• 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.
1 The Sling Psychrometer Measures water vapor content of air
• Web bulb temp < dry bulb temp for RH < 100% • The drier the environ, the larger the difference.
Expressing Water Vapor Content (2)
• 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.
2 Find Dew point temperature (Td, temperature condensation occurs)
x X 20oC (RH=25/42)
x X 10oC (RH=13/42)
• Temperature drops upward at 6.5 oC/km Æ Humidity decreases rapidly with height. • At ocean surface, RH~80% Æ Moisture content increases with temperature.
Which environment has higher water vapor content?
3 Subtropical storm
Water vapor channel (dark dry)Water VaporVisible (dark dry)
1510 15 μm
4 Relative humidity (that affects evaporation rate) and human discomfort (summer)
The Earth’s Hydrologic Cycle
70% in surface area.
On global average, 1 m water per m2 is evaporated and fall as precipitation each year.
5 Earth’s Water Distribution
6 Dew (Td > 0oC) Radiation cooling on ground at night
T < Td
Frost (Td < 0oC)
A decrease in temperature may cause condensation and cloud droplet formation, with the help of
7 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
8 Wilson’s cloud chamber a 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.
Brocken Spectre (Glory)
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.
CCN that Dissolve in Water
Hygroscopic aerosols readily absorb water from the surroundings. In the presence hygroscopic aerosols, condensation may occur at RH < 100%.
9 • Small droplets that may form at RH < 100% Haze (hygroscopic nuclei: salt, sulfuric and nitric acid). • Becomes visible by scattering light.
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.
10 Advection fog
Warm & Humid
Warm & HHuummiidd ld Warm & RH > 100% CCoold
11 Mixing fog • Your breath does not fog in summer because of small temperature difference. • It fogs in winter Å because of mixing with cold air.
Annual days of heavy fog
12 Key words
Evaporation, condensation, precipitation Saturated air, condensation nuclei, supersaturated air Humidity, actual vapor pressure Saturation vapor pressure, relative humidity, dew point Dew, frost, fog Radiation fog, advection fog, evaporation/mixing fog
16 Quiz #1 14 12
11. Temperature in the troposphere (increases, 2 decreases) with height and the moisture content 0 (increases, decreases) with height. 27-3024-27 21-24 <21
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.