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  2. Chapter Four Humidity, Condensation & Clouds

  3. Goal for this Chapter We are going to learn answers to the following questions: • Why eyeglasses often fog up after coming indoors on a cold day? • On a cold winter, why do you feel your skin to be very dry, lips crack, and have itching sensation on the skin? • You often itchy throat during winter than summer? • Why you have different types, colors and shapes of cloud during various days and times of the year? • Learn various terms (relative humidity, vapor pressure, dew, frost, haze, fog, cirrus clouds, cumulus clouds, stratocumulus clouds, etc.) used by Meteorologists

  4. Water Circulation in the Atmosphere • Evaporation:Conversion of liquid water into vapor • Condensation: Conversion of water vapor into liquid • Precipitation: Stuff that falls to the surface from the growth of cloud particles (rain, snow or hail) • Transpiration: Release of moisture from plants/trees • Hydrologic Cycle: Cycling of water molecules from liquid to vapor and back to liquid- ocean – atmosphere – land – ocean • Some Facts: 1.5 billion billion gallons of water vapor evaporate into the atmosphere in a year; 85% from ocean (70% of the earth’s surface area) & 15% from evaporation and transpiration • Total Mass of Water Vapor ~ 7 days of precipitation for the globe (Residence time ~ 7 days)

  5. Hydrologic Cycle

  6. Saturation & Condensation • Saturation: When number of molecules escaping a parcel of air = number of returning molecules • Wind can push away the molecules from a water body and hence will enhance evaporation • Warm temperature will aid the molecules to move faster leading to evaporation – warmer the water, greater the rate by evaporation • Condensation Nuclei: Microscopic particles such as dust, smoke, salt from ocean spray, etc – serve as surfaces on which water vapor can condense • Condensation occurs when air is cooled – speed of the vapor molecules decrease leading to more condensation

  7. Saturation & Humidity • Saturation is more likely to occur in cool air than warm air – Warm air can hold more water vapor molecules before getting saturated than cold air – Warm air has greater capacity to hold water vapor • Absolute Humidity or Water vapor density in a parcel of air = Weight of water vapor (g) / volume of air (m3) • Specific Humidity in a parcel of air = Weight of water vapor (g) / Total weight of all the air (kg) • Mixing Ratio in a parcel of air = Weight of water vapor (g) / Weight of the dry air (kg) • Relative Humidity = Amount of water vapor in the air / Maximum amount of water required for saturation at that temperature

  8. Water molecules at the surface of the water – more molecules are evaporating than condensing – net evaporation is taking place

  9. Number of water molecules escaping = number returning – air is saturated with the water vapor

  10. Water vapor content inside the parcel of air (Humidity)

  11. Humidity contd. • Relative Humidity (RH) tells us how close the air is to being saturated • RH = (vapor pressure / saturation vapor pressure) x 100 • Air RH >100%, is said to be supersaturated • Vapor Pressure: Total pressure inside a parcel of air = Sum of the pressures of individual gases • Total pressure (1000 mb) = Partial pressure (PP) exerted by N2 (78% or 780 mb) + PP exerted by O2 (21% or 210 mb) + PP exerted by water vapor (1% or 10 mb) • The partial pressure of water vapor (Actual vapor pressure) ~ 10 mb – only a small fraction of the total air pressure

  12. Vapor Pressure • High actual vapor pressure indicates large numbers of water vapor molecules • Saturation Vapor Pressure: Pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature • At higher air temperatures, it takes more water vapor to saturate the air (increase in temp leads to increased in the velocity – number of molecules escaping per second increases)

  13. Saturation vapor pressure increases with increasing temperature

  14. Relative Humidity – contd. Change in RH can come from • Change in Water pressure Content • Change in the air temperature (previous view graph) For a constant air temperature, RH increases with addition of water vapor For a constant water vapor, increase in air temperature lowers the RH Highest RH occurs in the early morning (coolest time corresponds to the highest RH); lowest RH occurs during the warmest part of the afternoon Rapid evaporation of moisture human body causes skin to crack, dry skin, itching, etc; itchy throat, etc.

  15. RH highest in the cool morning; lowest in warmest time

  16. Humidity • Assume: Air temperature in the morning is 10°C; the saturation vapor pressure is 12 mb; if the temp warms to 30 °C (water vapor content remains constant), actual vapor pressure remains the same – 12 mb, but the saturation vapor pressure has changed (temp effect) to 42 mb • RH = (12 mb/42 mb) x 100 = 29% - RH has decreased from 100% to 29% because of change in temp. • Dew Point: The temperature to which air would have to be cooled for saturation to occur (keeping air pressure or moisture content constant)

  17. Humidity – contd. • High dew point --- high water vapor content • Low dew point – low water vapor content • [Air temp – Dew Point] --- high values indicate RH is low; close to zero, RH is high; Zero implies air is saturated and the RH is ~100% • Polar Air is ‘dry’ when the RH is ~100% (Air temp and dew point are close together; low dew point temp means little water vapor in the air) • Wet-bulb Temperature: Lowest temperature that can be reached by evaporating water into the air • During summer, when wet-bulb temp is low, fast evaporation takes place from the skin

  18. Polar Air temp = -2°C; dew point = -2°C; RH=100%

  19. Desert Air temp = 35°C; dew point = -5°C; RH=16%

  20. Human body adjustment When the human body temp rises, the hypothalamus gland (gland in the brain that regulates body temp) activates the body’s heat regulating mechanism, and evaporation increases – we feel thirsty to keep up the fluid level in the body Heat Index: It is the apparent temperature of what the temperature ‘feels like’ – It takes air temp and RH into account (air temp of 100°F & RH ~60% - Heat index ~130 °F) Measurement of dew point and RH is done using Psychrometer

  21. Heat Index

  22. Sling Psychrometer • Two bulbs, one end has a piece of cloth (wick) covering the bulb – called the wet bulb is dipped in clean water – Other end is kept dry; both bulbs ventilated with by whirling the instrument or drawing air past it with an electric fan; water evaporates and thermometer cools; drier the air, greater the amount of evaporation & cooling – dry thermo gives air temp • Diff between dry and wet bulbs = wet bulk depression

  23. Humidity measurements • Large depression --- more water can evaporate into the air – RH is very low • Small depression --- little evaporation of water vapor is possible • Humidity are commonly called Hygrometers – Human humidity increases, the length of hair increases and the RH decreases, so does the hair length • Electrical Hygrometer: Plate coated with a film of carbon; electrical current sent across the plate; water vapor absorbed changes the electrical resistance of the carbon coating

  24. Hair hygrometer – changes in the length of a human or horse hair

  25. Dew, Frost, Fog • When air cools below the dew point, water vapor begins to condense upon surfaces forming tiny visible specks of water called ‘dew’ • If the air temp falls below freezing, the dew will freeze, becoming tiny beads of ice called frozen dew • When dew is seen, it will not rain; if grass is dry, rain may come • When air temp is much below freezing, water vapor can directly become ice – Deposition (opposite is sublimation) – White crystal is called frost • When RH >75%, some of the water vapor may condense onto condensation nuclei – size increases and can scatter light --- becoming haze

  26. Haze, Fog • Fog: Visibility < 1 km; air is wet with millions of tiny liquid droplets (or ice crystals), the haze becomes cloud with earth’s surface as the base • Radiation Fog or Ground Fog: Produced by the earth’s radiational cooling; most common over land in late fall & winter; ground cools so does the air directly above it, and a surface inversion forms --- moist lower layer quickly becomes saturated, and fog forms • Valley Fog: Radiation fog in a valley; heavy air drains downhill and collects in valley bottoms • How fog disappears when sun rises: Sunlight penetrates and warms the ground; temp of air increases; warm rises and mixes w/foggy air above; droplets evaporate

  27. Formation of dew; if the water freezes, frozen dew

  28. High RH of the cold air above the lake is causing the Formation of a layer of haze on a still winter morning

  29. Radiation fog nestled in a valley

  30. Fog – contd. • Advection fog: Warm, moist air moving over a colder air --- warm air cools to its saturation point, forming advection fog • Ice Fog: Marine air moves over an ice or snow surface, ice crystals form instead of water droplets – produce ice fog • Upslope fog: Fog that forms as moist air flows up along an elevated plain, hill or mountain • Evaporation (mixing) fog: Fog formed from the mixing of two unsaturated masses of air – e.g., fog produced from our breathing • Steam fog: Fog forming over lakes on autumn mornings, as cold air settles over water still warm from the summer

  31. Fog – contd. & Clouds • Precipitation (or frontal) fog: A warm rain falling through a layer of cold, moist air can produce fog • Heavy fog is more prevalent in coastal margins than in the center of the continent • In US, foggiest spot is Cape Disappointment, Washington (at the mouth of the Columbia River) • Clouds: A visible aggregate of tiny water droplets or ice crystals suspended in the air • Classification of Clouds: 10 Basic types

  32. Average annual # of days with heavy fogs in USA

  33. High Clouds (Cirrus, Ci, Cirrostratrus, Cs); Middle (Altostratus, As, Altocumulus, Ac); Low (Stratus, St, Stratocumulus, Sc, Nimbostratus, Ns); & Clouds w/vertical development (cumulus, Cu, & Cumulonimbus, Cb) • Approxi. Height of cloud bases above surface

  34. Classification of cloud • First classification (Luke Howard) based on they appear to a ground observer: • Sheetlike cloud – stratus (in Latin means ‘layer’) • Puffy cloud – cumulus (means ‘heap’) • Cirrus (means curl of hair’ • Nimbus (means ‘violent rain’) Other cloud description is based on the combination of these four basic cloud forms – e.g., nimbostratus Next classification based on the height of the cloud’s base above the surface: high clouds, middle clouds, low clouds and clouds that show vertical rather than horizontal development Altitudes separating high and middle cloud groups overlaps and varies with latitude – cirrus clouds at 4,000 m above Alaska will not be seen above Miami at the same height

  35. Cirrus clouds-6-18 km in tropics; 5-13 km in midlatitude; 3-8 km in polar region

  36. Cloud classification – contd. • High Clouds:Clouds composed of ice crystals and are thin; • Cirrus clouds usually move across the sky from west to east • cirrocumulus less seen frequently than cirrus • cirrostratus form ahead of an advancing storm; appearance of cirrostratus used to predict rain or snow within 12-24 hrs • Middle Clouds: Have bases 2-7 km; • Altocumulus: One part of the cloud is darker than the other • Altostratus: Gray or blue-gray cloud often covering entire sky or over an area that extends over many hundreds of square km; often forms ahead of storms having widespread and continuous precipitation; if precipitation takes place from this cloud, the base height lowers

  37. Cirrocumulus clouds: Less frequently seen, appear as small, rounded, white puffs

  38. Cirrostratus clouds with a halo: form ahead (12-24 hrs) of an advancing storm

  39. Altocumulus cloud: Presence in warm, humid summer morning – thunderstorms by late afternoon

  40. Altostratus cloud: Often form ahead of storms having widespread and relatively continuous precipitation

  41. Cloud classification – contd. • Low Clouds: Bases lie below 2000 m; composed of water droplets (in cold weather may contain ice crystals and snow) • Nimbostratus: Dark gray, ‘wet’-looking cloud layer; associated with continuously falling rain or snow; intensity of rain is light or moderate; never heavy, showery variety • Stratocumulus: Rain or snow rarely fall from stratocumulus • Stratus: When a thick fog ‘lifts,’ the resulting cloud is low stratus; generally no precipitation falls from the stratus Clouds with Vertical Development: Takes a variety of shapes, most often looks like a piece of floating cotton with sharp outlines and flat base; precipitation from cumulus congestus is always showery

  42. Nimbostratus cloud: associated with continuously falling rain or snow; Intensity: light to moderate

  43. Stratocumulus clouds: Rarely rain or snow fall from this cloud

  44. A layer of low-lying stratus clouds: no precipitation falls from the stratus clouds

  45. Cumulus clouds

  46. Cumulus congestus: Precipitation is always showery

  47. Cumulonimbus: Thunderstorm cloud

  48. Clouds with vertical development • Cumulonimbus is a thunderstorm cloud; condensation of water vapor --- release of energy – violent up- and down-drafts – lightning, thunder, violent tornadoes associated with the cumulonimbus Other Clouds: Lenticular clouds: formed in the wave crest ; wave is formed by the moist air crossing a mountain barrier 0rkednmoist air crossing Mammatus clouds: Formed in sinking air Condensation trail air (or contrail): Trail of Condensed vapor produced by a jet aircraft

  49. Generalized Classification of cloud type based on height

  50. Lenticular clouds (forms in crest) formed on the eastern side of the Sierra Nevada