7.Pressure & Wind

Question 1

define pressure

Answer 1

• a force exerted over a given surface area is pressure

• air molecules are pulled down toward Earth’s surface by gravity
• because these molecules are moving, they exert a force on anything they are in contact with

Question 2

1000 Pa=__kpa=__mb

Answer 2

1, 10

-all units of pressure

Question 3

Newtons 2nd law of Motion:

force= ____ x _____

Answer 3

mass x acceleration

• if 2 objects accelerate at the same rate, the heavier object will require more force to keep up
• we must also consider all of the forces, including the forces pushing the object and the forces resisting the motion of the forces
-vertical winds try and counter gravit

Question 4

The pressure at Earths surface is on average ______ mb

Answer 4

1013.2

Question 5

T OR F

pressure is proportional to the rate of collision between the molecules and a surface

Answer 5

T

• an increase in pressure implies that there is a greater rate of collision, while a decrease in pressure implies fewer collisions

Question 6

2 ways to increase rate of collisions, and therefore pressure

Answer 6

1. increase the density (add molecules, or decrease the volume) 2. raise the temperature (ie, the speed of the molecules

Question 7

Partial Pressure

Answer 7

a specific pressure that different molecules inidividually have based on Molar Mass

• C02 very high
• sum of all partial pressures is the total pressure

Question 8

Vertical Pressure Gradient

Answer 8

• counters gravity in that it makes molecules want to rise because they always want to go from high to low pressure in upper atmos
• ex: air baloons expanding tremendously at high altitudes

Question 9

You always need to correct pressure measurements for _____

Answer 9

altitude

Question 10

roughly,pressure decrease by __ mb for every 10 m you ascend

Answer 10

1

Question 11

_____plays a much larger role in pressure then temperature does

Answer 11

density

• although not contained in the ideal gas law water content also has an effect on air density
• really really humid days, air is actually lighter compared to dry days

Question 12

Does Dry or wet air create more pressure?

Answer 12

• dry air (ie, containing no water) has an average molecular weight of 28.5
• H2O has a molecular weight of 18.01, so as water content in the air (ie, humidity) increases, the molecular weight of the atmosphere decreases, and pressure decreases

Question 13

• the standard instrument for measuring atmospheric pressure is the ________ ________

Answer 13

mercury barometer

-• Hg is used because of its high density – if water were used, the barometer would have to be 10m tall

Question 14

3 corrections have to be made to barometer readings for them to be accurate:

Answer 14

1. Compensation for elevation
2. Compensation for temperature
   - as temperature increases, Hg expands (and vice versa) which can be mistaken for a change in pressure
3. compensation for gravity
   - acceleration due to gravity is greater at higher latitudes

• we therefore describe barometric pressure as being relative to sea level, 0 °C, and at 45 °N

Question 15

aneroid barometer

Answer 15

-much cheaper option
• air pressure collapses a chamber, and the attached needle reflects this change on a scale
• each aneroid barometer is calibrated to the specific location, so it automatically accounts for elevation and gravity
– temperature is irrelevant, since there is no expandable fluid in it

Question 16

isobars

Answer 16

lines of equal pressure

Question 17

Using isobars we can determine the pressure _____

Answer 17

gradient

Question 18

________ causes wind to blow

Answer 18

pressure gradient

• if the pressure gradient is strong, then the isobars are close together, the pressure difference is great, and wind speed is hgih
• if the pressure gradient is weak, then the isobars are far apart, the pressure difference is small, and wind speed is low

Question 19

Horizontal Pressure Gradient

Answer 19

• across Earth’s surface, pressure is often distributed as series of high and low pressure centres, with winds blowing between them from high to low • the typical pressure difference though is very small – low pressure is typically 980 mb while high pressure is typically 1020 mb; a difference of only 40 mb • the distance separating these high and low centres can be very large, on the order of 3000 km • this produces a relatively weak average horizontal pressure gradient of 1 mb per 75 km • sometimes, this pressure gradient can become much greater • the pressure gradient between the centre of a hurricane and the outer edge can be as much as 1 mb per 6 km

Question 20

• the typical pressure difference though is very small – low pressure is typically ___ mb while high pressure is typically ___ mb; a difference of only 40 mb

Answer 20

980

1020

Question 21

Vertical Pressure Gradients

Answer 21

-vertical pressure gradients are much greater than horizontal pressure gradients
• average sea level pressure is 1013.2 mb while at 500m up it is 500 mb, a pressure gradient of about 1 mb per 10 m
• compare this to the relatively strong horizontal gradient of a hurricane at 1 mb per 6 km
• if vertical pressure gradients are so much greater than horizontal gradients, why do we consider wind to be a horizontal process

Question 22

Why do we think of wind being a horizontal proccess?

Answer 22

• although air would prefer to flow upwards, gravity is pulling it back down to the surface
• vertical pressure gradient is balanced by the pull of gravity

Question 23

there is a greater vertical pressure gradient through ______ air because of higher density

Answer 23

colder

Question 24

Coriolis Force

Answer 24

to the right in north, left in south

Question 25

Coriolis is dependent solely on

Answer 25

wind speed

Question 26

4 important facts about coriolis

Answer 26

1. the Coriolis force produces an apparent deflection in all moving objects, regardless of their direction of motion (deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere)
2. the Coriolis force is zero at the equator and increases with increasing latitude, reaching a maximum at the poles
3. the Coriolis force acting on any object increases with the object’s speed
4. the coriolis force changes only the direction of a moving object, never its speed

Question 27

2 layers of earths atmosphere in terms of friction=

Answer 27

1. Planetary Boundary Layer <1.5 km
   - friction

2.Free atmos
>1.5 km
-almost non existant friction

Question 28

3 forces that effect wind speed and direction

Answer 28

1. Pressure Gradient= +
2. Coriolis effect= minor
3. Friction= -

Question 29

If earth didnt rotate winds would flow …

Answer 29

northward following upper atmos pressure surface

Question 30

Geostrophic flow/wind

Answer 30

-upper air movement caused by balance of pressure gradient and coriolis causing eastward flowing wind in North
• geostrophic flow is very stable, and is not easily disrupted
• eg, an increase in the pressure gradient would cause the wind to shift northward, but the associated increase in windspeed would enhance the Coriolis force, bringing the flow back to eastward