1. Background for Cloud Physics 1.1 Overview of atmospheric thermodynamics 1.2 Overview of cloud observations
1.1 Atmospheric Thermodynamics * Measures of vapor content Vapor pressure: The partial pressure of water vapor in the atmosphere Vapor density: Mass of water vapor per volume of air Mixing ratio: Mass of water vapor per mass of dry air Specific humidity: Mass of water vapor per mass of air Relative humidity: The ratio of the mixing ratio to its saturation value at the same temperature and pressure Virtual temperature: Accounts for both the effects of temperature and humidity on pressure and density of the air
* Clausius-Clapyron equation a. Saturation vapor pressure of water exceeds that of ice for all temperature below 273K; b. Any atmosphere saturated with respect to water is supersaturated relative to ice; c. The degree of supersaturation increases with the supercooling. Table (1): Saturation Vapor Pressure Over Water and Ice, and Latent Heats of Condensation and Saturation T (oC) es (Pa) ei (Pa) L (J/g) Ls (J/g) -40 19.05 12.85 2603 2839 -35 31.54 22.36 -30 51.06 38.02 2575 -25 80.90 63.30 -20 125.63 103.28 2549 2838 -15 191.44 165.32 -10 286.57 259.92 2525 2837 -5 421.84 401.78 611.21 611.15 2501 2834 5 872.47 2489 10 1227.94 2477 15 1705.32 2466 20 2338.54 2453 25 3168.74 2442 30 4245.20 2430 35 5626.45 2418 40 7381.27 2406
* Ways of reaching saturation Dewpoint temperature: temperature at which saturation is reached if cooled isobarically Wet-bulb temperature: temperature at which saturation is reached if cooled by evaporating water into it at constant pressure Equivalent temperature: temperature which moist air would attain if all the moisture were condensed out at constant pressure Isentropic condensation temperature: temperature at which saturation is reached when moist air is cooled adiabatically with mixing ratio held constant
* Mechanisms for Cooling the Air (1) Lifting – most clouds form when air is lifted. a) Convergence – low press center – stratus b) Mountains – lifting by terrain c) Fronts – lifting over denser air. d) Warm air relative to surroundings i) Fires, volcanoes – cumulus ii) Latent heat (2) Mixing – seeing your breath on cold day (3) Contact – with cold surface – advection fog (4) Radiation – ground fog
1.2 Clouds, Dew, Fog, and Precipitation As rising air cools, the temperature will approach the dew point, which is when condensation will occur. Clouds: Clouds are condensed water droplets, either in liquid or ice form. Dew: When high relative humidity air comes into contact with a cold surface, the closest air cools to the dew point, depositing the water as droplets onto the surface. Frost: If the temperature of the surface is below the freezing point (0° C), then water vapor may condense directly into ice crystals, by-passing the liquid phase altogether. Frozen dew is slightly different, in that the water first condenses into a liquid, and then further cooling freezes the liquid water into ice. Fog: A fog is nothing more than a low/surface layer cloud. The ground level humidity has reached saturation and water vapour condenses into water droplets, forming clouds.
Types of Precipitation Drizzle: Small (< 0.5 mm) raindrops falling from low lying stratus clouds. Rain: Larger drops (> 0.5 mm), usually falling from nimbus, or nimbostratus clouds. Such clouds will have more vigorous up-drafts and “convective cells”. Freezing Rain: Rain that falls onto surfaces with sub-zero temperature, so the rain freezes on contact. Results in layers of ice forming on all surfaces. Snow: Falling ice crystals. Larger snowflakes will form when cloud temperatures are greater than -10 °C and supercooled water aids in the growth of the ice crystal. Ice Pellets / Sleet: Forms when rain falls through cold air, freezing the droplets. Hail: Forms in clouds with vigorous convective cells, which allow repeated layers of water to freeze onto ice balls. The more vigorous the up-drafts (cells), the larger the hail stones. Virga: precipitation that evaporates before it hits the ground.
Dew Frost Sleet Hail