Moisture, Precipitation and the Hydrologic Cycle

1. Moisture, Precipitationand the Hydrologic Cycle The Interface between Atmosphere and Hydrosphere

2. First, Let’s Recall the • 5+ Basic Elements of the Atmosphere • – the main ingredients of weather and climate • -- Also called Elements of Weather and Climate • Solar Energy • -- Insolation and Heat Energy Transfer • Temperature • Pressure • Wind • In this segment, we’ll discuss the fifth element: • Precipitation • + Air Masses (and Fronts)

3. Atmospheric Water – Moisture → the Hydrosphere • Atmospheric water or moisture is the source of condensation, • cloud formation,and ultimately precipitation, which • is one of the most important elements of weather and climate, being the principal regulator and modifier of Earth’s temperature • These processes are also integral parts of the hydrosphere, and The Hydrologic Cycle. • Hence, we’ll first discuss thesignificance of water in the context of the hydrosphere and the hydrologic cycle, and • then specifically examine the precipitation process as a • component of weather and climate systems

4. The Hydrosphere • Water in its various forms • sustains life;without • water, livingthings • could not survive. • 73% of the earth surface is • covered with water • (1.33 billion cubic kilometers or 326 million cubic miles) • 97.1% of earth’s waters are in oceans (salt water) • 2.24% are in glaciers, including polar ice sheets (fresh water) • 0.61% are deep and shallow groundwater (fresh water) • 0.05% -- the remaining, are in fresh and salt water lakes, rivers, streams, soil root zones, and the atmosphere! • These bodies of water provide reservoirs of heat in the winter,and cooling effects in the summer

6. Properties of Water • A water molecule is composed of two atoms of hydrogen and one atom of oxygen,and it can exist in all three states or phases of matter. Phases of Water • Universal solvent – dissolves enormous numbers of substances • Transports many minerals and solid particles, even in suspension • Capillary action – ability of water to pull itself upward through small openings, against gravity • http://www.youtube.com/watch?v=dabPFNxo844&feature=related

7. What percent of the Earth’s total volume of water is stored in the atmosphere? • 0.001% • Water Vapor • Clouds (water vapor condensed on particulates)

8. Transfer of Water Into and Out of the Atmosphere • Evaporation [Contributes 90% of atmospheric water] The principal way in which water enters the air is through evaporation. Approximately 600 calories of heat must be added to a gram of water for it to evaporate into the air.  This energy is called "the latent heat of vaporization." Energy is transferred between the various spheres of the Earth system via phase changes of water. [See previous Figure] Evaporation, thus, is the phase change fromliquid water to a Gas form (vapor). Three important factors determine the Rate ofEvaporation: • 1) temperature ofavailable water; • 2) relative humidity of surrounding • air; and • 3)wind.

9. Transpiration [Contributes 10% of atmospheric water] Plants are another important source of atmospheric moisture. Plants contribute water to the atmosphere by transpiration. Transpiration is the transfer of water into the air via leaf pores. Together, evaporation and transpiration, or Evapotranspiration, accounts for virtually all the water vapor in the air [See: Potential Evapotranspiration]

10. Moisture and Humidity Humidity is a measure of the water vapor content of the air. There are several ways in which a meteorologist can express the humidity of the air. Each humidity measure is controlled to some degree by air temperature. Absolute humidity -- measure of the mass of water vapor per unit volume of air e.g., grams per cubic meter). [Often, specifichumidity is used, toexpress mass of water vapor in grams per kilogram of air] Relative humidity -- the ratio between the amount of water vapor in the air and the water vapor capacity of the air, at a given temperature; reported as a percentage that expresses how close the air is to saturation:

11. Saturation – when air of a given temperature holds all the water vapor it possibly can, it is said to be saturated, having reached its capacity. • Dew Point – is the critical temperature at which the air is fully saturated – the cooling of air to below its dew point temperature that brings about condensation that must precede any precipitation. • This progression from evapotranspiration to precipitation leads to a cycle – • Evapotranspiration Water Vapors  Condensation  Cloud Formation  Precipitation  Evapotranspiration • -- the cycle known as the Hydrologic Cycle!

12. The Hydrologic Cycle

13. The Condensation Process • Condensation is the process that changes water vapor into a liquid form. • It occurs when air saturated with water vapor is cooled; • So, condensation depends on: • the relative humidity of the air, and • the degree of cooling • During the process, water molecules lose the 600 cal/gm of latent heat that were added during the process of evaporation  Latent Heat of Condensation • Another factor necessary for the process – the presence of condensation nuclei* – minute particulates in the air, like sea-salt, dust or pollen • * Or a condensation surface, as in the case of dew formation

14. Fog andCloud appear when water vapor condenses on nuclei and a large number of these droplets form a mass – these masses of condensed water droplets, fog or cloud, assume any number of shapes and forms, usually in shades of white or gray;Fogs are essentially low level clouds that touch the ground • Dew is formed by the condensation of water vapor on a surface • Smogis formed when condensation takes place on chemical nuclei, making it corrosive and dangerous to human health • White Frost forms in below freezing temperatures when water vapor changes directly to the frozen state through the sublimationprocess • Rime – ice crystal formation on tree branches or airplane wings when supercooled droplets come in contact with the surface

15. CLOUDS classification based on height and shapehttp://www.youtube.com/watch?v=ULvujTon5tU

16. Cloud Forms • Based on Height or Altitude: • Strato (low) • Alto (mid) • Cirro (high) • Based on Shape or Form: • Cirrus (wispy) • Stratus (layers) • Cumulus (heap or pile) • Nimbo / Nimbus (rain is falling) • -- rain bearing clouds, e.g., Cumulonimbus • The cooling process that leads to Cloud Formation is quite different from that associated with fog, frost or dew (radiation or advection) • It is known as Adiabatic Cooling Adiabatic Lapse Rate (5.6oF/1000 ft) – Dry Adiabatic Lapse Rate vs. Wet Adiabatic Lapse Rate • Also, Adiabatic Heating relative to descending air

17. Precipitation Processes Precipitation from a meteorological stand point is water in some form, falling out of the air, and settling on the surface of the earth. This distinguishes between forms of condensation in the atmosphere and condensation that occurs at the surface. Dewis condensation at the surface and thus is not a form of precipitation. Forms of Precipitation • There are a variety of different types of precipitation. Rain, snow, hail, sleet, freezing rain are all forms of precipitation. The kind of precipitation received  depends on the variation of temperature above the surface. Rainis precipitation in liquid form. Snow is precipitation in solid form as (typically) a hexagonal crystal shape. If the air temperatures are below freezing on its journey toward the surface, precipitation will be in the form of snow.

19. Sleet occurs when snow falls through a warm layer of air and melts. • Freezing rain or glaze occurs when snow melts upon passing through a warm layer of air and then freezes on the surface whose temperature is at or below freezing. • Hail falls as rounded pellets or balls of ice from severe thunderstorms.  • In sum, Precipitation occurs when the droplets develop masses too great to be held aloft. • And, three conditions are necessary for precipitation to occur: • 1) Moist air, 2) Condensation nuclei, and 3) Uplift mechanism. Precipitation Types by Uplift Mechanism: • 1. Convectional Precipitation http://www.youtube.com/watch?v=RkgThul2El8&feature=related • 2. Relief /Orographic Precipitation • http://www.youtube.com/watch?v=BVykQfRC_aI&feature=related

20. 3. Frontal Precipitation • http://www.youtube.com/watch?v=D88dYNFyBq8&feature=related • 4. Cyclonic Precipitation • Occurs when air is lifted up into a low pressure system. • http://www.youtube.com/watch?v=75qAgSuMbzA&eurl=http%3A%2F%2Fwww.uwsp.edu%2Fgeo%2Ffaculty%2Fritter%2Fgeog101%2Ftextbook%2Fweather_systems%2Fhurricanes.html&feature=player_embedded

21. DISTRIBUTION OF PRECIPITATION – See Figure 6.23 in Textbook • Isohyet Map – Look for a) general pattern, b) evidence of orographic rain?

22. VARIABILITY OF PRECIPITATION – See Figure 6.25 in Textbook • Do the areas of greater variability generally correspond to the dryer areas of • the world, as shown on the other map? (Compare this map with Figure 6.23)