface • There are always abundant cloud condensation nuclei in the atmosphere —dust, salt, pollen, pollutants • The solute effect permits condensation at RH < 100%. This is called haze

Ice in clouds

• Deposits (deposition) on small particles called ice nuclei (clay minerals, tiny ice crystals) • There is a scarcity of ice nuclei at high subfreezing temperatures near but < 32ºF • Many water droplets do not freeze at subfreezing temperatures, called supercooling.

• Below -40ºC (or F), all water drops freeze

Ice in clouds (continued)

• Ice takes on different crystal shapes in clouds, depending on temperature and supersaturation • Clouds are saturated with respect to water, supersaturated with respect to ice • The saturation vapor pressure over ice is less than that over water • There can be ice fog (inland Alaska)

Nucleation

• Is the initial formation of a cloud droplet around any type of particle • Homogeneous nucleation – Droplet is formed only by water molecules – Only occurs at temperatures below -40°C • Heterogeneous nucleation – Uses cloud condensation nuclei (CCN) • Hygroscopic nuclei dissolve in water • Hydrophobic nuclei don’t dissolve in water

Ice Nucleation

• Ice nuclei are particles around which ice crystals form – Deposition nucleation—Ice forms from vapor by deposition onto the ice nucleus when the air is supersaturated with respect to ice – Freezing nucleation—A supercooled drop freezes without the air of a nonwater particle – Immersion nucleation—The nucleus is submerged in a liquid drop, causing the drop to freeze – Contact nucleation—Ice nuclei collide with supercooled drops and the drop freezes immediately

Types of Fog

• Radiation fog—cooling on clear nights – Light winds required – Common in valleys • Advection fog—warm air advected over a cold surface cools • Evaporation fog (frontal fog)—form when water evaporates from rain and saturates air beneath – Associated with inversions and warm fronts – Also when cold air flows over a warm lake (steam fog) • Upslope fog—rising air cools to saturation

Figure 08: Fog obscures the view of the Sears Tower Courtesy of Steven Ackerman

Figure 09: Number of days with fog across US

Figure 10: Fog in satellite image Courtesy of SSEC and CIMSS, University of Wisconsin-Madison

© Manamana/ShutterStock, Inc.

Figure 11: Advection fog

Figure 12: Steam fog © James Robertson, www.flickr.com/photos/shingen_au

Lifting mechanisms that form clouds

• Most clouds form when air cools to the dew point as a parcel of air rises vertically as an updraft • Orographic lifting—air flows up over a mountain • Frontal lifting—when less dense warm air is forced to rise over cooler, denser air • Convection—air near the surface warms and rises • Convergence—when air near the ground converges, or is squeezed together, and rises

Figure 13A: Orographic Lifting

Figure 13B: Frontal Lifting

Figure 13C: Convection

Figure 13D: Convergence of air at surface

The saturated adiabatic lapse rate

• Saturated air parcels have relative humidity 100% • Rising saturated air parcels expand and cool, but not at the dry adiabatic lapse rate of 10 °/km • Condensation in the saturated air parcel releases latent heat that acts to warm the air parcel • Cooling dominates, but at a lesser rate • The saturated adiabatic lapse rate is about 6 °/km

Figure 14: Saturated ascent of a parcel

Static stability and saturated air parcel

• Saturated air parcels can rise freely if their temperature is higher than that of their environment • Saturated air parcels can rise freely if their environment’s lapse rate is greater than the saturated adiabatic lapse rate – Thus the parcel will be warmer than its environment – An environmental lapse rate > the saturated adiabatic lapse rate is called conditionally unstable – The level at which rising air becomes saturated is the lifting condensation level

Figure 15: Stability diagram

Figure T02: Atmospheric Stability Summary

Cloud Classification

• Layered clouds are much wider than tall – Stratus describes layered clouds • Convective clouds are as tall or taller than wide – Cumulo describes convective clouds • High clouds of ice crystals – Cirro describes a high cloud • Middle clouds form below high clouds – Alto describes a middle cloud • Nimbus describes a cloud causing precipitation

Figure T03: Common Cloud Types

Figure 16: Schematic of cloud types

Ten Cloud Types

• Stratus—like fog hovering above the ground • Stratocumulus—low-lying cloud combining layered and convective cloud types • Cumulus—flat bases and intricately contoured domed tops – Fair-weather cumulus – Cumulus congestus—tall relative to their width • Can produce brief heavy rain for a short time

Courtesy of Lil Ackerman Figure 17: Stratus

© Olga Miltsova/ShutterStock, Inc.

Figure 18: Stratocumulus

Courtesy of Steven Ackerman Figure 19: Cumulus

Ten Cloud Types (continued)

• Precipitating clouds • Nimbostratus—deep precipitating cloud • Cumulonimbus—thunderstorm clouds – Extend to high altitudes – Produce large amounts of precipitation, severe weather, and even tornadoes – Flattened anvil shape of the top of the cloud – Under the anvil, sinking air may create pouches called mammatus

© Demydenko Mykhailo/Fotolia.com

Figure 20: Nimbostratus.

Figure 21: Cumulonimbus Courtesy of David W. |Martin, SSEC, University of Wisconsin-Madison

Courtesy of Anne Pryor Figure 22: Mammatus

Ten Cloud Types (continued)

• Middle clouds – Altostratus—layered clouds made up mostly of water droplets – Altocumulus—similar to stratocumulus with a higher base • High clouds – Cirrocumulus—similar to altocumulus but made of ice and have smaller elements – Cirrostratus—layerlike, uniform, made of ice – Cirrus—wispy, fibrous clouds made of ice

Courtesy of Ralph F. Kresge/NOAA Figure 23: Altostratus

Courtesy of Lil Ackerman Figure 24: Altocumulus

Courtesy of Steven Ackerman Figure 25: Cirrocumulus

© pzAxe/ShutterStock, Inc.

Figure 26: Cirrostratus

© Megmomster/Dreamstime.com

Figure 27: Cirrus

Clouds and the Greenhouse Effect

• Clouds play a critical role in the global warming debate • Clouds reflect solar radiation—a cooling effect • Clouds are good emitters and absorbers of longwave radiation —a warming effect • Which dominates?

– Cirrus have a net warming effect – Stratus have a net cooling effect – Small particles, net cooling; large particles, warming • Today’s distribution of clouds—net cooling

Figure 28: In the solar spectrum, clouds tend to cool Earth. In the longwave spectrum, they tend to warm the planet.

Cloud Composition

• Includes the phase(s) of water, size and number of particles, habit (shape) of ice crystals, if any – Continental clouds have more and smaller drops than marine clouds – Ice crystal habit, hexagonal plate, needle, column, dendrite – Warm clouds have temperatures above freezing throughout; cold clouds have temperatures below freezing • Precipitation is any liquid or solid water particle that falls from the atmosphere and reaches the ground

Figure 29: Ice crystal shapes

Precipitation

• Does not form by condensation like the process cloud particles form • Condensation and deposition are too slow to produce precipitation • It would take more than 2 days to grow raindrops by condensation • A single average raindrop has the mass of 1,000,000 cloud droplets

Precipitation processes in the atmosphere: there are two

• In warm clouds, with no ice crystals, raindrops form by the collision —coalescence process • A few cloud droplets are bigger than the others • The few large droplets fall faster and collide with the slower, smaller ones, continuing to grow • Collision helps create precipitation in cold clouds – Accretion when crystals sweep up water drops • Graupel when crystal disappears in a large ice particle – Aggregation when ice crystals collide and stick – Snowflake is an individual ice crystal or aggregate

Figure 30: Very small droplets may flow around the larger drops and avoid colliding with them.

Figure 31: Falling ice crystals

The Ice Crystal Process

• Occurs in cold clouds with tops colder than 0ºC • Also called the Bergeron-Wegener Process • These clouds contain mostly cloud droplets and a few ice crystals • The cloud is saturated for water and supersaturated for ice • The few ice crystals grow and the drops evaporate to feed the ice crystals

Figure 32: Beaker experiment for saturation

Figure 33: Ice crystals growing over time

Figure 34: The differences between the saturation vapor pressure over ice and over water

Figure 35: Fallstreaks are wisps of ice particles that fall out of a cloud Courtesy of Steven Ackerman

Precipitation Types

• Virga—rain that evaporates in the air – Fallstreaks are the ice equivalent of virga • Rain—drops at least 0.5 mm in diameter – Smallest drops are called drizzle • Snow—snowflakes and temperature from cloud base to ground is less than 0 °C • Freezing rain—cold air near the surface freezes melted precipitation on contact with surfaces • Sleet—frozen raindrops • Hail—lumps of ice that form in cumulonimbus

Figure 36A: Rain

Figure 36B: Snow

Figure 36C: Freezing rain

© Baudy/Dreamstime.com

Figure UN01: Results of an ice storm

Source: NOAA/NWS Figure UN02: Inches of freezing rain

Figure 36D: Sleet

Figure 37A: Climatology of annual precipitation over the land regions of the world.

Courtesy of University of Washington, Joint Institute for the Study of the Atmosphere and Ocean (JISAO)

Figure 37B: Climatology of annual rainfall across the world, including the oceans, based on satellite observations.

Courtesy of University of Washington, Joint Institute for the Study of the Atmosphere and Ocean (JISAO)

Figure 38: Snowfall map Prepared by Colorado Climate Center, Colorado Sate University, copyright © 1997.

Figure 39: 30-Year Climatology of Freezing Precipitation Adapted from Cortinas, V. J., et al., Monthly Weather Review, April 2004

Figure 40: Precipitation Ladder

Figure 41: Air flow over a mountain

Figure T04: Difference in Temperature, Cloud Cover, and Precipitation on the Windward and Leeward Sides of the Cascade Mountains in Washington State Data from the Western Regional Climate Center, http://www.wrcc.dri.edu/summary/lcd.html.