The Ocean-Atmosphere SystemThe oceans and the atmosphere are the two large reservoirs of water in the Earth's hydrologic cycle. The two systems are complexly linked to one another and are responsible for Earth's weather and climate.

Role of the atmosphere1. provides oxygen and moisture for life2. regulates heat by transferring energy (wind, clouds, storms, etc)3. retains enough heat to maintain acceptable surface temperatures4. protects surface from extraterrestrial bodies and solar radiation.5. Catalyzes decomposition of rock to form soil (Weathering)

Weather and ClimateWeather is the condition of the atmosphere at a particular time and place. It refers to such conditions of the local atmosphere as temperature, atmospheric pressure, humidity (the amount of water contained in the atmosphere), precipitation (rain, snow, sleet, & hail), and wind velocity. Because the amount of heat in the atmosphere varies with location above the Earth's surface, and because differing amounts of heat in different parts of the atmosphere control atmospheric circulation, the atmosphere is in constant motion. Thus, weather is continually changing in a complex and dynamic manner.

Climate refers to the average weather characteristics of a given region. The five weather variables are also used to measure the climate, but weather is a short—term event, whereas climate is a long-term one.Climate, although it does change over longer periods of geologic time, is more stable over short periods of time like years and centuries. The fact that the Earth has undergone fluctuation between ice ages and warmer periods in the recent past (the last ice age ended about 10,000 years ago) is testament to the fact that climate throughout the world has been changing through time.

The Earth's weather and climate system represent complex interactions between the oceans, the land, the sun, and the atmosphere. That these interactions are complex is evidence by the difficulty meteorologists have in predicting weather on a daily basis. Understanding climate change is even more difficult because humans have not been around long enough to record data on the long-term effects of these processes. Still, we do know that the main energy source for changing weather patterns and climate is solar energy from the Sun.

Composition of the atmosphereThe atmosphere is the gaseous envelope that surrounds the planet. It has structure. With increasing altitude, there are distinct variations in the composition, temperature, pressure, and humidity of the atmosphere.Air is the invisible, odorless mixture of gases and suspended particles. Because air pressure decreases with altitude, the amount of air per unit volume (density) also varies with altitude. However the relative proportions of the gases in the air are essentially constant regardless of altitude. Nitrogen, Oxygen, and argon make up 99.96% of the gases by volume.N2 78%O2 21%Ar 0.9%The relative amounts of the remaining gases are very small.

Natural trace gases – all <0.01%Neon, Helium, Methane, Krypton, Hydrogen, Radon

Variable ConstituentsCarbon Dioxide ~0.036%, but increasing due to burning fossil fuels Some transfer between oceans, organics, and soil Much greater in urban areas (up to 0.1%) Contributes to greenhouse

Water Vapor Up to 4% depending on availability

Water - liquid or solid Up to 1% (clouds, fog, rain, snow, etc) (H20 is the only substance which can exist in all 3 states at the

normal range of earth temperature and pressure)

Ozone < 0.7 ppm (7 x 10-6%) Much higher amounts in upper atmosphere (~10ppm) Natural from sunlight, lightning (higher amounts after t-storms) Vulnerable to CFC pollutants

Aerosols <0.1 ppm (1 x 10-6 %), depending on availability. Consist of tiny liquid droplets or solid particles that are so small

they remain suspended in the air. Dust, soot, smoke particles, sea-salt, pollutants are all types of

aerosols.

The greenhouse effectCarbon dioxide, water vapor, methane, ozone, and nitrous oxide are the greenhouse gases. They warm the air by preventing the heat from the sun’s radiation from escaping. They allow the sun’s visible short-wavelength radiation in, which is re-radiated as long-wavelength infrared radiation back into space warming the earth. The greenhouse gases absorb some of the infrared radiation that heats the atmosphere.

TemperatureTemperature is a measure of the average kinetic energy of all the atoms in a body. (Heat energy is the total kinetic energy of all the atoms in a substance. Not all the atoms in a given sample move with the same speed, and so there is a range of kinetic energies among them…temperature is a measure of the average of these kinetic energies).

Vertical Structure of the AtmosphereGeneral trends with increasing altitude

Constituent gases tend to be lighter in upper atmosphere Air pressure decreases Water vapor decreases dramatically Temperature in the troposphere, the lowest part of the

atmosphere where weather occurs, decreases with increasing altitude.

(Throughout the entire atmosphere temperature fluctuates and defines certain layers. There are four thermal layers of the atmosphere separated by boundaries called pauses. The four thermal layers of the atmosphere include the troposphere, stratosphere, mesosphere and thermosphere.)

Troposphere Details Lowest part of the atmosphere. Extends to variable altitudes of

10 to 16km. Varies over latitude - >16km over equator where more heat per unit area reaches the earth’s surface, <9km over poles. The upper boundary of the troposphere is the tropopause.

Temperature decreases with altitude. Absorption of reradiated long-wavelength infrared rays is most effective at the base of the atmosphere where the air is most dense. It is continually warmed by the ground and ocean.

Horizontal (wind) and vertical (convection) mixing of atmosphere. Location of almost all weather.

Contains almost all atmospheric water and dust, 90% of atmospheric mass.

Air PressureAir pressure decreases with altitude. As air pressure decreases, air density decreases. (D=m/v; with lower pressure, the mass per unit volume decreases…the composition of the gases are the same, but the density is less).

Air pressure is measured with a barometer. Air pressure decreases with altitude. At any given altitude, the air pressure is caused by the weight of air above. This means that the air near the ground is compressed by the weight of the air above it. As a result, half of the mass of the atmosphere lies below an altitude of 5.5km and 99 percent lies below 32 km (within the troposphere). The 1- percent of the atmosphere that lies above 32 km continues out to an altitude of about 500km (extent of the thermosphere).

Moisture in the AtmosphereH20 is the only substance that can exist in all 3 states (solid, liquid, and gas) at the normal range of earth temperature and pressure.

Whenever matter changes from one state to another, energy is either absorbed or released. In going from a more ordered state (solid) to a less ordered state (liquid or gas) heat energy is absorbed. The reverse is true when the change is from a less ordered to a more ordered state – heat energy is released. The amount of heat released or absorbed per gram during a change of state is known as the latent heat. Latent heat of condensation (from gas to liquid) releases 2260 J/g Latent heat of evaporation (from liquid to gas) absorbs 2260 J/g Latent heat of freezing (from liquid to solid) releases 330 J/g Latent heat of melting (from solid to liquid) absorbs 330 J/g

-Why you sweat – to evaporate water requires heat. The heat from your skin is used to evaporate the water… the loss of heat from your skin cools you down.-Why steam burns are worse…absorbed more heat to turn to the gas phase.

Evaporation and condensation play vitally important roles in the weather.(1) they give rise to clouds, fog, rain(2) they are the means by which heat is moved from equatorial

regions to the poles.

Relative HumidityWater vapor gets into the air by evaporation. Liquid -> Gas by absorbing energy. Because molecules in a vapor move at random in all directions, some of the gas molecules in the vapor will move back into the liquid. When the number of molecules that evaporate equals the number condensing back into the liquid the vapor is said to be saturated. This is the maximum concentration of water molecules in the vapor phase at any specified temperature.

Partial pressure is a measure of the volume percent of a specific gas in a mixture. Every gas in a mixture will exert a certain amount of pressure and the pressures in a mixture of gases are additive…Therefore the total pressure of a mixture of gases is the sum of the partial pressures exerted by all the individual gases present. Water vapor is only one of the gases present in air. Therefore water vapor content of air is often reported as a pressure rather than a percent.

The saturation vapor pressure, also known as the water vapor capacity of air at any given temperature, cannot be exceeded. The vapor pressure can, however, be lower than the saturation value.

Exercise on a dry day, the water quickly evaporates and you get cool. Exercise on a humid day, little perspiration evaporates into the air, which

means little heat is carried away, and you aren’t cooled. Swimming on a hot dry day, water quickly evaporates from your body and

can carry away so much heat you get goose bumps even if the temperature is 90F.

The relative humidity is the ratio of the vapor pressure in a sample of air to the saturation vapor pressure at the same temperature, expressed as a percentage. Note that the relative humidity does not refer to a specific amount of water vapor in the air….It refers to the ratio of what is present at a given temperature to the maximum possible amount that the air could hold at the same temperature. The amount of water vapor needed to saturate air increases as the air’s temperature increases.

Relative humidity can be changed in two ways – by addition of water vapor or by change of temperature. Decrease temperature – increase relative humidity, Increase temperature – Decrease relative humidity.

House is dry in winter because even if the cold air outside is saturated, when brought in the house and warmed, relative humidity decreases.

Can be more humid at night- as temperature drops, relative humidity increases.

Dew PointThe temperature at which the relative humidity reaches 100 percent and condensation starts is called the dew point. If the amount of

water vapor in the air is kept constant and the temperature drops, the relative humidity will rise. When the ground is cold and the air warm (ground cools faster than water), the layer of air in contact with the ground may cool sufficiently for the dew point to be reached, so that dew is formed. If the ground temperature is below freezing, frost forms.

Adiabatic ProcessesWarm air is less dense than cold air and therefore rises, creating a convection cell in the process. Because it is warmest at ground level, air in the troposphere is continually rising or, after cooling falling. However, because air pressure decreases with increasing altitude, the rising are expands. Conversely, sinking air is compressed.

Compressional warming – when air is compressed, the temperature rises. Compression increases the temperature and density in the parcel.

Expansional Cooling – when air expands, the temperature decreases. Heat is consumed in the expansion process.

These are adiabatic processes…processes that occur without the addition of subtraction of heat from an external source.

An ascending (rising) parcel of air tends to cool by expansion.A descending (falling) parcel of air heats by compression.

NOTE that Temperature changes in both ascending and descending air occur without any significant heat exchange between the surrounding environment and the vertically moving parcel of air. Adiabatic.

When a parcel of unsaturated air rises and expands adiabatically, the temperature drops. Conversely, if unsaturated air sinks toward the surface, it is compressed and the temperature. The way temperature changes with altitude in rising or falling air is called the adiabatic lapse rate.

Eventually, when air rises far enough, it will cool to the dew point where it will become saturated and condensation will occur. This point along it’s ascent is the lifting condensation level.

When a parcel of air rises, reaches saturation at the lifting condensation level, condensation begins and clouds form.

There are four principle reasons for the upward movement of air.1. Density lifting, which occurs when warm, low-density air rises

convectively and displaces cooler, denser air.2. Frontal lifting, which occurs when two flowing air masses of

different density meet. The boundaries between air masses of

different density are called fronts. When warm humid air advances over cold air it is called a warm front. When denser, cooler air advances and displaces warm air by pushing it upward it is called a cold front.

3. Orographic lifting occurs when flowing air is forced upward over a mountain range.

4. Convergence lifting occurs when flowing air masses converge and are forced upward.

Atmospheric StabilityIn our discussion we have made two assumptions a) certain process force air to rise from the earth’s surfaceb) that rising air does not mix substantially with the surrounding

atmosphere.Once an initial lifting force ceases, the fate of a rising air parcel depends on the state of the atmosphere through which it rises, or atmospheric stability. The relation among the dry adiabatic rate, moist adiabatic rate, and the environmental lapse rate determines the stability of the atmosphere over an area.

The environmental lapse rate is the average decrease in temperature with increasing altitude.

Basically, if a parcel of air that is forced to rise stayed cooler than the surrounding atmosphere as it expands, it would be more dense than the surrounding air, and if allowed to do so, would sink to its original position. Air of this type, called stable air, resists vertical movement.If, however, the rising parcel of air were warmer and therefore less dense than the surrounding air, it would continue to rise until it reached an altitude having the same temperature. This type of air is classified as unstable air.

In summary, stability is a property of air that describes its tendency to remain in its original position (stable) or to rise (unstable).

Absolute StabilityThe atmosphere is considered absolutely stable if an air parcel that is forced aloft cools faster than the surrounding environment. That is, the parcel rate of cooling (the dry or moist adiabatic lapse rate) is faster than the environmental lapse rate. If the force lifting the parcel continued to act, condensation would eventually occur when the dew point temperature is reached, but clouds would be layered clouds without much vertical development. If the force ceased, the parcel would have a density to sink.

Absolute InstabilityThe atmosphere is considered absolutely unstable if the parcel rate of cooling is slower than the environmental lapse rate. That is, the environmental lapse rate is greater than the parcel’s adiabatic rate. The parcel will continue to rise because it is warmer and more buoyant than its surroundings. If the air parcel rises to its condensation level, clouds with vertical development will form as the buoyant air rises on its own. (Thunderstorms).

Absolute Instability occurs most often during the warmest months and on clear days when solar heating is intense. Under these conditions, the lowermost layer of the atmosphere is heated to a much higher temperature than the air aloft. This results in a steep environmental lapse rate and a very unstable atmosphere.

We discussed the upward movement of air and its importance in cloud formation. As important as vertical motion is, far more air is involved in horizontal movement, the phenomena we call wind. Although we know that air will move vertically if it is warmer and thus more buoyant than surrounding air, what cause air to move horizontally.

Simply stated, wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. Wind is nature’s attempt to balance larger-scale inequalities in air pressure. Because unequal heating of Earth’s surface continually generates these pressure differences, solar radiation is the ultimate driving force of wind.

Wind is a horizontal air movement arising from differences in air pressure. Wind results when air flows from a place of high pressure to one of low pressure.

Air pressure is related to density.High-pressure means the air is more compressed and therefore more dense. Low pressure means less compression and lower density.

Wind SpeedWind Speed measurements are averaged over a specific period, commonly five minutes. Most places around the world have wind speeds that average between 10 and 30 km/h (6 and 19 mi/h).

The Windchill FactorThe windchill factor measures the heat loss from exposed skin as a result of the combined effects of low temperature and wind speed.

The human body is a heat generator that continually releases energy. Immediately adjacent to the body is a thin layer of still air called the boundary layer. This layer is still because friction prevents movement. Heat escaping from the body must pass through the boundary layer, making the boundary layer an insulator. However, as wind speed increases, the thickness of the boundary layer decreases, as does its effectiveness as an insulator. As a result, the wind is acting to carry heat away from the body by constantly replacing warmer air next to the body with colder air.

During a weather forecast, when mentioning the windchill factor, they are actually reporting the windchill equivalent temperature. It is the temperature it ‘feels like’. It is the temperature at which exposed parts of the body would lose heat at the same rate if there were no wind.

Factors affecting wind speed and directionIf the earth did not rotate and there was no friction, air would flow directly from areas of higher pressure to areas of lower pressure. Because both factors exist, however, wind is controlled by a combination of factors.

Wind is controlled by1. The air pressure gradient, which is the drop in air pressure/unit

distance.2. The Coriolis effect, which is the deviation from a straight line in the

path of a moving body due to the earth’s rotation.3. Friction, which is the resistance to movement when two bodies are

in contact.

Pressure Gradient forceTo get anything to accelerate (change in velocity) requires an unbalanced force in one direction. The force that generates winds results from horizontal pressure differences. When air is subjected to greater pressure on one side than on another, the imbalance produces a force that is directed from the region of higher pressure toward the area of lower pressure. Thus, pressure differences cause the wind to blow, and the greater the differences, the greater the wind speed.

Variations in air pressure over Earth’s surface are determined from barometric readings taken at hundreds of weather stations. These pressure data are shown on surface weather maps by means of isobars. Isobars are lines connecting places of equal air pressure. The spacing of the isobars indicates the amount of

pressure change occurring over a given distance and is expressed as the pressure gradient.

You can compare the pressure gradient to the slope of a hill. A steep pressure gradient, like a steep hill, causes greater acceleration of a parcel of air than does a weak pressure gradient, like a shallow hill. Thus, the relationship between wind speed and the pressure gradient is straightforward. Closely spaced isobars indicate a steep pressure gradient and strong winds, widely spaced isobars indicate a weak pressure gradient and light winds. The pressure gradient force is always directed perpendicular to the isobars.

Pressure differences are simply a result of unequal heating of Earth’s land-sea surface.For example – surface temperatures over the ocean change only slightly on a daily basis whereas land surfaces and the air above can be substantially warmed during a single daylight period. As air over the land warms, it expands, causing a reduction in density and an area of low pressure. A sea breeze then develops, as cooler air over the water moves onto the land. At night, the reverse may take place. The land cools more rapidly than the sea and a land breeze develops.

Temperature variations create pressure differences – and hence wind – and the greater these temperature difference, the stronger the pressure gradient and resultant wind.

In summary, the horizontal pressure gradient is the driving force of wind. It has both magnitude and direction. Its magnitude is determined from the spacing of isobars, and the direction of the pressure gradient force is always from areas of higher pressure to areas of lower pressure (perpendicular to the isobars).

Once the air starts to move, the Coriolis force and friction come into play, but only to modify the movement, not to produce it.

Coriolis Effect. The Coriolis effect influences all moving bodies. Because wind is moving air, the directions of all winds are subject to the Coriolis effect.All free moving objects, including wind, are deflected to the right of their path of motion in the Northern Hemisphere and the left in the Southern Hemisphere.

Example.Imagine the path of a rock launched from the North Pole toward a target on the equator. If the rocket took an hour to reach its target,

the earth would have rotated 15 o to the east during its flight. To someone standing on Earth, it would look as if the rocket veered off its path and hit earth 15 o west of it’s target. But it was earth turning under the rocket that gave it its apparent deflection. Note that the rocket was deflected to the right of its path of motion because of the counterclockwise rotation of the Northern Hemisphere. Clockwise rotation produces a similar deflection in the Southern Hemisphere, but to the left of the path of motion.

Winds flowing West to East are also effected by the Coriolis effect. This deflection is caused by Earth’s rotation, which changes the orientation of the surface over which the winds are moving. The amount of deflection is greater at 60o latitude than at 40o latitude, which is greater than at 20 o. Furthermore, there is no deflection observed for the airflow along the equator. Therefore, the magnitude of the deflecting force (Coriolis deflection) is dependent on latitude. It is strongest at the poles and it weakens equatorward where it eventually becomes nonexistent.

The amount of Coriolis deflection increases with wind speed because a fast-moving object covers a greater distance in a given time than a slow moving object.

The longer the trajectory , the greater the change in angular velocity and therefore the greater the Coriolis deflection.

In conclusion, we attribute this apparent shift in wind direction to the Coriolis force. It is hardly a ‘real’ force but rather the effect of Earth’s rotation on a moving body. This deflecting force:1. Is toward the right in the Northern Hemisphere and to the

left in the Southern Hemisphere.2. Affects only wind direction, not wind speed3. Is affected by wind speed (the stronger the wind, the

greater the deflecting force)4. Is strongest at the poles and weakens equatorward,

becoming nonexistent at the equator.

Where airflow is concerned, the Coriolis effect is of greatest importance in large-scale wind systems, but of only minor importance in small scale, local wind systems.

Convergent and Divergent FlowAs air near the ground flows inward from all directions toward a low pressure center, frictional drag causes the flow direction to be across the isobars at an oblique angle. As a consequence, winds around a low

pressure center develop an inward spiral motion. In the northern hemisphere, where the coriolis force deflects the flow to the right, the resulting wind blows counterclockwise about the low.

By the same process, air flow spirals outward from a high pressure area. Around a high pressure cell, the outward directed pressure gradient force is opposed by the inward directed Coriolis force, and a clockwise flow results in the northern hemisphere.In the Southern Hemisphere, the reverse is true. Because the Coriolis force deflects the winds to the left in the Southern Hemisphere, the flow is reversed there – clockwise around low pressure centers and counterclockwise around high pressure centers.

Centers of low pressure are called cyclones and exhibit cyclonic flow. Cyclonic flow has the same direction of rotation as earth, counterclockwise in the Northern Hemisphere and clockwise in the southern hemisphere.

Centers of high pressure are called anticyclones and exhibit anticyclonic flow. Anticyclonic flow has the opposite rotation as Earth, clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

Vertical Flow Associated with Cyclones and AnticyclonesAround a surface low pressure system (cyclone) air is spiraling inward. This leads to an upward flow of air at the center of the low and divergence aloft. The surface convergence about a cyclone causes a net upward movement. Rising air often result in cloud formation and precipitation, and therefore the passage of a low-pressure center is generally related to unstable conditions and stormy weather.

Around a surface high pressure system (anticyclone) air is spiraling outward. This leads to a downward flow of air at the center of the high and convergence aloft. Because descending air is compressed and warmed adiabatically, cloud formation and precipitation are unlikely in an anticyclone. Thus, fair weather can usually be expected with the approach of a high-pressure system.