METEOROLOGY

GEL-1370

Chapter FourChapter Four

Humidity, Condensation & CloudsHumidity, Condensation & Clouds

Goal for this ChapterWe 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

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)

Hydrologic Cycle

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

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

Water molecules at the surface of the water – more molecules are evaporating than condensing – net

evaporation is taking place

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

Water vapor content inside the parcel of air (Humidity)

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

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)

Saturation vapor pressure increases with increasing temperature

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 vaporFor a constant water vapor, increase in air temperature

lowers the RHHighest 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.

RH highest in the cool morning; lowest in warmest time

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)

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

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

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

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

Heat Index

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

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

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

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

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

Formation of dew; if the water freezes, frozen dew

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

Radiation fog nestled in a valley

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

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

Average annual # of days with heavy fogs in USA

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 surfaceCloud Group

Tropical Region

Mid-lati. region

Polar Region

High (Ci, Cs, Cc)

6000 – 18000 m

5000 – 13,000 m

3000 – 8000 m

Middle (As, Ac)

2000 – 8000 m

2000 – 7000 m

2000 – 4000 m

Low (St, Sc, Ns)

0 – 2000 m

0 – 2000 m

0 – 2000 m

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

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

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

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

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

Altocumulus cloud: Presence in warm, humid summer

morning – thunderstorms by late afternoon

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

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

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

Stratocumulus clouds: Rarely rain or snow fall from this cloud

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

Cumulus clouds

Cumulus congestus: Precipitation is always showery

Cumulonimbus: Thunderstorm cloud

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

Generalized Classification of cloud type based on height

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

Mammatus clouds: forms in sinking air

Contrail forming behind a jet aircraft – 10 km from ground