1. Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt Anthony J. Vega

2. Part 2. Water in the Atmosphere Chapter 5. Atmospheric Moisture

3. Introduction Over 70% of the planet is covered by water Water is unique in that it can simultaneously exist in all three states (solid, liquid, gas) at the same temperature Water is able to shift between states very easily The hydrologic cycle refers to the regular cycle of water through the earth-atmosphere system Liquification of water occurs frequently at normal Earth temperatures Occurs when air is saturated with respect to water vapor The addition of water vapor, or the lowering of temperature, in saturated air will lead to condensation

4. Evaporation Occurs if energy is available to a water surface Water vapor increases in air as surface water evaporates Upon saturation, condensation will begin and water will return to the surface Saturation marks an equilibrium between evaporation and condensation Saturation may occur in the presence or not in the presence of dry air, so that the statement that air “holds” water is erroneous

5. Changes of state may also occur with regard to water vapor changing directly to ice Deposition Or, the inverse situation Sublimation Indices of Water Vapor Content Humidity indicates the amount of water vapor in air Humidity expressed through a variety of ways Each has advantages and disadvantages All indices refer solely to water vapor and exclude liquid and frozen states Vapor Pressure Simply the amount of pressure exerted on the atmosphere by water vapor Dependent upon both temperature and density of the vapor with density most important

6. The maximum water vapor pressure which can occur is termed saturation vapor pressure Saturation vapor pressure is solely temperature dependent It exponentially increases with temperature such that high temperatures may have extremely high saturation vapor pressures compared to lower temperatures The movement of water vapor molecules exerts vapor pressure on surfaces

7. Absolute Humidity Indicates the density of water vapor expressed in g/m 3 Changes as air volume changes Specific Humidity Represents a given mass of water vapor per mass of air in g/kg Term does not vary with air volume fluxes Does not change with temperature changes Saturated air has the highest specific humidity for a given temperature and pressure = saturation specific humidity Exponential increase in saturation vapor pressure with increase in temperature

8. Mixing Ratio Very similar to specific humidity in that it expresses the mass of water vapor relative to air mass However, mixing ratio expresses the amount of water vapor relative only to a mass of dry air Maximum mixing ratio = saturation mixing ratio Relative Humidity Most commonly used expression of water vapor content Indicates the amount of water vapor in the air relative to the possible maximum Given as a percentage Does not indicate the amount of air which is water vapor but instead describes the amount present relative to a saturation point The saturation point, thus the relative humidity term, is relative to air temperature and total water vapor present

9. More water vapor can exist in warm air than cold, the term is sometimes misleading An example involves the diurnal distribution of RH in which the highest RH occurs in the morning during the coolest time of the day The lowest RH values will be recorded in late afternoon, the time of greatest air temperature This makes high temperature/high relative humidities (90 o F, 90% RH, or so) impossible Because of temperature dependency the term cannot be used to compare moisture content at different locations having different temperatures

10. The relationship between RH and temperature

11. Dew Point The dew point temperature is the temperature at which saturation occurs in air Reached either by increasing water vapor content or by chilling air (while holding moisture content constant) Good indicator of moisture content in air Relatively high dew points indicate abundant atmospheric moisture Dew points can be only equal or less than air temperatures If saturation is reached and air temperatures cool further, water vapor is removed from the air through condensation When air reaches saturation at temperatures below freezing the term frost point is used

12. Dew point/temperature relationships in a) unsaturated air b) and c) saturated air

13. Methods of Achieving Saturation Air may become saturated through the addition of water vapor to air at a constant temperature Example: light fogs formed beneath clouds as vapor is added through falling raindrops Or by mixing cold air with warm, moist air Example: Contrails and steam fogs which develop as cold air passes over warm water bodies Or by cooling air to the dew point The most common way Effects of Curvature and Solution Condensed water suspended in the atmosphere is typically curved Impurities also exist Both factor into phase shifts

14. Effect of Curvature Small drops exhibit greater curvature than larger ones Curvature influences saturation vapor pressure with highly curved drops requiring RHs in excess of 100% to remain liquid For very small drops, supersaturation may approach 300% Hygroscopic aerosols acting as condensation nuclei help keep RHs below these extremes Condensation onto such particles, called heterogeneous nucleation, causes dissolution of the aerosol Larger drops have less curvature than smaller ones

15. Effect of Solution Evaporation from solutions is less than from pure water This directly opposes curvature influences such that condensation typically occurs at RHs near 100% Hygroscopic nuclei abound in the atmosphere from many natural (salt, dust, ash, etc.) sources and anthropogenic (combustion derivative) sources Very small condensation nuclei lead to very tiny water drops = haze Small droplets require higher RHs to remain liquid

16. Ice Nuclei Atmospheric water does not freeze at 0 o C (32 o F) Leads to the presence of supercooled water Ice crystal formation requires ice nuclei A rare temperature dependent substance similar in shape to ice Examples: clay, ice fragments, bacteria, volcanics, etc.) Ice nuclei become active at temperatures below -4 o C Between -10 o and -30 o C (14-22 o F), saturation may lead to ice crystals, supercooled drops, or both Below -30 o C, clouds are composed solely of ice crystals At or below -40 o C (-40 o F) spontaneous nucleation, the direct deposition of ice with no nuclei present, occurs

17. Measuring Humidity The easiest way to measure humidity is through use of a sling psychrometer A pair of thermometers one of which has a wetted cotton wick attached to the bulb The two thermometers measure the wet and dry bulb temperature Swinging the psychrometer causes air to circulate about the bulbs When air is unsaturated, evaporation occurs from the wet bulb which cools the bulb Once evaporation occurs, the wet bulb temperature stabilizes allowing for comparison with the dry bulb temperature The wet bulb depression is found with a greater depression indicative of a dry atmosphere Charts gauge the amount of atmospheric humidity Aspirated and hair hygrometers are alternatives

18. High Humidities and Human Discomfort Temperature extremes account for more fatalities than severe storms, of all types, combined High temperature extremes are compounded by humidity (and other factors such as wind and intensity of sunlight) The effect of humidity and high temperatures can be expressed in a heat index Humans are cooled by the release of perspiration which cools the body by evaporating into air When the atmosphere has a high moisture content, the rate of evaporation is effectively reduced This leads to a reduction in the cooling power of perspiration This increases the apparent temperature of the air leading to heat related health risks Muscle cramps, heat exhaustion, heat stroke (potentially fatal)

19. Cooling Air to the Dew or Frost Point Most condensation processes occur as air is chilled to the dew point Air temperature changes either from direct energy exchanges (diabatic processes) or from those involving no net energy exchange (adiabatic processes) Diabatic Processes Involve the direct addition or removal of heat energy Example: Air passing over a cool surface loses energy through conduction Energy is always transferred from areas of high temperature toward those of lower temperatures The Second Law of Thermodynamics Diabatic processes are typically associated with fog development

20. Adiabatic Processes Cloud formation typically involves temperature changes with no net exchange of energy Such processes occur according to the First Law of Thermodynamics Rising air expands through an increasingly less dense atmosphere causing a decrease in internal energy and a corresponding temperature decrease Parcels expand and cool at the dry adiabatic lapse rate 1 o C/100 m (5.5 o F/1000 ft) Sinking parcels experience exactly proportional compression warming Parcels may eventually reach the lifting condensation level, the height at which saturation occurs Parcels then cool at the saturated adiabatic lapse rate ~0.5 o C/100 m (3.3 o F/1000 ft)

21. Dry adiabatic cooling

22. The Environmental Lapse Rate The environmental (ambient) lapse rate (ELR) refers to an overall decrease in air temperature with height This rate, which changes diurnally from place to place, stems from the fact that air located farther from surface heating is typically cooler than that nearer the surface A comparison of adiabatic and environmental cooling rates

23. Forms of Condensation Many forms of either liquid or solid condensation can occur depending on particular process characteristics Dew Liquid condensation on surface objects Diabatic cooling of surface air typically takes place through terrestrial radiation loss on calm, cool, clear nights Surface air becomes saturated and condensation forms on objects acting as condensation nuclei Frost Similar to dew except that it forms when surface temperatures are below freezing Deposition occurs instead of condensation May be referred to as white or hoar frost

24. Frozen Dew Occurs when normal dew formation processes occur followed by a drop in temperature to below freezing Ensures a tight bond between ice and the surface Causes “black ice” on roadways Fog Simply a surface cloud when air either cools to the dew point, has moisture added, or when cooler air is mixed with warmer moister air Radiation Fog Occurs when near surface air chills diabatically to saturation through terrestrial radiation loss on clear cool nights Require a slight breeze to vertically mix air through a shallow column

25. Dew and Frost

26. Radiation fog in the Central Valley of California If winds exceed about 5km/hr (3 mph) warmer air from aloft will mix with the near surface air and evaporate the fog After sunrise, the fog evaporates from below due to surface heating

27. Advection Fog Occurs when warm moist air moves across a cooler surface Air is chilled diabatically to saturation Common on the U.S. west coast as warm, moist air from the central Pacific advects over the cold California ocean current Frequently develop near boundaries of opposing ocean temperatures Example: Off the northeast coast of the U.S. Upslope Fog The only fog developed through adiabatic cooling Occur when air is advected over land surfaces which increase in elevation A common occurrence in the Great Plains of the U.S. where warm, moist air advects from the Miss. River Valley towards the Rocky Mountains

28. Different types of fog found throughout the U.S.

30. Formation and Dissipation of Cloud Droplets Clouds are mainly associated with adiabatic cooling of rising air Dew points decrease as air rises at the shallow dew point lapse rate 0.2 o C/100 m (1.1 o F/1000 ft) Approximately 50 m above the lifting condensation level, all condensation nuclei have condensed water attached Leads to additional growth of those drops over the creation of new drops Process soon stops leaving drops to slowly evaporate or sublimate

31. End of Chapter 5 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt