AOSCI 3 Midterm

Density (r)

mass (kg) per unit volume (m^3)

Mean free path

Average distance a molecule travels before colliding with another molecule

Pressure

force per unit area

What causes pressure?

The force caused by collisions of molecules with the surface

What would happen to the molecules in a column of air if I increased the density of a gas?

higher density → more collisions → higher pressure

True or False: Pressure is inversely proportional to density.

False

Suppose that instead we keep the density constant and increase the temperature of the gas. What happens to the pressure of the gas?

Pressure increases as we increase temperature because Temperature is a measure of kinetic energy, which is proportional to the speed of molecules squared.

What would happen to the molecules in a column of air if we increased the temperature in a gas?

the higher temperatures would result in stronger collisions (bc temp is directly proportional to kinetic energy), which would lead to an increase in pressure

We usually hear people say that “hot air rises.” Using the ideal gas law, can we explain this statement? Are there other conditions that must exist for this to be true?

Air rises when it is buoyant, which happens if the density is lower than its surroundings. (Thick helium in a balloon). Density of an air parcel may decrease with increasing temperature, thereby causing it to be buoyant, but the ideal gas law tells us this would only occur if the pressure of the air parcel stays constant.

What happens to pressure with changing altitudes?

It decreases most rapidly at low elevations and gradually tapers off at greater altitudes; i.e. pressure and altitude are inversely proportional

Suppose I’m at a location where the sea level pressure is lower than the surface pressure? Where could I be?

Possibly in the Death Valley, because it is located below the sea level. Since pressure decreases with increasing altitude, the only way the sea

Name and describe the four thermal layers.

Troposphere (layer of virtually all weather processes), Stratosphere (ozone layer), Mesosphere (ozone layer), Thermosphere (low density, almost no mass).

Name and describe the three ways thermal energy (heat) can be transferred.

Radiation (the sun heats the ground) 2. Conduction (the ground heats the air) 3. Convection (the warm air rises due to fluid motion)

In the troposphere, temperatures are highest near the surface and cooler above. If “hot air rises, cool air descends” in convection, how does the atmosphere keep this structure?

If a parcel of air is lifted, its temperature decreases even if it does not give off heat. This happens because the parcel expands so that its pressure always equals the environmental pressure, which is decreasing as the altitude is increased – the parcel temperature drops just because it expands. Since the pressure, temperature and therefore density change as the parcel is lifted, the parcel can remain warmer than the air above it without causing convection.

The pressure gradient force forms an angle of with the isobars, pointing in the direction of pressure

90 degrees, lower

Vertical PGFs in the atmosphere are much larger than horizontal PGFs, but vertical velocities in the atmosphere typically much smaller than the horizontal velocities. How is this so?

Gravity is a large force that only operates in the vertical direction. The drop in pressure with altitude is there so that a vertical PGF is large enough to balance gravity. The balance is very precise, so that very little net force is present in the vertical direction.

If heat is added to a column so that its temperature increases (without changing pressure or mass), how does its density decrease?

its volume increases

What happens to the density when a column of air is heated?

it causes a density decrease; the column contains the same amount of air but has a lower density to compensate for its greater height

How does air flow from high to low pressure work?

In high pressure, winds spiral outward and flow between the isobars to low pressure, where they spiral inward

An airplane is flying forward at constant speed. At a given moment the forces acting on the airplane are such that Thrust is balanced by Air Friction, but Lift is smaller than the Weight. How is the airplane motion going to respond?

the plane will continue moving forward at the same speed, but will accelerate downward. this is because if thrust is equal to friction (which it is here), it wouldn’t change the speed forward or backward bc there’s no net force. However, there is net force in the vertical direction because it’s given that weight \> lift.

When does geostrophic balance occur?

When PGF and the Coriolis Force are in balance because there’s no net force.

Cyclones

enclosed areas of low pressure with roughly circular isobars; air circulates inward in the counterclockwise direction

Anticyclones

enclosed areas of high pressure with roughly circular isobars; air circulates outward in the clockwise direction

The velocity in the 500 mb map below is parallel to the isobars (represented as lines of constant height). Can we say that the wind is in Geostrophic Balance?

No. For the wind flow to curve, a net force causing acceleration is needed.

In the upper atmopshere, the ridge axis is \_____ than geostrophic balance.

faster

In the upper atsmophere, the trough axis is \____ than the geostrophic balance.

slower

Evaporation

Molecules escape from liquid into the overlying volume as water vapor

What relationship does evaporation rate have to the temperature of the liquid?

Directly proportional

Condensation

Water vapor molecules randomly collide with the water surface and bond with adjacent molecules

What relationship does condensation rate have to the amount of vapor?

directly proportional

What are two ways to quantify humidity?

Vapor pressure (the portion of atmospheric pressure exerted by water vapor molecules) and mixing ratio (mass of water vapor in a unit mass of dry air)

We know that evaporation rate increases with temperature. How does temperature affect vaporpressure at saturation?

vapor pressure at saturation increases with increasing temperature because the evaporation rate is increasing relative to the condensation rate, but condensation increases by vapor itself increasing.

What would happen if water temperature increased?

the evaporation rate would increase, and we’d need higher “humidity” for the condensation rate to match the evaporation rate, meaning that SVP would increase.

Dew point temperature

temperature to which air must be cooled to reach saturation

Is the dew point temperature a measure of amount of water vapor in the air? Does it change when we change the amount of water vapor? Does it change if we fix the amount of water vapor and change temperature?

Despite having units of temperature, the dew point temperature only depends on the amount of water vapor. It does change when we change the water vapor amount. It does not change if we fix the amount of water vapor and change the temperature.

What processes cause saturation?

the addition of water vapor 2. cooling the air to the dew point 3. mixing cold air with warm moist air

Suppose we do our “saturation” experiment. Assume the air, the container, and the liquid water are all at the same temperature. Where is the condensation going to occur?

Water molecules stick to surface (adhere) and to each other (cohere). Condensation occurs whenever water vapor comes into contact with either type of surface.

Homogeneous nucleation

only water vapor molecules coming together to form water

Heterogeneous nucleation

water vapor molecules adhere to a solid surface on a condensation nucleus

What is the difference between diabatic and adiabatic processes?

Diabatic processes consist of changes in temperature caused by heat transfer, and adiabatic processes consist of changes in temperature caused by changes in pressure.

Dew

liquid condensation on surface often occurring during the early morning hours; surface air becomes saturated and condensation forms on objects acting as condensation nuclei

Frost

forms when surface temperatures are below freezing; deposition (vapor to solid) occurs instead of condensation

Frozen dew

when normal dew formation processes occur, followed by a drop in temperature to below freezing

Lifting condensation level (LCL)

the height of cloud formation

Convection

vertical motion caused by buoyancy

Buoyancy

principle that a parcel of air that is less dense than its environment will rise, and one that’s denser will sink

How will the pressure of an air parcel compare to the pressure of its environment when it’s rising?

it’ll be equal, which lets us relate density to temperature. We know from that that the temperature of the parcel is greater than the environmental temperature, and therefore the environment is denser than the parcel (in a case where the air parcel rises).

How will the pressure of an air parcel compare to the pressure of its environment when it’s sinking?

The temperature of the parcel will be greater than the environmental temperature, and the density of the parcel is greater.

How can we gauge the stability of an environment?

By comparing lapse rates; if the lapse rate is less than the environment, the equilibrium is unstable.

How does density change with height?

Decreases

(1) The atmosphere is a mixture of

gas molecules, small particulates, and moisture.

(1) Radiosondes

allow us to measure weather conditions in the upper atmosphere.

(1) According to the scientific method, a hypothesis

must lead to predictions.

(1) Atmospheric motions are mostly horizontal because

its shallowness doesn't allow large vertical motions.

(1) The troposphere

is thicker in summer.

(1) What will increase given an increase in pressure?

density

(1) What are used to identify places of equal atmospheric pressure?

isobars

(1) Wind direction is properly referred to as

the direction the wind is blowing from.

(2) Radiation, as an energy transfer mechanism, is especially important because

radiation energy can travel without an intervening medium. radiation provides us with visible light. everything radiates.

(2) Your hand feels cold when you hold a cold can of soda. Energy is being transferred from your hand to the can by

conduction.

(2) Warm air rises. This is an example of energy transfer by

convection.

In 1991, Mt. Pinatubo in the Philippines erupted. The accompanying photo shows the ash and debris that the volcano propelled high into the atmosphere. What meteorological impact could these aerosols have on the global weather and climate?

Global temperatures could decrease. The ash and smoke could lead to an increase in clouds and fog.

(4) The relationship between air pressure and elevation is nonlinear because

air is compressible.

(4) Horizontal pressure changes are \________ than vertical pressure changes.

less than

(4) According to the Ideal Gas Law, the pressure will increase if

the air density increases while the temperature is held constant.

(4) According to the equation of state p \= RT,

an increase in density requires an increase in pressure or a decrease in temperature or both.

(4) Consider a closed container filled with air at a fixed temperature. Suppose we double the number of molecules inside the container and keep the temperature the same. What can we say about the changes in pressure and density in the container?

Density and pressure will both double.

(4) Consider a closed container filled with air at a fixed pressure. Suppose we double the number of molecules inside the container and keep the pressure the same. What can we say about the changes in temperature in the container?

Temperature will be reduced by half.

True or False: the spacing of isobars indicates the strength of the pressure gradient.

True

(4) Hydrostatic equilibrium occurs when

the force of gravity and the vertical pressure gradient have equal value and oppose each other.

(4) The heights of the 500 mb surface in air column A is 5700 meters and 5640 meters in air column B. Choose the best answer from the following.

the temperature in air column A is higher while the density is lower

(4) If a column of air is heated, its pressure gradient will be…

smaller

(4) The Coriolis Effect is zero at

the Equator.

(4) This is the layer in the atmosphere where friction is important in determining wind speed.

planetary boundary layer.

(4) This prevents wind from following the direction of the horizontal pressure gradient force

the Coriolis effect

(4) If the pressure gradient force exactly balances the Coriolis force above the boundary layer, which of the following is the result?

A geostrophic wind occurs. When the airflow becomes unaccelerated with unchanging speed and direction, such nonaccelerating flow is called the geostrophic flow (or geostrophic wind), and it occurs when the pressure gradient force equals the Coriolis force.

(4) Which of the following would most likely happen to Hurricane Isabel if it were to approach the equator? (Northern Hemisphere)

Hurricane Isabel would weaken dramatically. Because the Coriolis Force is zero at the equator, if Hurricane Isabel were to approach the equator, it would be very weak, since hurricanes are greatly influenced by the Coriolis Force.

(4) Which of the following is most likely why Hurricane Isabel's wind speed decreases after it makes landfall? (Northern Hemisphere)

friction. Friction values are far higher on land than over water. Because of this, Hurricane Isabel decreases in wind speed and therefore weakens slightly as it makes landfall.

(4) When an air parcel moves north in the Northern Hemisphere, it gets deflected to its right due to the Coriolis force. What is the reason for this deflection?

The parcel is moving toward the axis of rotation of the planet, so it must spin faster to conserve angular momentum

(4) An updraft is a region of \_______ air where clouds are likely to form. In an updraft we typically have that the vertical pressure gradient force is \________ then the gravitational force.

rising / larger

(4) If the wind is parallel to the isobars, we can say that

the wind could be geostrophic, supergeostrophic or subgeostrophic.

(4) If we hypothesize that pressure gradient forces are the only forces driving horizontal winds in the upper atmosphere, what would be an observation that proves our hypothesis to be wrong?

Winds are mostly parallel to isobars in the upper atmosphere.

(4) Suppose the wind is initially in geostrophic balance, and then frictional forces are added. As a consequence, we would observe that

the Coriolis force decreases. the winds speed decreases. the wind direction shifts towards the low pressure

(In class Ch 5) We know that evaporation rate increases with temperature. How does temperature affect vapor pressure at saturation?

Vapor pressure at saturation increases with increasing temperature

(5) Water is unique because

it exists in all three states naturally at the same time.

(5) Advection fog

occurs when warm air passes over a cool surface.

(5) When air within a cloud begins to sink, it

warms at the saturated adiabatic lapse rate.

(5) Frost forms on your windshield through which process?

deposition

(5) Saturation vapor pressure is dependent upon this variable.

temperature

(5) Vapor pressure is

the part of atmospheric pressure due to water vapor.

(5) When the relative humidity is 100 percent

water will condense out of the air.

(5) The dew point can be used to…

indicate the value of relative humidity.

(5) As the temperature increases, generally, the relative humidity

decreases

(5) This is not one of the three processes for saturating air with water vapor.

increasing the carbon dioxide content of air

(5) When the atmosphere contains more moisture than theoretically possible, the condition of \________ results.

supersaturation

(5) Which of the following best describes the process in which water droplets form onto hygroscopic particles?

Heterogeneous nucleation. The formation of water droplets onto hygroscopic particles is called heterogeneous nucleation, and the particles onto which the droplets form are called condensation nuclei.

(5) Which of the following best describes the lifting condensation level (LCL)?

The altitude at which clouds begin to form in a rising parcel of air. If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude at which this occurs is known as the lifting condensation level (LCL).

(5) Which of the following is the relative humidity of an air parcel with a vapor pressure of 8 mb and a saturation vapor pressure of 16 mb?

50%

(5) Suppose we increase the temperature of an air parcel without changing its mass of water vapor. The air parcel is not saturated. We can say that:

Its dew point temperature will not change. The dew point temperature is dependent only on the mass of water vapor

(5) During Santa Ana winds, air blows from the high deserts towards Los Angeles. Suppose the mass of water vapor in the air does not change between the desert and L.A.

The relative humidity in L.A. will be lower than in the desert.