Water and Atmospheric MoistureWater and Atmospheric Moisture

Hydrologic Cycle

Humidity Humidity is a general term for the amount of water vapor in

air. Meteorologist use RELATIVE HUMIDITY and DEW POINT to

measure the amount of water vapor in the air. Saturated

Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure.

When saturated, warm air contains more water vapor than cold saturated air.

ABSOLUTE HUMIDITY

Absolute humidity (expressed as grams of water vapor per cubic meter volume of air)

DEW POINTDEW POINT

The The dewpoint dewpoint is the temperature of air which is the temperature of air which is needed for condensation or dew (at that is needed for condensation or dew (at that particular temperature). particular temperature).

RELATIVE HUMIDITYRelative humidity is a measure of how much water vapor the air actually could "hold" at a certain temperature.

The relative humidity represents how close the air is to saturation.

Saturated air will have an RH of 100 %. You need the RH of 100% to have rain form in clouds.

To measure relative humidity use a sling

psychrometer (hygrometer)

Dry bulb thermometer and a wet bulb thermometer mounted together

Dry bulb tells actual temperatureWet bulb shows how much water can be

evaporated – temperature lowers as water is evaporated

The difference in temperature on the 2 thermometers is an

indication of the amount of water vapor in the air.

Dry air: the water will evaporate quickly and cause a large drop in the wet-bulb temperature.

This makes the difference in readings on the 2 thermometers greater.

Moist air: little water will evaporate from the wet-bulb and the temperature decrease will be small.

The difference between the wet bulb and dry bulb will be small.

When the Wet bulb temperature = the dry bulb temperature………

100% HUMIDITY!!!

Wet- and Dry Bulb Psychrometer Psychrometric Chart.

If the difference between wet and dry bulb is 6º F and the temperature is 72º F (dry bulb), then the RH is 54%.

•Capacity of air is primarily a function of temperature•Relative Humidity (RH) =

(actual water vapor content) x 100 (max. water vapor capacity of the air)

•Heated air becomes lower in RH because denominator gets larger•Cooled air becomes higher in RH

Saturation vs Air Temperature

The actual amount ofWater air can hold changesWith air temperature

Air at 104 F can hold 3 timesAs much water as 68 F air !(47 grams vs only 15 grams)

Air at 68 F can hold 4 timesAs much water as air at 0 F(15 grams vs only 4 grams)

32 F

68 F

104 F

4 grams

15 grams

47 grams

Saturation and Dew Point•Saturated v. unsaturated air•Dew-point temperature• temperature to which air must be cooled to reach saturation (100% RH)• water on outside of

drinking glass• ice on your car

window• dew and fog

Adiabatic Cooling

Relative Humidity and Temp.RH fluctuates over a day or season.

Moisture, Clouds & Precipitation

Question: What Causes Air to Precipitate?

After Saturation Occurs the AirMust Release Extra Water as Fluid

Water forms on the outside of a cold glass as the coldAir surrounding the glass chills the air to the Dew Point Temperature

The resulting water is not from the glass, the water is from condensation of moisture in the air around the glass.

Atmospheric Lifting Mechanisms

Air lifting processes create clouds & precipitation.Saturation of air and cloud condensation nuclei (dust/pollen) required.

Four types of lifting are recognized:

1. Convectional Lifting2. Convergence3. Orographic Lifting4. Frontal Lifting

Convectional Lifting

Anywhere air is warmer than its surrounding air, it will rise.

In this example an island heats more than the surrounding water and causes a massive cumulus cloud to form.

Convectional Lifting Over FloridaWarmer temperatures over the peninsula of

Florida, which is land, cause air to rise compared to the cooler oceans nearby

Rising air in thisShuttle Picture isShown by a Cloud patternwhich generallyfollows the shape of the southern Florida peninsula

Convectional Lifting in the Desert

Extremely high afternoon temperatures in late summer often leads to thunderstorms throughout the world’s arid regions.

The Grand Canyon in August

Mojave Desert

ConvergenceConvergence occurs when large air masses

meet & are forced to rise vertically by crowding of molecules.

This process is best seen at the ITCZ where the Trades Winds meet & rise to form towering clouds & heavy precipitation

Orographic Lifting of Air

When air movingHorizontally Encounters aMountain it mustRise over the crest

As it rises, it coolsTo create clouds,And most oftenprecipitation

MoistAir

MoistureLost

DryAir

Run off NO Run off

Frontal Lifting of Air

Although not a mountain range, masses of moving airCreate the same effect – Unlike mountains air massesCan provide lifting in many different locations

Fronts can lift air Which is stable,Creating clouds& large amountsOf precipitationAs rain, snow,Sleet or hail

Precipitation Types / Properties

Global Precipitation

Variation in Precipitation:

The monthly and annual precipitation affect the characteristics of the climate of different places.

•The distribution of annual precipitation can be classified as ☺humid, ☺sub humid, ☺wet, ☺semi-arid, and ☺arid.

Global Distribution of Precipitation The area of maximum annual precipitation, over

2000mm per year, extends in a band through the equatorial regions.

The subtropical deserts and the polar regions have values below 250 mm.

The mid-latitude regions have intermediate values, general about 1000 mm per year.

Factor affecting the distribution of precipitation:

Effects of Latitude

• The global variation in the thermal environment in turn determines the pressure distribution.

•At the high latitudes, e.g. poles, the low temperatures result in the contraction of air and hence the development of high pressure.

•At the low latitudes, the high temperatures along the equator result in the expansion of air and hence the development of low pressure.

• The spatial variation in the distribution of pressure results in the motion of air (winds) which plays an important role in determine the amount and seasonal distribution of precipitation.

Pressure system

• The global distribution of precipitation is considered to be basically correlated with the planetary pressure systems.

•Near to the Horse Latitude and the Polar regions where the pressure is high, precipitation is scarce due to the subsiding air stream.

•However, near to the equatorial low pressure belt and sub-polar low pressure belt, precipitation is abundant due to the strong updraft.

Prevailing Wind

• In general, the rainfall will be abundant and evenly distributed if the regions are of prevailing onshore wind all the year.

• The rainfall will be little or scarce when the regions are of

prevailing offshore wind all the year.

•When regions, usually monsoon regions, lie in the path of rain-bearing onshore wind for one season and in the path of non-bearing offshore wind in other season, the distribution of rainfall will vary seasonally.

Mountain barrier

• In temperate latitudes, rainfall increases with elevation (wind slope) because air expands and cools as it rises, probably resulting in orographic rainfall.

•However, in tropical areas, precipitation may decrease with increasing altitude since much of the tropical rainfall is of convective type.

•As altitude increases, there will be a corresponding decrease in convection, The amount being received will be smaller.

Effect of Continentality

•Under the influence of prevailing onshore wind, coastal areas will generally receive more precipitation than that of continental regions.

• The amount of precipitation will steadily decrease moving inland.

• Some coastal areas receive scarce precipitation when there are offshore winds.

• The effect of continentality should be considered together with the direction of prevailing winds.

Ocean Current

• It is regarded as local factor as it operates over small area.

•A wind blowing over a cold current becomes cooled and may lose most or all of its water vapor through condensation. When the wind crosses over the land, it is not likely to produce rain. This happens off the coasts of southern California

•A wind blowing over a warm current is warmed and the rate of evaporation increases. The wind becomes moist and when it crosses over the land, it will yield rain if it is made to rise.

• e.g. if it crosses a mountain range, westerlies winds crossing the warm North Atlantic Drift bring heavy rainfall to north western Europe.

Summary: Factors affecting the world distribution of rainfall

Factors Rainfall is possible Rainfall is impossible

Temperature High temperature causes air to rise

Low temperature causes air to sink

Pressure Low pressure, cyclonesHigh pressure with sinking air, e.g. anticyclones

Slope Windward slope Leeward slope

Wind Onshore wet wind Offshore dry wind

Distance from the sea Coastal area Inland

Ocean currents and onshore wind Warm ocean current Cold ocean current

Acid Rain

ACID RAIN AND ITS EFFECT ON THE ENVIRONMENT

What is acid rain and what is the scale of the problem?

How do we know if the rain is acid? What causes acid rain (chemical reactions)? Where do the acid rain precursors come from? How does acid rain affect the environment? What can we do to protect our planet form acid

rain?

Acid rain, or acid deposition, is a mixture of sulphuric and

nitric acids. Wet deposition

rain hail snow fog

Dry deposition gases and particles

The valid risk assessment

The net acid deposition flux

pH Scale

(pH value - below 5.6 is acidic).

SOURCES The main precursors of acid rain are emissions of sulphur

dioxide (SO2), nitrogen oxides (NOx) and Carbon dioxide (CO2)

Fuel combustion in power plants (SO2)

Exhaust pipes in automobiles (NOx)

MAN MADE

Acid rains appear when

Sulphur dioxyde Nitrogen oxide

Release sulphuric acid and nitric acid !

They move up into the air and are released as acid rains

NATURAL:

Terrestial, tidal, and nutrient-rich oceanic areas

Volcanic eruptions Forest fires Outgassings from anaerobic wetlands Breakdown of amino acids in organic wastes

ENVIRONMENTAL EFFECTS Water and marine life

Extinction of fish in lakes

Flora Plants, forests, crops

leaf necrosis, tissue damage, death of the whole tree

Soil Improper growth of plants and trees

Human Health respiratory problems

POLICIES AND TAKING ACTION

Strategies for mitigating acid rain

Liming bodies of acidified water

This process of neutralizing acidity in endangered lakes and streams is through the addition of lime (calcium carbonate and calcium hydroxide). Chemicals such as caustic soda, sodium carbonate, slacked lime and limestone help raise pH of acidified water.

Drawbacks:

Very expensive Treatment needs to be repeated every 3-6 years Remote lakes are inaccessible LLarge pieces of lime dissolve slowly Doesn’tDoesn’t solve the problem for soil acidification

Reducing SO2 and NOx emissions

Changing from high to low sulfur coal

Moving to non-fossil fuels such as nuclear power

Using Solar, Wind or Hydro resources

COAL CLEANING

IMPROVING IN-PROCESS TECHNOLOGY

REDUCTION OF COMBUSTION TEMPERATURE (NOX)

CHANGING THE FUEL USED: