atmospheric forces or why does the wind blow?

ATMOSPHERIC FORCESor

Why Does the Wind Blow?

1. Acceleration =

2. Pressure gradient force +

3. Coriolis force +

4. Friction force.

1ˆ1

rFVkfpdt

Vd h

1 2 3 4

Air motion is well described by the Navier-Stokes equation,

The change in pressure measured across a given distance is called a "pressure gradient".

PG = change in pressure / distance

The pressure gradient results in a net force that is directed from high to low pressure and this force is called the "pressure gradient force".

The closer the isobars or height contours, the greater the PGF magnitude. Larger PGF result in stronger winds, so strong winds are associated with closely spaced contour lines.

PGF points at a right angle to the local isobar or height contour line. It also points from high pressure to low pressure (or high heights to low heights).

Reasons for pressure variation:Pressure differences are ‘forced’ by uneven absorption of solar radiation at the Earth's surface. (remember P=ρ*R*T, pressure = density x constant x temperature)

warmer air is less dense and typically exerts less pressure than cooler air however, for a given surface pressure, a column of cold air will decrease in pressure with height more rapidly than will a warm column of air (see next slide for example)

Moisture content of air also affects pressure the greater the amount of water vapor, the less dense the air will be, and the less pressure it will exert

Wind develops as a result of spatial (i.e. horizontal) differences in atmospheric pressure.

Wind speed tends to be at their greatest during the daytime when the greatest spatial extremes in atmospheric temperature and pressure exist.

Coriolis Force - an artifact of the earth's rotation.

V = wind speedΩ = earth angular velocity, 7.27 x 10-5

φ = latitude

Once air has been set in motion by the pressure gradient force, it undergoes an apparent deflection from its path, as seen by an observer on the earth. This apparent deflection is called the "Coriolis force" and is a result of the earth's rotation.

ˆcorF fk V

##############

2 sinf

The Coriolis force is zero at the equator.

An air parcel initially at rest will accelerate from high pressure to low pressure because of the pressure gradient force (PGF).

As the air parcel begins to move, it is deflected by the Coriolis force (to the right in the northern hemisphere, to the left in the southern hemisphere). Coriolis force always acts at 90° to the wind motion.

As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force. This balance is called geostrophic balance. The geostrophic wind blows parallel to contours. (Note: friction is not a component!)

What hemisphere is depicted in this figure?

FRICTION

The surface of the Earth exerts a frictional drag on the air blowing just above it. This friction can act to change the wind's direction and slow it down -- keeping it from blowing as fast as the wind aloft. The difference in terrain conditions directly affects how much friction is exerted.

For example, a calm ocean surface is pretty smooth, so the wind blowing over it does not move up, down, and around any features. By contrast, hills and forests force the wind to slow down and/or change direction much more.

Net result is that surface winds blow across isobars!

This cross-isobar flow allows the surface wind to spiral into low pressure and out of high pressure.

Ekman turning has two results:

1. Surface winds around high and low pressure systems are determined: winds spiral cyclonic into Low Pressure and anticyclonic out of High Pressure

Surface wind crossing isobars is known as “Ekman turning” or “Ekman pumping”. It is due to friction.

z

wu

x

pvf

Dt

uD

''

1

z

wv

y

puf

Dt

vD

''

1

2. Surface winds are oriented approximately 45° to right of geostrophic wind at surface. Top of “Ekman layer” is defined where v wind component is zero. (And thus wind is now geostrophic).

Another consequence of Ekman turning is the response of the upper-atmosphere to the low-atmosphere surface layer.

Low pressure system: convergence at surface, divergence aloft

High pressure system: divergence aloft, convergence at surface

upper level

surface

We have now seen three very important force balances in the atmosphere. We will see a fourth soon.

PGF = Coriolis force Geostrophic balance (Earth turning)

PGF = Centripetal force Cyclostrophic balance (Tornado)

PGF = gravity Hydrostatic balance

PGF = CO + Friction Ekmann Balance (PBL)

Summary: What is wind? Simply a balance of forces acting on air parcels

Geostrophic wind: case where pressure gradient force is equally and oppositely balanced by coriolis force. Resulting wind blows parallel to isobars / height contours

At the surface, friction force induces a drag and creates imbalance between PGF & CF. Resulting wind crosses isobars, diverging from high and converging into low pressure

Sea Breeze

Land Breeze

Local wind circulations

During day, land and ocean heat up differently (why?)

By afternoon, land is warmer than ocean

Warm air has lower pressure than cold air (assume density remains constant)

A local pressure gradient sets up and air moves from H to L, resulting in a sea breeze

At night, land cools down faster than ocean, reversing the pressure gradient

Wind blows off shore, resulting in land breeze Land Breeze

Valley Breeze• During the day the air close to the mountain sides becomes warmer than the surrounding air, thus lowering pressure relative to the cooler air at the same height over the valley.  • This warm air rises up the mountains and is replaced by air from within the valley.  • So during the day valley air moves up the mountain sides. 

Warmer, local low pressure

Cooler, local high pressure

This process leads to clouds and rain in the mountains in the summer, particularly in the late afternoon. 

Mountain Breezes

• At night the sides of the mountain cool down and cool air is pulled down by gravity. • In the morning, the coldest air is, therefore, often found within the valley.

The large volcanoes on the Big Island of Hawaii are the source of strong nighttime mountain breezes

General Circulation, Monsoons

Circulation scales• We know that weather is essentially nothing more than moving air (&

the accompanying heat & moisture transport)• We also know that air parcels are acted on by several forces, the net

result being motion:– pressure gradient, Coriolis, friction, gravity

• To help us understand air motion, we categorize “disturbances” by size and time

Understanding global-scale atmospheric motion is important because it shows us how and why the planet is divided into different zones, distinguished from one another by their typical weather and climate

These divisions of the global climate system have had a significant impact on the development of civilization on Earth, because climate has been more or less the same for the last 10,000 years or so

However, there are climatic fluctuations on timescales of years to decades that can have significant effects On even longer timescales, we find that the atmospheric general circulation and climate have undergone very large fluctuations.

In a simplistic world model, heated air in the equatorial regions becomes less dense, more buoyant and rises. At the same time, cold air in the Polar Regions becomes denser, heavier and sinks. In the lower levels of the atmosphere, cold dense air from the Polar Regions flows towards the equator. It replaces the heated equatorial air. A return flow of warm air at higher levels of the atmosphere completes this simple circulation model. This type of “direct” circulation is called a Hadley Cell

•Convergence is coming together. •Divergence is going apart.•Subsidence is sinking air.

The planet Venus rotates very slowly on its axis (a “day” on Venus is longer than a “year”!)

So the general circulation of the lower atmosphere of Venus is essentially large Hadley cells on either side of the equator

The circulation in the northern and southern hemispheres is broken into 3 bands in each hemisphere

The large-scale rising or sinking of air tells us (at least in general) where the rainiest and driest latitudes are:

Global (visible + infrared) satellite image

Tropical RainforestA dense evergreen forest occupying a tropical region with an annual rainfall of at least 2.5 meters (100 inches). Cover about 6% of earth’s land area

Temperate rainforest is defined as any forest in the mid-latitudes that receives more than 50-60 inches of rainfall a year. They are found along the Pacific coast of the USA and Canada (from northern California to Alaska), in New Zealand, Tasmania, Chile, Ireland, Scotland and Norway. They cover less area than tropical rainforests.

Westerlies and trade winds blow away from the 30 degrees latitude belt. Over broad regions centered at 30 degrees latitude, surface winds are light or calm. Air slowly descends to replace the air that blows away. Descending air warms and is dry. The subtropical deserts, such as the Sahara of Africa and the Sonoran of Mexico, occur under these regions of descending air.

A jet stream is a current of fast moving air found in the upper levels of the atmosphere. This rapid current is typically thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Jet streams are usually found somewhere between 10-15 km (6-9 miles) above the earth's surface. The position of this upper-level jet stream denotes the location of the strongest SURFACE temperature contrast

Consequences of three large circulation scales:- Jet streams & fronts

There are two main jet streams at polar latitudes, one in each hemisphere, and two minor subtropical streams closer to the equator. In the Northern Hemisphere the streams are most commonly found between latitudes 30°N and 70°N for the polar jet stream, and between latitude 20°N and 50°N for the subtropical stream.

Technically the wind speed has to be higher than 90km/h or 55 mph to be called a jet stream.

The wind speeds vary according to the temperature gradient, averaging 55km/h or 35 mph in summer and 120km/h or 75 mph in winter, although speeds of over 400km/h or 250

mph are known.

Jet streams can be explained as follows: If two air masses of different temperatures meet, the resulting pressure difference is highest at altitude. If one of the air masses lies north of the other one, then the wind will not flow directly from the warm to the cold area, but is deflected by the Coriolis force and flows along the boundary of the two air masses.

The general circulation serves to transport heat energy from warm equatorial regions to colder temperate and polar regions. Without such latitudinal redistribution of heat, the equator would be much hotter than it is whilst the poles would be much colder.

So far we have used a simple rotating earth with a smooth uniform surface to describe the general circulation of the atmosphere.

However, the earth does not have a uniform surface, and this fact plays an important role in modifying the pattern of airstreams.

January wind patterns. Note location of ITC(Z)

July wind patterns. Note location of ITC(Z)

Land and oceans absorb and reradiate heat differently. Consequently, land heats up rapidly and loses heat just as rapidly. In contrast, oceans distribute thermal energy through a much greater depth and take longer to change temperature.

Northern Hemisphere Southern Hemisphere

Atmospheric circulation does not account for all the energy that needs to be redistributed to maintain the global energy balance. Winds drive surface ocean circulation, and the cooling and sinking of waters in the polar regions drive deep circulation. Surface circulation carries the warm upper waters poleward from the tropics.

Heat is disbursed along the way from the waters to the atmosphere. At the poles, the water is further cooled during winter, and sinks to the deep ocean.

This is especially true in the North Atlantic and along Antarctica. Deep ocean water gradually returns to the surface nearly

everywhere in the ocean. Once at the surface it is carried back to the tropics, and the cycle begins again. The more efficient the cycle, the more heat is transferred, and the warmer the climate.

Large-scale ocean currents

Monsoon is used to describe seasonal reversals of wind direction, caused by temperature differences between the land and sea. They occur in a number of countries around the world.

The Monsoon Circulation

The Monsoon climate in Southeast Asia and India is most well known and most potent. This circulation is known to bring about long drought periods from September to March. It is hard to grow crops during this period due to the lack of water. The summer monsoon is known to do the opposite, bringing huge amounts of moisture from the Indian Ocean causing massive flooding in many areas.

During summer, the Tibetan Plateau acts like a gigantic exposed brick, absorbing summer heat and heating the atmosphere above it. Hot air rises, and cool, moist air—drawn in from over surrounding oceans—rushes in to replace it. That moist air is the source of monsoon rains.

During the winter dry period, the airflow comes from high pressure to the north—the dry Himalayas and Siberia.

The two key ingredients for the Asian Monsoon are a large land mass and a large ocean – namely, southern Asia and the surrounding Arabian Sea and the Indian Ocean.

About 60% of the earth's population live under the influence of the Asian monsoon climate.

The Mexican Monsoon (also known as the North American monsoon, the Southwest monsoon or Arizona monsoon) is a well-defined meteorological event

•The Mexican Monsoon is a regional-scale circulation that develops over southwest North America during the months of July through September. • It is associated with a dramatic increase in rainfall that occurs over what is normally an arid region of North America.

Seasonal distribution of precipitation for southwestern North America. The largest peaks in mid-to-late summer rainfall (i.e., monsoonal)are found in northwestern Mexico. Note that northeastern Mexico and Texas display early summer/early fall precipitation peaks. Areas south of the broken line receive 50% of their annual rainfall during July, August and September (after Douglas et al. 1993).

Föhn wind

Graphic courtesy of USA Today

•Called “chinook” in N. America (native word for ‘snow eater’)• The winds are caused by when a deep layer of prevailing wind is forced up, over, and down a mountain range.

Black Hills of South Dakota are home to the world's fastest recorded rise in temperature, a record that has held for six decades.

• On January 22, 1943, the northern and eastern slopes of the Black Hills were at the western edge of an Arctic airmass and under a temperature inversion. A layer of shallow Arctic air hugged the ground.• At about 7:30am MST, the temperature in Spearfish, SD was –20o C. The chinook kicked in, and two minutes later the temperature was 7.2o C above zero. The 27.2 degree rise in two minutes set a world record that is still on the books.

• By 9:00am, the temperature had risen to 12.2o C.• Suddenly, the chinook wind ceased and the temperature tumbled back to –20o. • The 32.2 degree drop took only 27 minutes.

Santa Ana WindsHot, dry winds that often sweep through the L.A. Basin in the fall and winter.

•Develop when the desert is cold, and are thus most common during the cool season stretching from October through March.

•High pressure builds over the Great Basin (e.g., Nevada) and the cold air there begins to sink.

• Winds descend from the higher desert terrain down in to the L.A. Basin - parcels become warmer and drier due to compressional heating (about 10oC per kilometer of descent)• As its temperature rises, the relative humidity drops; the air starts out dry and winds up at sea level much drier still.•The air picks up speed as it is channeled through passes and canyons.

• The fast, hot winds cause vegetation to dry out, increasing the danger of wildfire. Once the fires start, the winds fan the flames and hasten their spread.

Wind – a renewable resource for energy!

Air has mass, and when it is in motion, it contains the energy of that motion (kinetic energy) Some portion of that energy can converted into other forms — mechanical force or electricity — that we can use to perform work

Wind energy has been used by people for hundreds of years

Definition:Kilowatt hour (kWh) = amount of electricity required to light a 100 Watt light bulb for 10 hours

Average energy usage per person per year:

Alaska (highest in U.S.) = 340,000 kWhRhode Island (lowest in U.S.) = 6,200 kWhAustria = 52,000 kWh

USA: 86% of energy comes from fossil fuels, 8% from nuclear, 6% from renewableAustria: 62% of energy comes from fossil fuels, 38% from other (mostly renewable)

Equivalent oil consumption comparison: kg/person/year

Austria AUT 3,551.1

Canada CAN 8,155.5

China CHN 895.9

France FRA 4,341.0

Germany DEU 4,173.9

Norway NOR 5,766.2

Sweden SWE 5,363.8

Switzerland CHE 3,692.0

United Kingdom GBR 3,938.6

United States USA 8,083.5

% of world population % of world energy

U.S. < 5% ~ 24%

China ~ 20 % ~ 10%

The considerable wind electric potential has not been tapped to its fullest in modern times because wind turbine technology was not able to utilize this resource.

However, during the past decade, increased knowledge of wind turbine behavior has led to more cost-effective wind turbines that are more efficient in producing electricity.

The price of electricity produced from wind by these advanced turbines is estimated to be competitive with conventional sources of power, including fossil fuels.