Chapter 6 - Air Pressure and Winds

Question 1

Wind

Answer 1

The result of horizontal differences in atmospheric pressure that causes air to move horizontally.

Question 2

In what direction does air flow?

Answer 2

Air flows from areas of higher pressure to areas of lower pressure. Wind is nature’s attempt to balance inequalities in air pressure.

Question 3

Atmospheric Pressure aka Air Pressure

Answer 3

The force exerted against a surface by the continuous collision of gas molecules.

The force exerted by the weight of a column of air above a given point.

Question 4

What is the average air pressure at sea level?

Answer 4

1 kg/cm2 or 14.7 lbs/in2

Question 5

If air pressure is so heavy why don’t things on Earth collapse?

Answer 5

Air pressure is exerted in all directions - up, down, and sideways. Thus, the air pressure around all sides of the desk is exactly balanced.

Question 6

Newton

Answer 6

A unit of force used in physics. One newton is the force necessary to accelerate 1 kilogram of mass 1 meter per second squared.

Question 7

Under average (sea level) conditions how much force does the atmosphere exert?

Answer 7

101,325 newtons per square meter or

1013.25 millibars

Question 8

Millibar (mb)

Answer 8

The standard unit of pressure measurement used by the National Weather Service. One millibar (mb) equals 100 newtons per square meter.

Question 9

What name is given to a newton per square meter (N/m2)?

Answer 9

The standard unit of pressure in the SI system is the pascal, which is the name given to a newton per square meter (N/m2). A standard atmosphere then has a value of 101,325 pascals, or 101.325 kilopascal.

Question 10

Mercury Barometer

Answer 10

A mercury-filled glass tube in which the height of the column of mercury is a measure of air pressure.

Question 11

How does a mercury barometer work?

Answer 11

One end of a glass tube is enclosed and filled with mercury. This tube is then inverted into a dish or mercury. Mercury flows out of the tube until the weight of the mercury column is balanced by the pressure exerted on the surface of the mercury in the dish by the air above. When air pressure increases, the mercury in the rube rises; conversely, when air pressure decreases, so does the height of the column of mercury.

Question 12

What is the measurement on a mercury barometer at sea level?

Answer 12

29.92 inches

Question 13

Aneroid Barometer

Answer 13

An instrument for measuring air pressure; it consists of evacuated metal chambers that are very sensitive to variations in air pressure. The chamber compresses as pressure increases and expands as pressure decreases.

Question 14

Barograph

Answer 14

A recording barometer; a recording mechanism attached to an aneroid barometer.

Question 15

In general, what kind of weather is associated with high and low air pressures?

Answer 15

Fair weather corresponds with high-pressure readings, whereas rain is associated with low pressures. Falling pressure is often associated with increasing cloudiness and the possibility of precipitation, whereas rising air pressure generally indicates clearing conditions.

Question 16

What can an aneroid barometer be used for other than measuring barometric pressure?

Answer 16

Another important adaptation of the aneroid barometer is its use to indicate altitude for aircraft, mountain climbers, and mapmakers.

Question 17

Explain why air pressure decreases with an increase in altitude.

Answer 17

The relationship between air pressure and the air’s density largely explains the drop in air pressure that occurs with altitude. As we ascend through the atmosphere, we find that the air becomes less dense because of the continual decrease in the amount (weight) of the air above. Therefore, there is a corresponding decrease in pressure with an increase in altitude.

Question 18

U.S. Standard Atmosphere

Answer 18

The idealized vertical distribution of atmospheric pressure, temperature, and density that represents average conditions in the atmosphere.

Question 19

Is the rate at which pressure decreases with altitude constant? What is the rate?

Answer 19

The rate at which pressure decreases with altitude is not constant. The rate of decrease is much greater near Earth’s surface, where pressure is high, than aloft, where air pressure is low.

Near Earth’s surface, air pressure decreases by about 10mb for every 100-meter increase in elevation, or about a 1in of mercury for every 1000-foot rise in elevation. Further, atmospheric pressure is reduced by approximately one-half for each 5km increase in altitude. Therefore, at 5km the pressure is 500mb, about one-half its sea-level value, at 10km it is one-fourth, at 15km it is one-eight, and so forth. Thus, at the altitude at which commercial jets fly (10km), the air exerts a pressure equal to only one-fourth that at sea level.

Question 20

What is the influence of temperature on air pressure?

Answer 20

Recall that temperature is a measure of the average molecular motion (kinetic energy) of a substance. As a result, cold air is composed of comparatively slow-moving gas molecules that are packed closely together. These cold, dense air masses are associated with high surface pressures and are labeled highs (H) on weather maps. By contrast, high temperatures are accompanies by low surface pressures labeled lows (L) on weather maps.

Also, air pressure drops more rapidly with altitude in a column of cold (dense) air than in a column of warm (less dense) air. Therefore, warm air aloft tends to exhibit a higher pressure than cold air at the same altitude.

Question 21

Pressure Altimeter

Answer 21

An instrument that allows a pilot to determine the altitude of a plane.

It is essentially an aneroid barometer makes in meters instead of millibars and, as such, responds to changes in air pressure.

Height based on the fact that air pressure is usually a certain amount at certain altitudes.

Question 22

What disadvantage do altimeters pose for pilots?

Answer 22

Because of temperature variations and moving pressure systems, actual conditions are usually different from that shown by an aircraft’s altimeter. When the air is warmer than predicted by the standard atmosphere, the plane will fly higher than the height indicated by the altimeter. By contrast, in cold air the plane will fly lower than indicated. To avoid dangerous situations, pilots make altimeter corrections before takeoffs and landings, and in some cases they make corrections en route as well.

Question 23

What is the influence of water vapour in air pressure?

Answer 23

Water vapour reduces the density of air. The molecular weights of nitrogen (N2) and oxygen (O2) are greater than that of water vapour (H2O). In a mass of air the molecules of these gases are intermixed, and each takes up roughly the same amount of space. As the water content of an air mass increases, lighter water vapour molecules displace heavier nitrogen and oxygen molecules. Therefore, humid air is lighter (less dense) than dry air. Nevertheless, even very humid air is only about 2% less dense than dry air at the same temperature.

Question 24

What is largely responsible for the pressure variations observed at Earth’s surface?

Answer 24

Cold, dry air produces higher surface pressures than warm, humid air. Further, a warm, dry air mass exhibits higher pressure than an equally warm, but humid, air mass. Consequently, differences in temperature and to a lesser extent moisture content are largely responsible for the pressure variations observed at Earth’s surface.

Question 25

What influence does airflow have on air pressure?

Answer 25

The movement of air can cause convergence and divergence. The pressure at the surface will increase when there is a net convergence in a region and decrease when there is a net divergence.

Question 26

Convergence

Answer 26

Where there is a net flow of air into a region and air accumulates. As it converges horizontally, the air is squeezed into a smaller space, which results in a more massive air column that exerts more pressure at the surface.

Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favourable to cloud formation and precipitation.

Question 27

Divergence

Answer 27

Where there is a net outflow of air and the surface pressure drops.

In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavourable to cloud formation and precipitation.

Question 28

What factors affect wind?

Answer 28

1. Pressure Gradient Force
2. Coriolis Force
3. Friction

Question 29

Isobar

Answer 29

Lines connecting places of equal atmospheric pressure (barometric readings taken at hundreds of weather stations) usually corrected to sea level.

Iso = equal, bar = pressure.

Question 30

Pressure Gradient Force

Answer 30

The amount of pressure change occurring over a given distance as indicated by the spacing of isobars.

The pressure gradient force is always corrected at right angles to the isobars.

Question 31

What is the relationship between the spacing of isobars, pressure gradient force, and winds?

Answer 31

Closely spaced isobars indicate a strong pressure gradient and high wind speeds, whereas widely spaced isobars indicate a weak pressure gradient and low wind speeds.

Question 32

How do pressure differences create wind?

Answer 32

If an object experiences an unbalanced force in one direction, it will accelerate (experience a change in velocity). 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 these differences, the greater the wind speed.

Question 33

How and why do meteorologists adjust atmospheric pressures at weather stations of different elevations?

Answer 33

Meteorologists must compensate for the elevation of each station, otherwise, all high-elevation locations would always be mapped as having low pressure. This compensation is accomplished by converting all pressure measurements to sea-level equivalents. Doing so requires meteorologists to determine the pressure that would be exerted by an imaginary column of air equal in height and temperature to the elevation of the recording station and adding it to the station’s pressure reading.

Question 34

How does a sea breeze form?

Answer 34

1. Before sunrise, temperatures and pressures do not vary horizontally therefore there is no wind.
2. After sunrise, the temperature of Earth’s land surface begins to increase, while the temperature over the ocean remains nearly constant.
3. As air over the land warms, it expands and forms a less dense air column.
4. Although the air pressure at the surface remains essentially the same, at higher altitudes the warm air aloft tends to have a higher pressure compared to a column of cooler air.
5. Consequently, above the land a high pressure develops, and the air aloft begins to flow away from the land.
6. The mass transfer of air aloft creates a surface high-pressure area over the ocean, where the air is collecting, and a surface low over the land.
7. The surface circulation that develops from this redistriibution of mass is from the sea toward the land (sea breeze). Thus, a simple thermal circulation develops, with a seaward flow aloft and a landward flor at the surface. Note that the vertical movement is also required to make the circulation complete.

Question 35

How does temperature affect global air pressure?

Answer 35

On a global scale variations in the amount of solar radiation received in the polar versus the equatorial latitudes generate the much larger pressure systems that in turn produce the planetary atmospheric circulation. Therefore, the underlying cause of global pressure differences and, by extension, wind, is mainly the unequal heating of Earth’s land-sea surface.

Question 36

Cyclones

Answer 36

An area of low atmospheric pressure characterized by rotating and converging winds and ascending air.

Represented on a weather map by somewhat circular-closed isobars and a red letter L denoting a low pressure system.

Tend to produce stormy weather.

Question 37

Midlatitude Cyclones

Answer 37

A large low-pressure centre with a diameter often exceeding 1000km that moves from west to east and may last from a few days to more than a week and usually has a cold front and a warm from extending from the central area of low pressure.

Question 38

Anticyclones

Answer 38

An area of high atmospheric pressure characterized by diverging and rotating winds and subsiding air aloft.

Represented on a weather map by a blue letter H denoting a high pressure system.

Tend to be associated with clearing conditions.

Question 39

Coriolis Force (Effect)

Answer 39

The deflective effect of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and the the left in the Southern Hemisphere.

This deflecting force is always directed at right angles to the direction of airflow; affects only wind direction, not wind speed; is affected by wind speed (the stronger the wind, the greater the deflecting force); and is strongest at the poles and weakens equator ward, becoming nonexistent at the equator.

Question 40

How do you tell the direction of wind speed on weather maps? What do “wind flags” represent?

Answer 40

The “wind flags” indicate the expected airflow surrounding pressure cells and are plotted as “flying” with the wind (that is, the wind blows toward the station circle).

Question 41

Why doesn’t wind speed increase indefinitely?

Answer 41

The pressure gradient force is the primary driving force of the wind. As an unbalanced force, it causes air to accelerate from regions of lower pressure. Thus, we might expect winds to continually increase (accelerate) as long as this imbalance existed. But friction, which acts to slow moving objects comes into play and stops this from happening.

Question 42

Why does wind strength increase with altitude?

Answer 42

Because it is less affected by friction from objects near Earth’s surface.

Question 43

Geostrophic Winds

Answer 43

Aloft (a few km above Earth’s surface), the Coriolis force is responsible for balancing the pressure gradient force and thereby directing airflow. As the wind flows with the pressure gradient force from areas of high to low pressure the Coriolis force acts on it. Eventaully the wind turns so that it is flowing parallel to the isobars. When this occurs, the pressure gradient force is balanced by the opposing Coriolis force. Under these idealized conditions, when the Coriolis force is exactly equal in strength but acting in the opposite direction of the pressure gradient force, the airflow is said to be in geostrophic balance. The winds generated by this balance are called geostrophic (“turned by Earth”) winds. They flow in relatively straight paths, parallel to the isobars, with velocities proportional to the pressure gradient force.

This is an idealized model of air aloft as winds are never purely geostrophic, but it provides a good estimate.

Question 44

Buys Ballot’s Law

Answer 44

For air aloft, in the Northern Hemisphere, if you stand with your back to the wind, low pressure will be found to your left and high pressure to your right. In the Southern Hemisphere, the situation is reversed.

Question 45

Does Buys Ballot’s Law apply to surface airflow?

Answer 45

Although Buys Ballot’s Law holds for airflow aloft, it must be used with caution when applied to surface winds. At the surface, if you stand with your back to the wind and then turn clockwise about 30*, low pressure will be to your left and high pressure to your right in the Northern Hemisphere and vice versa in the Southern Hemisphere.

Question 46

Gradient Winds

Answer 46

Isobars are generally not straight; instead they make broad, sweeping curves. Occasionally the isobars connect to form roughly circular cells of either high or low pressure. Thus, winds around these cells follow curved paths in order to parallel the isobars. Winds of this nature, which blow at a constant speed parallel to curved isobars, are called gradient winds.

Question 47

Cyclonic Flow

Answer 47

Flow around a centre of low pressure. As soon as the flow begins from the pressure gradient force (which is always towards the low) the Coriolis force causes the air to be deflected. In the Northern Hemisphere, it is deflected to the right, resulting in wind that blows counterclockwise, and in the Southern Hemisphere it deflects to the left resulting in wind that blows clockwise around the low.

Question 48

Anticyclonic Flow

Answer 48

Flow around a centre of high pressure.

As soon as the flow begins from the pressure gradient force (which is always outwards from a high) the Coriolis force causes the air to be deflected. In the Northern Hemisphere, the outward-directed pressure gradient force is opposed by the inward-directed (to the right) Coriolis force, and a clockwise flow results. In the Southern Hemisphere, it deflects to the left resulting in wind that blows counterclockwise around the high.

Question 49

Trough

Answer 49

Occurs whenever isobars curve to form elongated regions of low pressure.

The flow around a trough is cyclonic.

Question 50

Ridge

Answer 50

Occurs whenever isobars curve to form elongated regions of high pressure.

The flow around a ridge is anticyclonic.

Question 51

What forces produce the gradient flow associated with cyclonic and anticyclonic circulations?

Answer 51

Wherever the flow is curved, a force has deflected the air (changed its direction), even when no change in speed results. This is a consequence of Newton’s first law of motion, which states that a moving object will continue to move in a straight line unless acted upon by an unbalanced force. In a low-pressure centre, the inward-directed pressure gradient force is opposed by the outward-directed Coriolis force. But to keep the path curved (parallel to the isobars), the inward pull of the pressure gradient force must be strong enough to balance the Coriolis force as well as to turn (accelerate) the air inward. The inward turning of the air is called centripetal acceleration. Stated another way, the pressure gradient force must exceed the Coriolis force to overcome the air’s tendency to continue moving in a straight line. The tendency of a particle to move in a straight line when rotated creates an imaginary outward force called centrifugal force.

The opposite situation exists in anticyclonic flows, where the inward-directed Coriolis force must balance the pressure gradient force as well as provide the inward acceleration needed to turn the air. The pressure gradient force and Coriolis force are not balanced (CF stronger) as they are in geostrophic flow. This imbalance provides a change in direction (centripetal acceleration) that generates curved flow.

Question 52

Unlike winds aloft, which blow nearly parallel to the isobars, surface winds generally cross the isobars. Explain what causes this difference.

Answer 52

Friction as a factor affecting wind is important only within the first few kilometres of Earth’s surface and it acts to slow the movement of air. By slowing air movement, friction also reduces the Coriolis force, which is proportional to wind speed. Because the pressure gradient force is not affected by wind speed, it wins the tug of war against the Coriolis force. The result is the movement of air at an angle across the isobars, toward the area of lower pressure.

The roughness of the surface determines the angle at which the air will flow across the isobars and influences the speed at which it will move. Over relatively smooth surfaces, where friction is low, air moves at an angle of 10* to 20* to the isobars and at speeds roughly two-thirds of geostrophic flow. Over rugged terrain, where friction is high, the angle can be seen as great as 45* from the isobars, with wind speeds reduced by as much as 50%

Question 53

Explain airflow around a cyclone and anticyclone in relation to friction.

Answer 53

We have learned that above the friction layer in the Northern Hemisphere, winds blow counterclockwise around a cyclone and clockwise around an anticyclone (and vice versa in the Southern Hemisphere) with winds nearly parallel to the isobars. This is the same at the surface where friction is considered except that the airflow crosses the isobars at varying angles, depending on the terrain, but always from higher to lower pressure.

Question 54

How is friction the same in regards to airflow in both hemispheres?

Answer 54

Regardless of hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone because it reduces the Coriolis force.

Question 55

In what direction does pressure increase/decrease in a cyclone and an anticyclone?

Answer 55

In a cyclone, pressure decreases inward (low pressure) and in an anticyclone pressure increases inward (high pressure).

Question 56

Explain vertical airflow associated with cyclones and anticyclones.

Answer 56

In a surface low-pressure system (cyclone) the air spirals inward (from the surface anticyclone of high-pressure system). The net inflow of air causes a horizontal convergence that results in the air piling up increasing its pressure. So, for the surface low to endure, compensation must occur aloft so that the piled up air at the surface flows vertically and diverges aloft.

Anticyclones at the surface must also be maintained from above. Outflow near the surface is accompanies by convergence aloft and general subsidence of the air column.

Question 57

Pressure Tendency aka Barometric Tendency

Answer 57

The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction.

Question 58

What general weather conditions are to be expected when the pressure tendency is rising? When the pressure tendency is falling?

Answer 58

Because rising air often results in cloud formation and precipitation, the passage of a low-pressure centre (lowering pressure tendency) is generally accompanied by bad weather. Because descending air is compressed and warmed, cloud formation and precipitation are less likely in an anticyclone. Thus, fair weather can usually be expected with the approach of a high-pressure system (increasing pressure tendency).

Question 59

What factors contribute to surface convergence and surface divergence and how?

Answer 59

1. Friction

Friction can cause both convergence and divergence. For example, when air moves from the relatively smooth ocean surface to land, the increased friction causes an abrupt drop in wind speed. This reduction of windspeed downstream results in a pileup of air upstream. Thus, converging winds and ascending air accompany flow from the ocean to land and the opposite is true for air moving from the land to the ocean.

2. Mountains

As air passes over a mountain range, it is compressed vertically, which produces horizontal spreading (divergence) aloft. On reaching the leeward side of the mountain, the air experiences vertical expansion, which causes horizontal convergence.

Question 60

What causes “mountain sickness”?

Answer 60

At high altitudes our bodies try to compensate for the oxygen deficiency of the air by breathing more deeply and increasing the heart rate, thereby pumping more blood to the body’s tissues. The additional blood is though to cause brain tissues to swell, resulting in headaches, insomnia, and nausea - the main symptoms of acute mountain sickness.

Question 61

How is wind direction described?

Answer 61

Winds are always labeled by the direction FROM which they blow. A north wind blows from north toward the south; an east wind blows from the east toward the west, and so on. It is also described by a scale of 0* to 360* 0* (or 360) is north, 90 is east, 180* is south, and 270* is west.

Question 62

Wind Vane

Answer 62

An instrument used to determine wind direction.

Question 63

Prevailing Wind

Answer 63

A wind that consistently blows from one direction more than from any other.

During the construction of an airport, the runways are aligned with the prevailing wind to assist in takeoffs and landings.

Greatly affect the weather and climate of a region - for example, if they run into a mountain.

Question 64

Cup Anemometer

Answer 64

An instrument used to determine wind speed that has a dial much like a speedometer.

Question 65

Aerovane

Answer 65

A device that resembles a wind vane with a propeller at one end. Used to indicate wind speed and direction.

Question 66

Wind Sock

Answer 66

A cone-shaped bag that is open at both ends and free to change position with shifts in wind direction. The degree to which the sock is inflated indicates the strength of the wind.