Atmospheric Moisture

Earth is the Water Planet >70% of the surface is covered by liquid water (oceans and lakes) Only planet in which water occurs in all three states –Solid –Liquid –Gas

Earth is the Water Planet >70% of the surface is covered by liquid water (oceans and lakes) Only planet in which water occurs in all three states –Solid -ice, snow –Liquid - oceans, lakes, rivers, rain –Gas - water vapor in the atmosphere

Earth is the Water Planet >70% of the surface is covered by liquid water (oceans and lakes) Only planet in which water occurs in all three states –Solid -ice, snow –Liquid - oceans, lakes, rivers, rain –Gas - water vapor in the atmosphere

Hydrologic cycle THydrologic cycle - the cycling of water between the land, oceans and atmosphere and includes the entire hydrosphere (all forms and location of water). Processes involved in the hydrologic cycle Precipitation condensation Evaporation Infiltration Runoff Transpiration

Some characteristics of Water All life needs water Everything dissolves in water to some extent - universal solvent. Water has a high specific heat capacity –Specific heat capacity - amount of energy needed to increase the temperature of a substance by 1 o C. Water expands upon freezing

Oxygen Hydrogen H2OH2O H H O

Phase Diagram for Water

States of Water Evaporation - process whereby water molecules break free of the liquid. –Liquid to vapor Condensation - process whereby water molecules bond with other water molecules. –Vapor to liquid Saturation - maximum amount of water that can exist in the atmosphere as vapor, at a given temperature. At saturation a balance exist between the rate of evaporation and rate of condensation.

States of Water ( Water Vapor and Ice) Sublimation - phase change of ice (solid) directly to vapor. –Solid (ice) to vapor Deposition - phase change of vapor directly to solid. –Vapor to Solid (ice)

Latent heat heat energy absorbed or released during change of state Latent heat of vaporization - heat energy absorbed during evaporation. Latent heat of condensation - heat energy released when vapor changes to liquid. Latent heat of fusion - energy absorbed during melting.

Measuring Water Vapor in the Atmosphere Humidity - amount of water vapor in the air Can be expressed as: –Density –Pressure –Amount (%)

Measuring Water Vapor in the Atmosphere 1)Vapor Pressure - part of the total atmospheric pressure due to water vapor. Units of pressure - pascal (Pa) or millibar (mb) increases with temperature abundance Saturation vapor pressure - maximum vapor pressure that can exist in the atmosphere at a given temprature. Not widely used in weather reports. Mostly used in scientific applications

Measuring Water Vapor in the Atmosphere 2) Absolute Humidity - density of water vapor in the air Expressed as: g water vapor / m 3 air (grams of water vapor per cubic meter of air)

Measuring Water Vapor in the Atmosphere 3) Specific Humidity - mass of water vapor per unit mass of air Expressed as: g water vapor / Kg air (grams of water vapor per kilograms of air) Specific humidity = Mass of water vapor Mass of air Saturation Specific humidity - maximum specific humidity the air can have before it will condense.

Measuring Water Vapor in the Atmosphere 4) Mixing Ratio - mass of water per unit mass of dry air Expressed as: g water vapor / Kg dry air (grams of water vapor per kilograms of dry air) Mixing Ratio = Mass of water vapor Mass of dry air Saturation Mixing Ratio - maximum ratio the air can have before it will condense.

Measuring Water Vapor in the Atmosphere 5) Relative Humidity - measure of the amount of water vapor in the air relative to the maximum amount the air can hold. Most familiar method - used in weather reports Relative humidity = Specific HumidityX 100 Saturation Specific Humidity Given as %

Measuring Water Vapor in the Atmosphere 6) Dew Point - temperature to which the air needs to be cooled to become saturated. called dew point because dew forms temperature of saturation Important method used in weather reports and by meteorologists. The closer the dew point is to the air temperature, the higher the relative humidity

Measuring Water Vapor in the Atmosphere 6) Dew Point - temperature to which the air needs to be cooled to become saturated. Dew Point as Indicator of humidity a)When the dew point is high there is abundant moisture in the air. b)Indicator of relative humidity Dew point is much lower than the air temperature, RH is low. Dew point is close to air temperature, RH is high. Dew point equals air temperature and the air is saturated, RH=100%. Frost point - when the temperature is below 0 o C, the term “frost point” is applied.

Measuring Humidity 1) Sling Pyschrometer - instrument used for measuring humidity –Consist of two thermometers Wet bulb Dry bulb Wet bulb thermometer has a cotton wick wrapped around the bulb that is saturated with water. Dry measures air temperature If the air is unsaturated, water evaporates from the wet bulb and the temperature falls because latent heat is consumed to evaporate the water from the cotton wick.

Measuring Humidity Wet bulb depression - difference in temperature between the dry and wet bulbs. Depends on the moisture content (humidity) of the air.

Measuring Humidity 2) Hair hygrometer - uses human hair attached to a lever. Human hair will expand and contract in response to humidity. Hygrometer - any instrument used to measure humidity.

Ways in which the air becomes saturated 1.Adding water vapor to the air. Example: rain drops evaporating produces precipitation fog. 2.Mixing cold air with warm air. Example: When warm moist air mixes with cool air, the new air may be saturated. In steam fog cold air mixes with warm moist air over a lake. 3.Lowering air temperature to the dew point. Example: At night the air cools down. If it cools to the dew point temperature the air will condense producing dew or radiation fog.

Nucleation Even of the air is saturated condensation may not occur without the aid of solid particles. Effects of impurities (aerosols) on nucleation Homogeneous nucleation - water droplets condense from chance collisions and bonding of water molecules Heterogeneous nucleation - formation of water droplets onto aerosols. Hygroscopic aerosols - particles in the atmosphere that attract water and serve as nuclei.

Ice Nucleation Even if the air temperature is below 0 o C, it may condense as water droplets and not ice. Supercooled liquids - water cooled to below the ice point but existing as liquid. 0 to -10 o C - supercooled liquids Formation of ice at these temperatures requires ice nuclei. Below -10 to -30 o C - both ice crystals and supercooled liquids may condense

Cloud Formation Lowering the air temperature to the dew point is an important mechanism for cloud formation. Adiabatic process - process in which temperature changes but heat energy is not added or removed.

Thermodynamics - science concerned with the relationship between heat energy and temperature. First law of thermodynamics Heat added to a system will result in some combination of volume change (expansion of a gas) and increase in its temperature. ∆ H = p. ∆V + Cv. ∆T ∆ H - Change in heat p - pressure ∆V - Volume Change Cv - Specific heat (energy needed to change the temperature of one gram of a substance by 1 o C. ∆ T - Temperature Change

For adiabatic systems ∆H = 0 ∆ H = p. ∆V + Cv. ∆T ∆ H - Change in heat p - pressure ∆V - Volume Change Cv - Specific heat ∆ T - Temperature Change 0 = p. ∆V + Cv. ∆T p. ∆V = - Cv. ∆T For adiabatic systems gases cool as they expand. For adiabatic systems gases warm as they contract.

As air rises, it encounters lower surrounding pressure, it expands and therefore cools adiabatically. Adiabatic Lapse rate - rate at which air cools as it rises. dry adiabatic lapse rate (DALR) - rate at which a rising of dry (unsaturated) air cools. about 1 o C/100m (5.5 o F/1000 feet) No heat is removed but rising air will cool 1 o C for every 100m that it rises into the atmosphere.

dry adiabatic lapse rate (DALR) - rate at which a rising parcel of dry (unsaturated) air cools. about 1 o C/100m (5.5 o F/1000 feet) Lifting Condensation level (LCL) - the height at which a rising parcel of air undergoes condensation or deposition. Saturated adiabatic lapse rate (SALR) - rate at which a rising parcel of wet (saturated) air cools. about 0.5 o C/100m (3.3 o F/1000 feet) Environmental Lapse Rate (ELR) - rate at temperature in the troposphere decreases with height.

Forms of Condensation 1) Dew - liquid condensation onto a surface. Forms in the morning Dew is most likely to form on cloudless and windless nights. 2) Frost - similar to dew but saturation temperature is below 0 o C. When the air is lowered to the frost point, the water vapor in the air may go directly to ice crystals.

Forms of Condensation 3) Frozen Dew - Differs from frost Frozen dew is a continuous layer of ice. Forms when liquid dew freezes Frozen dew on roadways is called “black ice” 4) Fog - a cloud whose base is near the surface. Can be a meter to tens of meters high.

Forms of Condensation Types of fog: (a) Precipitation fog - results from evaporation of falling raindrops. (b) Steam Fog - forms when cold air mixes with warm moist air above a water surface.

Forms of Condensation Types of fog: (c) radiation fog (ground fog) - results from loss of longwave radiation and cooling of the surface at night. Forms on cloudless nights with gentle winds (d) advection fog - forms when relatively warm moist air moves horizontally over a cold surface. As the warm air moves over the surface it transfers heat downward causing it to cool. If it cools to the dew point, fog will form. Examples: San Francisco Bay area - Warm Pacific air moves over the cool California Ocean current. London in winter - Warm air from the Gulf Stream moves over England where it is chilled to the dew point.

Forms of Condensation Types of fog: (e) Upslope Fog - formed by adiabatic cooling of air as it moves up a slope.