Atmospheric Moisture

Recap: daily temperature variations During the day, the Earth’s surface and air above will continue to warm as long as incoming energy (mainly sunlight) exceeds outgoing energy from the surface. At night, the Earth’s surface cools, mainly by giving up more infrared radiation than it receives – a process called radiational cooling. The coldest nights of winter occur when the air is calm, fairly dry (low water-vapor content), and cloud free. The highest temperatures during the day and the lowest temperatures at night are normally observed at the Earth’s surface. Therefore, the greatest daily variation in air temperature also occurs at the surface. Radiation inversions exist usually at night when the air near the ground is colder than the air above. Both the diurnal and annual ranges of temperatures are greater in dry climates than in humid ones. Even though two cities may have similar average annual temperatures, the range and extreme of their temperatures can differ greatly. How cold the wind makes us feel is usually expressed as a wind-chill index (WCI).

Wind-Chill Index

Wind-Chill Equivalent Temperatures WINDS WIND CHILLS DEPARTURE FROM NORMAL

Water, Water Vapor and Weather PRECIPITATION CLOUDS SNOW COVER WATER VAPOR

Phase Changes

The Hydrological Cycle Different phases of water and phase transitions: solid (ice), liquid (water), gas (water vapor). melting, evaporation, sublimation; condensation, freezing, deposition. The hydrological cycle. Advection latent heat absorbed latent heat released Ep -> Ek Convection

Evaporation The amount of water in the air is controlled by the process of evaporation, condensation and transportation of H2O molecules. Saturated water vapor: the number of evaporating H2O molecules is exactly equal to the number of condensing H2O molecules. The effect of T (P constant): warm water evaporates more readily. The effect of pressure (T=const): water evaporates easier at a lower pressure. Food takes longer to cook in the mountains (see focus section) The effect of the wind (advection): it enhances evaporation by blowing away the water vapor molecules already in the air, allowing additional water molecules to evaporate Wind chill effect

Vapor Pressure and Boiling At P=1 bar water boils at T=100 C At high altitudes (lower P) boiling occurs at lower T. At P=750 mbar T(boiling)=92 C At T=92 C it takes longer for food to cook.

Condensation Condensation occurs when the air is cooled. In the atmosphere, H2O molecules condense on small particles (nuclei), cold or open water surfaces. Condensation nuclei: bits of dust, smoke, salt, ice, and other particles. The effect of T Warm molecules move too fast -> bounce off the nuclei and do not stick to them Cold air -> molecules move slower and are more likely to stick and condense to the nuclei

Humidity Humidity: the amount of water in the air. Absolute humidity: the mass of water vapor in a unit volume of air. Specific humidity: the mass of the water vapor compared to the total mass of the air parcel. Water (mass) mixing ratio: The mass of water vapor compared to the mass of the rest of the air parcel.

Specific Humidity/Latitudes Warm latitudes (equator, tropics) have higher specific humidity. Cold latitudes (polar regions) have low specific humidity.

Vapor pressure Partial pressure: the pressure of each gaseous component in a mixture of gases. Dalton’s law of partial pressure: the total pressure of a mixture of gases is the sum of the partial pressures of each gas component. Vapor pressure: the partial pressure of H2O vapor. What is the H2O vapor pressure if 1% of the air is H2O and the total air pressure is 1bar? The pressure of a gas is proportional to the number of molecules and to the temperature of the gas.

Saturation Vapor Pressure What is it? This is the partial pressure of H2O when the air is saturated. This is the maximum H2O partial pressure before the H2O molecules condense out. Supersaturation: P(H2O)>Ps It is an unstable condition It occurs in the absence of condensation nuclei.

Saturation Vapor Pressure The saturation vapor pressure Ps, depends on the temperature. It increases with temperature. Ps over water is larger than it is over an ice surface at the same temperature.