GEOG220 Study Guide - Midterm Guide: Automated Airport Weather Station, Vapor Pressure, Latent Heat
18 May 2018
School
Department
Course
Professor
Exam 2 Study Guide
Chapter 4
4.1 Water on earth
● Movement of water through atmosphere
○ Hydrologic cycle is continuous exchange of water among the oceans, and
continents
■ Evapotranspiration - small amount of groundwater is taken up by
plants and released into atmosphere by plants
○ Total amount of water in atmosphere remains constant
■ Avg annual precipitation over earth = quantity evaporated
○ Over continents, precipitation is greater than evaporation
4.2 Water’s Changes of State
● Latent Heat
○ Heat involved when water changes state is measured in units called
calories
○ One calorie = amount of energy required to raise 1g of water 1C
■ 1 calorie = 4.184J
○ Energy can be added without raising temp
■ Ice melting, temp does not change until lice is completely melted
■ Breaking bonds
○ Latent heat - energy used does not change temp
■ Melting 1g of ice requires 80 calories (latent heat of melting) and
freezing it would release 80 calories per gram to environment
(latent heat of fusion)
● Evaporation and Condensation
○ Latent heat is involved in evaporation
○ More energy needed for ice to water than water to gas
○ Latent heat of vaporization give energy needed for molecules to escape
surface of liquid
■ Varies from 600 (OC)-600 (100C) calories
○ During evaporation, avg molecular motion (temp) of remaining water is
lowered
■ cooling process
○ Condensation - water vapor releases latent heat - latent heat of
condensation equal to what was absorbed during evaporation
○ Formation of clouds - heat is released - giving it buoyancy
● Sublimation and Deposition
○ Sublimation - solid to gas
○ Deposition - vapor to solid
4.3 to 4.4 Humidity
● Absolute humidity
○ Expresses quantity of water vapor in specific amount of air
○ Mass of water vapor given volume of air
○ = mass water vapor / volume of air
○ Changes in pressure and temp can change its volume - when this
changes, absolute humidity changes
○ Can change without water being added/taken out of system
● Mixing ratio
○ Expresses quantity of water vapor in specific amount of air
○ Mass of water vapor in a unit of air compared to remaining mass of dry air
○ =mass of water vapor / mass of dry air
○ Since uses mass, not affected by changes in pressure or temp
○ Preferred over absolute when expressing water vapor of air
● Vapor pressure
○ Most Used to describe moisture content & relative humidity
○ Part of the total atmospheric pressure attributable to its water vapor
content
○ Saturation is hit when a balanced is reached so that number of particles
returning to surface = number leaving
○ Saturation vapor pressure is temp dependent
■ Higher temp = higher temp takes more water vapor to saturate
○ For every 10 C increase in temp, amount of water vapor needed for
saturation almost doubles
○ More often than not, more water vapor leaving surface than returning - net
evaporation
○ Formation of fog is the opposite - net condensation
○ Temp determines which is happening more (condensation or evaporation)
■ At higher temps water vapor can escape easier
■ When air is dry (low pressure) rate water escapes is high
■ vapor pressure increases = rate of condensation increases
● Relative humidity
○ Most Used to describe moisture content & vapor pressure
○ Is ratio of air’s actual water vapor content compared with amount of water
vapor required for saturation at that temp (and pressure)
○ How close to saturation rather than actual quantity of water vapor in air
○ = mixing ratio / saturation mixing ratio x 100
○ When 100% = saturation
○ Based on airs water-vapor content and amount of moisture required for
saturation - 2 factors
■ Changes when water vapor is added or removed
■ Changes bc of amount of moisture required for saturation is a
function of air temps
○ Decrease in temp = increase in relative humidity
○ Natural changes in RH
■ Daily changes in temp (daylight vs night)
■ Temp changes that result with air moves horizontally from one
location to another
■ Temp changes caused when air moves vertically in atmosphere
○ Mid Afternoon RH richest lowest and evenings reaches highest
● Dew point
○ The temp at which WV begins to condense or when reaches saturation
○ Directly related to actual moisture content in a parcel of air
○ For every 10 C increase in dew point - air contains about twice as much
water vapor
● Measuring humidity
○ Hygrometers - used to measure moisture content of the air
■ Most measure RH or dew point temps
○ Psychrometers - simplest, 2 thermometers mounted side by side
■ Dry and wet bulbs (wet has thin cloth wick tied to bottom that is
saturated with water)
■ Continuous current of air passes over wick -> water evaporates
absorbing heat energy from wet bulb causes temp to drop
■ Amount of cooling is proportional to dryness of air
■ Dryer air = greater cooling
■ Greatest difference between wet and dry = lower humidity
○ Hair hygrometers
○ Electric hygrometers
■ Chilled mirror
■ Automated Weather Observing System works on the principle of
capacitance - materials ability to hold electrical charge
■ Higher capacitance = higher RH
4.5 Adiabatic Temp Changes and Cloud Formation
● Condensation happens when enough water vapor is added to atmosphere or
more often when temp drops to dew point
Document Summary
Hydrologic cycle is continuous exchange of water among the oceans, and continents. Evapotranspiration - small amount of groundwater is taken up by plants and released into atmosphere by plants. Total amount of water in atmosphere remains constant. Avg annual precipitation over earth = quantity evaporated. Over continents, precipitation is greater than evaporation. Heat involved when water changes state is measured in units called calories. One calorie = amount of energy required to raise 1g of water 1c. Energy can be added without raising temp. Ice melting, temp does not change until lice is completely melted. Latent heat - energy used does not change temp. Melting 1g of ice requires 80 calories (latent heat of melting) and freezing it would release 80 calories per gram to environment (latent heat of fusion) More energy needed for ice to water than water to gas. Latent heat of vaporization give energy needed for molecules to escape surface of liquid.
