Chapter Six - Test Three

Question 1

Although pressure is the “least sensible” of the six weather elements we examine in this class, its role in meteorology is vital because:

Answer 1

• Spatial variation of pressure generates wind

Question 2

Wind is responsible for

Answer 2

advection of temperature and moisture

Question 3

Air Pressure [p]:

Answer 3

the force (per unit area) exerted by the weight of a column of air above a given point
- the force is created by the continuous
collision of gas molecules (N2, O2, Ar…)

Question 4

At mean sea level (msl), the average p is:

Answer 4

• 14.7 ibs in^-2

Question 5

Measuring Air Pressure

Meteorological applications do not use lbs in-2; rather we use:

Answer 5

Inches of mercury (in Hg): reported to the public

• Millibars (mbs): used by meteorologists
  
  1. 0 lbs in-2 = 2.04 in Hg = 68.9 mbs

Question 6

At msl the average pressure equals:

Answer 6

14.7 lbs in-2 = 1013.25 mb = 29.92 inches Hg

Question 7

Typical values and ranges of barometric pressure (mbs, in Hg) observed at mean sea level

Answer 7

Highest ever: 32.01 in Hg

Lowest ever: 25.70 in Hg

Question 8

Instruments used to measure air pressure are called:

Answer 8

Barometers- Instruments that measure “bars”

Question 9

Since we measure pressure with barometers, we refer to it as:

Answer 9

Barometric Pressure

Question 10

The first barometer was invented in ____ by ____ and is very similar to liquid barometers still used today.

Answer 10

1643, Torricelli

Question 11

Mercury barometer:

Answer 11

a column of mercury in a glass tube is in balance with the weight of the air above it.
Advantages: very accurate
Disadvantages: expensive mercury is toxic

Question 12

Why use Hg in a barometer, especially if it’s a toxin?

Answer 12

Any liquid could be used, such as water. But, since water is much less (13.6x) dense than Hg, the height of the barometer would have to be much greater (13.6x)!
So: a typical barometric pressure of 30.00” Hg would equal 408.0” water (34’)…….
Necessitating a very large instrument!

Question 13

Aneroid Barometer:

Answer 13

consists of a small metal “box” called an aneroid (w/o liquid) cell that has had some of its air removed before sealing

An increase (decrease) in pressure cause the box to contract (expand)

This “signal” is then amplified with levels.
Advantages: relatively inexpensive, contains no toxic liquid, most common home barometer
Disadvantages: needs frequent calibration

Question 14

Two variations of aneroid barometers are :

Answer 14

Barograph & Pressure Altimeter

Question 15

Pressure Altimeter:

Answer 15

an aneroid barometer labeled to indicate altitude (instead of barometric pressure)
- used in aircraft

Question 16

Barograph:

Answer 16

a recording aneroid barometer that uses strip charts and timing mechanism (Fig. 6.5)
- provides trend information

Question 17

Pressure ____ very rapidly wrt altitude

Answer 17

decreases; (∆p/ ∆z) is much less than 0

Question 18

In the troposphere, where the decrease is linear, we can approximate the lapse rate using:

Answer 18

(∆p/ ∆z) = (-1 mb/10 meters)

Due to this rapid change, even a small difference in elevation between two surface observations can result in a large, misleading difference in barometric pressure.

Question 19

To compensate for different station elevations, we make: Elevation corrections

so that all station pressures are adjusted to an altitude of 0 meters (msl) which we call:

Answer 19

Equivalent Sea Level Pressure (ESLP)

Question 20

Station Station Pressure Elevation
A 1008 mb 0 m
B 915 mb 1000 m
C 840 mb 1800 m

Answer 20

ESLP
1008+ 0= 1008 mb
915+100 = 1015 mb
840+ 180 = 1020 mb

Question 21

Horizontal Variations in Air Pressure

After making the corrections to equivalent sea level pressures, we then plot the values on a map and then analyze the horizontal variations using:

Answer 21

Isobars: lines of equal pressure

Question 22

Isobars allow us to…

Answer 22

locate areas of high and low pressure, and strong and weak pressure differences.

Question 23

Wind:

Answer 23

nature’s attempt to balance inequalities in air pressure which are due to inequalities in surface heating

Question 24

In order to determine wind direction and speed, we must identify all of the influencing forces

Answer 24

1) Pressure Gradient Force (Fpg)
2) Coriolis Force (Fco)
3) Frictional Force (Ff)

Question 25

Pressure Gradient Force (Fpg)

Answer 25

This Force results from differences in pressure represented by pressure gradients (Fig 6.9, 6.10)

The Fpg is this force that causes the air to move (wind)

Question 26

The Fpg is this force that causes the air to move (wind)

Answer 26

It is directed from high pressure to low pressure at right angles to the isobars
- its magnitude is directly proportional to the pressure gradient

Question 27

closely spaced isobars indicate:

Answer 27

Strong gradient, high winds

Question 28

widely spaced isobars indicate:

Answer 28

Weak gradient, light winds

Question 29

If the FPG was the only force acting on the atmosphere, the wind would blow directly from areas of…

Answer 29

high pressure to areas of low pressure; however, the instant the air starts moving, its path (direction) is deflected by the: Coriolis Force (Fco)

Question 30

The Fco is an apparent force due to earth’s rotation

Answer 30

all free moving objects (planes, projectiles, air) seem or appear to deflect from a straight-line path because the earth rotates underneath them (Fig 6.12; 6.13)

• impacts wind direction only, not wind speed
• in the NH the wind is deflected to the right

Question 31

The magnitude of the FCO depends on:

Answer 31

1) rotation rate of the earth (Omega)
2) Latitude (O mega)
3) Speed of the wind (V)

Question 32

Fco can be calculated using:

Answer 32

Fco = 2 x V x Ω x sin (latitude)
Where V = wind speed and…
Ω = 360º/ 24 hours = 7.29 x 10 -5 s-1

Question 33

We can see from the Fco equation, that:

Answer 33

• The stronger the wind, the greater the Fco
• The higher the latitude, the greater the Fco
• At the equator (0 ), Fco = 0
• At the poles (90), Fco is maximum (for a given V)

Question 34

Note that under any conditions Fco is a very small force, and subsequently only comes into play when considering large time and space scales…

Answer 34

therefore it does not influence water going down drains!

Question 35

Within ~ 1500 meters of the surface, an area we call the Friction Layer. A third force, called the ____ must be considered.

Answer 35

Friction Force (Ff)

Question 36

The Friction force (Ff) is dependent on:

Answer 36

• Height: as you near the surface, the Ff maximizes (fig. 6.14a)
• Surface roughness: the rougher the terrain (i.e. mountains) the stronger the Ff

Question 37

Above the Friction Layer (where FF = 0) Fig. 6.15 illustrates that the FPG and the FCO evolve into a balanced scenario resulting a wind called the:

Answer 37

Geostrophic Wind (Vg)

Question 38

The direction of the Vg is always

Answer 38

parallel to the isobars, with its speed proportional to the gradient of the isobars.

Question 39

Using Buys-Ballot’s Law, we can see that if you stand with your back to the wind (in the NH), the Vg blows with:

Answer 39

• Low pressure on your left

- High pressure on your right

Question 40

In Geostrophic wind examples…
Fco-
Vg-
Fpg-

Answer 40

Fco - always directed 90º to the right of the wind
Vg- always parallel to the isobars
Fpg- always directed from high to low pressure at right angles to the isobars

Question 41

An aloft high pressure center has what characteristics?

Answer 41

• Clockwise (Anticyclone) Circulation

- Parallel Flow

Question 42

An aloft low pressure center has what characteristics?

Answer 42

• Counterclockwise (Cyclonic) Circulation

- Parallel Flow

Question 43

Within the Friction Layer (less than 1500 m) we must take into account the Ff

Answer 43

• reduces the wind speed, which in turn

- reduces the Fco (which is a function of velocity)

Question 44

Since the FCO is reduced in the Friction Layer, it can no longer balance the FPG; as a result the wind within the friction layer flows:

Answer 44

• At a reduce speed

- Across the isobars, toward low pressure

Question 45

Surface Winds

The average angle at which the wind crosses the isobars at the surface:

Answer 45

• 30 degrees

but it can vary depending upon the surface roughness and range from:

• 10 degrees over the oceans,
• 45 degrees over mountains

Question 46

In Surface wind examples...
Fco-
Ff-
Fpg-
V-

Answer 46

Fco- always directed 90º to the right of the wind
Ff- always directed in the opposite direction of the wind
Fpg- always directed from high to low pressure at right angles to the isobars
V- cross isobar flow towards low pressure

Question 47

A surface high pressure center has what characteristics?

Answer 47

• Clockwise (Anticyclone) outflow

- Surface Divergence

Question 48

A surface low pressure center has what characteristics?

Answer 48

• Counterclockwise (Cyclonic) inflow

- Surface Convergence

Question 49

Although the vertical transport of air is generally much ____ than the horizontal transport, it is nonetheless critical to most of what we consider weather (Fig 6.22).

Answer 49

slower

Question 50

Surface winds associated with a Cyclone:

Answer 50

Converge toward the center.

Since the converging air can’t go into the ground it must rise, thereby:
Producing clouds and precipitation

Question 51

Surface Winds associated with an Anticyclone:

Answer 51

Diverge from the center.

To replace the laterally diverging air, air aloft must sink, thereby:
Inhibiting clouds and precipitation.

Question 52

Meteorologists measure and report wind:

Answer 52

speed : { ms. -1, mph, knots}

direction : (FROM which it blows) {degrees, quadrants. (i.e. nw, se)

Question 53

Wind measurement instruments include:

Answer 53

Wind Vanes: measure direction
Cup Anemometers: measures speed
Aerovane: measures speed and direction

Question 54

Wind Roses

Answer 54

provide a method of representing prevailing winds by indicating the % of times the wind blows from a particular direction.

Question 55

Prevailing winds from RDU are coming from the ____ direction

Answer 55

southwest