Water Vapor & Heat Imbalances 3&4

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CHAPTER 3

Water Vapor

Water Vapor

• It constitutes only a small percentage of the Earth’s atmosphere, varying from only trace amounts to 4 percent by volume.
•  Approximately half of all of the atmospheric water vapor is found below 2 kilometers (6,500 feet) altitude.
• Most variable component
• Residence time of a water molecule in the atmosphere = 10 days

WATER VAPOR THEORY

• Each individual gas exerts pressure, referred to as partial pressure, which then makes up the entire pressure of 1013.2mb
• Vapor Pressure is the term used to describe the pressure (force per unit area) exerted by water vapor in the atmosphere
• When the amount of molecules leaving a water surface equal the number arriving, the air is saturated
• The vapor pressure exerted by the water vapor in this equilibrium state is known as Saturated Vapor Pressure

Partial Pressure

Vapor Pressure of Water

Partial Pressure and Vapor Pressure

• Partial pressure = Pressure water vapor actually exerts
• Vapor pressure = Pressure water vapor could exert if the atmosphere were saturated
• Most of the time the partial pressure is less than the vapor pressure

WATER VAPOR THEORY

• Relative Humidity is % of actual water vapor present to that which could be present
• It is found by taking vapor pressure divided by saturation vapor pressure and multiplying by 100
• Or you can use a Sling Psychrometer then compare the wet bulb temp and the dry bulb temp to a chart and presto you have RH
• Dew point is the point to which the air must be cooled to become saturated
• Dew point is always colder than the temp except when saturation (100% RH is achieved), then they are equal

Vapor Pressure and Humidity

• The higher the vapor pressure, the more moisture the air can hold
• Vapor pressure increases with temperature
• If partial pressure (water vapor actually in air) stays constant, then –
• Humidity decreases as it gets hotter
• Humidity increases as it gets cooler
• This is the primary concept behind cloud formation

WATER VAPOR THEORY

• The higher the temp the more moisture can be present
• H2O molecules change state constantly
• If more leave than arrive then net evaporation takes place
• If more arrive than leave net condensation

WATER VAPOR THEORY

• This exchange takes place all the time
• Vapor arrival depends upon:
• Vapor pressure
• The higher the pressure the greater the chance for a net condensation to occur

WATER VAPOR THEORY

• Vapor departure depends on 4 things:
• 1.  It’s state
• molecules leave a liquid easier than a solid
• 2.  The shape of the boundary
• molecules escape from curved surfaces easier than small droplets

WATER VAPOR THEORY

• 3.  The purity of the boundary
• foreign substances dissolved in the liquid or ice diminish their escape e.g. salt
• 4.  The temperature of the boundary
• higher temp, molecules have more energy and can escape easier

Boiling Water

• When the vapor pressure of water = atmospheric pressure, water boils
• Since air pressure drops with altitude, so does the boiling point of water

Hydrologic Cycle

• This is the continuous circulation of water in the Earth-atmosphere system.

Hydrologic Cycle

• Evaporation. Evaporation is the phase transition by which a liquid is changed to a vapor (gas).
• On average, about 120 centimeters (47 inches) is evaporated into the atmosphere from the ocean each year.
• For evaporation to take place, energy is required. The energy can come from any source: the sun, the atmosphere, the Earth, or objects on the Earth, such as humans.

Hydrologic Cycle

• Transpiration. Transpiration is the evaporation of water from plants.
• In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil.
• Of the transpired water passing through a plant, only 1 percent is used in the growth process of the plant. The remaining 99 percent is passed into the atmosphere.

Hydrologic Cycle

• Sublimation. Sublimation is the phase transition by which a solid is changed into vapor (a gas) without passing through the liquid phase.
• In the atmosphere, sublimation of water occurs when ice and snow (solids) change into water vapor (a gas).

Hydrologic Cycle

• Condensation. Condensation is the phase transition by which vapor (a gas) is changed into a liquid.
• In the atmosphere, condensation may appear as clouds, fog, mist, dew, or frost, depending upon the physical conditions of the atmosphere.

Hydrologic Cycle

• Transportation. Transportation is the movement of solid, liquid, and gaseous water through the atmosphere.
• Without this movement, the water evaporated over the ocean would not precipitate over land.
• Precipitation. Precipitation results when tiny condensation particles grow through collision and coalescence.

Hydrologic Cycle

•  Runoff. Runoff occurs when there is excessive precipitation and the ground is saturated (i.e., cannot absorb any more water).
• Evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and into the ground water, only to be drawn into plants again for transpiration to take place.
• Infiltration. Infiltration is the movement of water into the ground from the surface.

Hydrologic Cycle

• Groundwater Flow. Groundwater flow is the flow of water underground in aquifers. The water may return to the surface in springs or eventually seep into the oceans.
• Plant Uptake. Plant uptake is water taken from the groundwater flow and soil moisture.

Humidity

• Saturation. Saturation is the maximum possible quantity of water vapor that an air parcel can hold at any given temperature and pressure
• Relative Humidity. Relative humidity is the ratio, usually expressed as a percentage, of water vapor actually in the air parcel compared to the amount of water vapor the air parcel could hold at a particular temperature and pressure.

Humidity

• Dewpoint. Dewpoint is the temperature an air parcel must be cooled at constant pressure and constant water vapor pressure to allow the water vapor in the parcel to condense into water (dew).

Humidity

• Temperature-Dewpoint Spread (Dewpoint Depression). The difference between an air parcel’s temperature and its dewpoint is the dewpoint depression, or commonly referred to as the spread.
• As the spread decreases, relative humidity increases.
• When the spread decreases to zero, relative humidity is 100 percent, and the air parcel is saturated.
• Surface temperature-dewpoint spread is important in anticipating fog, but has little bearing on precipitation. To support precipitation, air must be sat urated through thick layers aloft.

WATER VAPOR THEORY

• So, for the air to become saturated, either
• 1. Cool the Temp
• 2. Raise the Humidity
• Temp Dew Point spread is 3º or less usually fog will form

DEW AND FROST

• If the temp of a surface cools below the dew point, dew will form on that surface
• Frost happens when the dew point is below freezing and the surface is colder than freezing

WATER VAPOR THEORY

• Water vapor must condense for fog to form
• If there is no condensation nuclei present then even with 100% relative humidity, fog or clouds may not form
• Salt, dust combustion by products, smoke are all classified as condensation nuclei

CHANGE OF STATE, DEFINITIONS

• Sublimation – ice to vapor, vapor to ice
• Evaporation – water to vapor
• Condensation – vapor to water (must happen to get clouds or fog)
• Deposition – vapor directly to ice

Change of state

• There are 6 phase changes:
• Latent heat absorption
• Melting
• Evaporation
• Sublimation
• Freezing
• Condensation
• Deposition

CHANGE OF STATE, Definitions

• Virga – precipitation that never reaches the ground (not to be confused with Viagra)
• Virga is a perfect example of condensation then evaporation

CHANGE OF STATE

• Latent heat of vaporization – when water changes to vapor it takes heat from the surrounding environment
• Like when sweat cools  your body
• Latent heat of fusion – when water changes to ice, it gives off heat to the surrounding environment
• Like when Orchardists use water to keep crops from freezing
• Sensible heat – Heat that can be felt and measured when molecules pass to a lower energy state.

Fun fact – SUPER COOLED WATER

• Water droplets can exist below 32º F
• We find them aloft between 0º C and -15º C
• They may exist at temps lower than -15º C
• They have been known to exist up to -40º C



Chapter 4

Earth-Atmosphere Heat Imbalances

• The energy balance
• About 30% of the energy from the sun is reflected back into space
• This is know as albedo
• About 19% is absorbed by the atmosphere
• The earth soaks up the rest
• These numbers vary on a daily/nightly basis
• This process results in a stable average temperature

The energy balance

Greenhouse effect

• Say 51 units of the suns energy make it to the ground
• 23 units lost to evaporate water
• 7 units lost to convection and conduction
• That leaves 21 units to be radiated by the earth back to the atmosphere
• However 117 units are radiated back to the atmosphere because of the emission through the night hours
• Only 6 are lost to space
• 111 are absorbed by CO2 and water vapor
• 96 units are re-radiated back at earth
• This means that the earth receives twice as much long wave radiation from the atmosphere as it receives short wave radiation from the sun
• This covers the relationship between the earth and the sun, but another similar example exists between the atmosphere and the earth
• This relationship is also balanced in a similar fashion

Greenhouse effect

The Earth/atmosphere balance

• There are 2 main processes at play here
• Sensible heating
• Latent heating
• Sensible heating is a process of heating where no change of state takes place
• Latent heating of course is the due to the change of state
• Sensible heating involves transport of heat through the air by means of convection and conduction

The earth/atmosphere balance

• Sensible Heat example:
• A great example of this process is a thermal
• Air at the surface becomes hotter (conduction), less dense and rises
• This transports heat upwards (convection)

The earth/atmosphere balance

• Latent heat involves using the energy to achieve evaporation, rising to a higher altitude then condensing and releasing the energy

Heat imbalances

• Latitude Variation plays a role in how much short wave radiation is absorbed by the earth
• The higher the latitude the less energy
• Seasonal Variation has the same effect
• During summer there is less angle on the sun’s rays

Heat imbalances

• Diurnal Variation is the difference between the maximum day time temp and the minimum night time temp.
• Influenced by:
• Surface type
• Latitude
• Sky cover
• Water vapor
• Wind speed
• Temperature difference is maximized by:
• Being over land (as opposed to water)
• Low latitudes
• Clear sky
• Dry air
• Light wind
• Opposites are true for minimization
• Handy for prediction of fog