The Water Cycle. Water in the Atmosphere AOSC 200 Tim Canty. Class Web Site:

Transcription

1 Water in the Atmosphere AOSC 200 Tim Canty Class Web Site: Topics for today: Latent Heat Evaporation & Saturation Relative Humidity Dew Point Lecture 11 Oct The Water Cycle Fig 4.1: Essentials of Meteorology 2

2 The Water Cycle Fig 5.1 Weather: A Concise Introduction 3 Ways to get water to condense: 1) Decrease the air temperature 2) Increase the amount of water vapor 4

3 Air Pressure Fig 1.7: Essentials of Meteorology 5 Atmospheric Composition (What are you breathing?) Table 1.1: Essentials of Meteorology 6

4 Vapor Pressure The pressure you feel is due to the atmosphere pushing against your body. The air is made up of a collection of different gases. The total pressure is the sum of the pressure due to each gas. Fig 1.7: Essentials of Meteorology 7 8

5 Example: Air pressure is = mb If H 2 O (water vapor) is 2.22% The pressure due to H 2 O is x mb = 22.7 mb This is what is called the vapor pressure (e) If the air can t hold any more water vapor, it is saturated This is the saturation vapor pressure (e s ) 9 Vapor Pressure This boundary indicates that the air is completely saturated with water (100% relative humidity) Actual vapor pressure: the atmospheric pressure due to water Saturation vapor pressure: the atmospheric pressure if the air was saturated Fig 4-3 Meteorology: Understanding the Atmosphere 10

6 Different ways to think about humidity 1. Absolute humidity: the mass of water vapor per volume 2. Specific humidity: the mass of water vapor per mass of dry air 3. Relative humidity: percent of water vapor present in air compared to the maximum at saturation; RH= e/e s 11 Relative Humidity Temperature and relative humidity are anti-correlated Fig 4.7: Essentials of Meteorology 12

7 Relative Humidity As T and T d get closer, relative humidity increases The blue arrows indicate when a cold front passed through Fig 5.8 Weather: A Concise Introduction 13 Relative Humidity As T decreases, saturation vapor pressure (e s ) decreases while vapor pressure (e) stays constant Relative humidity increases Vapor pressure does start to decrease after RH reaches 100% and e s continues to drop Why? Table 5.1 Weather: A Concise Introduction 14

8 Dew Point Fig 5.5 Weather: A Concise Introduction 15 Frost Point (when T d is at or below freezing) Fig 5.6 Weather: A Concise Introduction 16

9 Fog If air in contact with the ground cools enough, water will condense and form a suspension of tiny water drops This is called a fog 17 Radiation Fog The surface cools as heat radiates away. Air close to the ground cools enough for water to condense in the air Fig 4.21: Essentials of Meteorology 18

10 Advection Fog Warm, moist air blows over a cold surface (land or water) and cools enough for droplets to form Fig 4.21: Essentials of Meteorology 19 Upslope Fog Winds push moist air uphill to an altitude where temperatures are cold enough for condensation to occur Fig 4.22: Essentials of Meteorology 20