1. Course Schedule Overview Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Feb-16-07 (1 of 1) GG101 Overview (L01) Part 1: –General context/physical geography (L02) –Global radiation and energy system (L03-L09) –Temperature regimes and temperature cycles (L10-L11) –Midterm-01 (Feb-14-07) Part II: –Focus on the dynamics/processes associated with weather (L12-L22 or L23) Clouds, pressure, winds, fronts, air masses, etc… Circulation (Atmos and Oceans) Midterm-02 (Mar-28-07) Part III: –Global climates and Climate Change (L23 or L24-L32) –Last class on April 30th FINAL Thursday May 10th 9-11 am

2. Outline Further Reading: Chapter 06 of the text book - the hydrosphere and the hydrological cycle - humidity - consequences Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (1 of 13)

3. Introduction Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (2 of 13) Focus on dynamics of the atmosphere (Part II) Consists of three components: –Moisture in the atmosphere Clouds and precipitation –Atmospheric motions Winds and global circulation –Weather systems Mechanisms behind day to day weather Moisture in the Atmosphere –Key component of climate and weather systems Hydrologic cycle Atmospheric humidity Cloud formation Precipitation Two important points made earlier Water can take three forms within the earth system: liquid, solid, gas Evaporation/condensation involves significant amounts of energy

4. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (3 of 13) Hydrosphere-1 –Note that 97% of all water on Earth is tied up in the oceans This water remains in the oceans for long periods of time (~1000yrs) However the ocean’s circulation patterns are important for global climate, climate change, and geographic patterns in weather - we’ll get to this later

5. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (4 of 13) Hydrosphere-2 –Only ~3% of water takes other forms Most is trapped in the form of ice and glaciers - again this is an inert form in that it doesn’t allow for transfer of water between systems 0.6% is actually stored in ground water in the bedrock, more than in all the lakes and streams combined This leaves only 0.02% of all water which is “active” in that it actively cycles between the oceans, land, and atmosphere This cycling is called the “hydrologic cycle” and takes place mainly within streams, the atmosphere, and soil water

6. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (5 of 13) The Hydrological Cycle-1 Represents the constant cycling and movement of water between the oceans, land and atmosphere

7. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (6 of 13) The Hydrological Cycle-2 –Represents the constant cycling and movement of water between the oceans, land and atmosphere –Ocean/land/atmosphere represent “reservoirs” of water –Fluxes between reservoirs occurs via: Evaporation Condensation (cloud formation) Precipitation Runoff (i.e. flow from the land to the ocean) –Note that the Hydrologic cycle dynamics involves only a very small percentage of water in the total earth system Atmosphere holds 0.001% of all water Land holds 0.02% of all water –But the change of state of water (i.e. its thermodynamics involves large amounts of energy: Latent heat is ~22% of total solar energy

8. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (7 of 13) Phases of Water –Energy is removed from water when it goes from gas to liquid to solid; this represents a release of energy by the water –Energy is put into water when it goes from solid to liquid to gas; this represents a absorption of energy by the water Let’s examine the gas phase of moisture first …

9. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (8 of 13) Humidity –Water is one of the key constituents of the atmosphere –This is how we define how much water is in the atmosphere (two key variables) How much is actually in the atmosphere How much can the atmosphere actually hold Specific Humidity: –How much water vapor is in the atmosphere –Given as kg(water)/kg(air) –We measure this

10. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (9 of 13) Saturation Humidity Saturation humidity: Maximum amount of water the atmosphere can hold for a given temperature –Saturation humidity depends primarily on temperature of the atmosphere –Cold temperatures Can hold little water Large temperature change doesn’t add much water –Warm temperatures Can hold lots of water Small temperature changes produces large change in how much water atmosphere can hold –We estimate this, i.e. we measure temperature then calculate what this number is

11. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (10 of 13) Relative Humidity Ratio of Specific Humidity to Saturation Humidity –Given as percentage –Highest percentage is 100%, i.e., specific humidity is equal to saturation humidity Atmosphere is saturated If water is added, it must go into liquid, not vapor; i.e. it condenses out –For unsaturated air, if we fix the temperature and increase the water vapor, relative humidity goes up –If we fix the water vapor amount and increase the temperature, relative humidity goes down –Now if we have a fixed amount of water vapor, and the temperature decreases we find that some of the water has to condense out as liquid water (clouds and rain) because the atmosphere can’t hold as much in the form of vapor

12. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (11 of 13) Dew Point Temperature –Given a fixed amount of water vapor, how much would we have to cool the parcel to have water begin to condense out –Always less than or equal to the actual temperature

13. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (12 of 13) Consequences-1 –We can chart the three types of humidity over their diurnal cycle –We can look at dew point and see that it stays fairly constant - this (generally) means the specific humidity is fairly constant –We can also look at the temperature which increases during the day; this means that the saturation humidity is also increasing –Because the specific humidity is constant and the saturation humidity is increasing, we expect the ratio, i.e. the relative humidity to decrease over the course of the day

14. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Myneni Lecture 12: Moisture-and-Precipitation Feb-16-07 (13 of 13) Consequences-2 –Specific humidity maximum at the equator and minimum at the poles -> warm air can hold more water than can the cold air –Also specific humidity in deserts more than at the poles –At poles, the air is closer to saturation –Deserts have lower relative humidity than at the poles -> we consider the deserts “dry” because relative humidity is low