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• Relative Humidity (RH) is ratio of actual vapor pressure e to saturation vapor pressure eS– 100 * e/eS– Range: 0-100% (+)– Air with RH > 100% is supersaturated

• RH can be changed by– Changes in water vapor content.– Changes in temperature, which alter the

saturation vapor pressure.

Expressing Water Vapor Content (1)

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The Sling PsychrometerMeasures water vapor content of air

Wet bulb

Dry bulb

• Web bulb temp < dry bulb temp for RH < 100%• The drier the environ, the larger the difference.

• Mixing ratio: ratio of the weight of water vapor to the weight of the air in the parcel (unit: g/kg).

• Dew point temperature: temperature to which air must to be cooled (without changes in moisture content) for saturation to occur.

Expressing Water Vapor Content (2)

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x X 20oC (RH=25/42)

Find Dew point temperature(Td, temperature condensation

occurs)

x X 10oC (RH=13/42)

• Temperature drops upward at 6.5 oC/km Humidity decreases rapidly with height.

• At ocean surface, RH~80% Moisture content increases with temperature.

Which environment has higher water vapor content?

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Water Vapor (dark dry)

http://cimss.ssec.wisc.edu/tropic/real-time/mosaic/images/images.html

1 5 10 15 μm

VisibleWater vapor channel (dark dry)

Subtropical storm

Cold front

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Relative humidity (that affects evaporation rate) and human discomfort (summer)

The Earth’s Hydrologic Cycle

15%85%

On global average, 1 m water per m2 is evaporated and fall as precipitation each year.

70% in surface area.

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Earth’s Water Distribution

Condensation (RH > 100%)fog, cloud, precipitation

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Dew (Td > 0oC)

Frost (Td < 0oC)

Radiation cooling on ground at night

T < Td

A decrease in temperature may cause condensation and cloud droplet formation, with the help of

condensation nuclei (dust, smoke, sea salt, SO2 …)

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Cloud Condensation Nuclei• 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 CCN clouds would not form in the

atmosphere, because a RH of several hundred percent is required for pure water drop formation. Why?

Very Small Drops Tend to Evaporate!

• Small drops have large curvature

• Large curvature gives a high saturation vapor pressure because of effect of surface tension.

• Very high RH required for equilibrium with very small drops: Supersaturation(RH > 100%)– ~300% RH for a 0.1 µm

pure water drop

Typical cloud droplets: 20 μm

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Wilson’s cloud chambera sealed environment containing a supercooled, supersaturated water vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it. The resulting ions act as condensation nuclei, around which a mist forms.

Charles Thomson Rees Wilson (1869-1959), a Scottish physicist, is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centres for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911.

Brocken Spectre (Glory)

CCN that Dissolve in Water

Water molecule

Salt molecule

Hygroscopic aerosols readily absorb water from the surroundings. In the presence hygroscopic aerosols, condensation may occur at RH < 100%.

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• Small droplets that may form at RH < 100% (hygroscopic nuclei: salt, sulfuric and nitric acid).

• Becomes visible by scattering light.Haze

Fog: droplets larger than haze and visibility < 1 km.

Radiation fog

• Forms at night due to longwave radiation cooling (often observed in valleys);

• Dissipates under sunlight as ground warms up by absorbing solar radiation.

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Advection fog

Fog Drip

Warm & Humid

Evaporation/mixing fog

Warm & HumidWarm & HumidColdColdRH > 100%

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Mixing fog• Your breath does not fog in

summer because of small temperature difference.

• It fogs in winter because of mixing with cold air.

Annual days of heavy fog

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Key words

Evaporation, condensation, precipitationSaturated air, condensation nuclei, supersaturated airHumidity, actual vapor pressureSaturation vapor pressure, relative humidity, dew pointDew, frost, fogRadiation fog, advection fog, evaporation/mixing fog

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Quiz #1

2. The wavelengths of maximum radiant energy emitted by the sun are in the visible/shortwaverange of the electromagnetic spectrum.

11. Temperature in the troposphere (increases, decreases) with height and the moisture content (increases, decreases) with height.

18.The term used for a line of constant pressure is Isobar.

26. The sling psychrometer is used to measure air humidity. The wet bulb temperature is always (smaller than, greater than, equal to) the dry bulb temperature in an unsaturated atmosphere.

Of the four most abundant gases in our atmosphere, which one shows the greatest variations from place to place on the earth’s surface? (Nitrogen, oxygen, water vapor& Argon)