Weather and Climate Exam 2

Latent Heat

The heat required to melt or evaporate a substance

How does latent heat affect weather?

When things cool or condense, latent heat is released back into the environment

What is an example of latent heat?

When you sweat, latent heat is taken from the environment to evaporate your sweat to cool you off

Condensation

when water vapor hits the surface through collisions and random movement

Saturated

evaporation and condensation in the air are equal

What are two ways to get water to condense?

Decrease the air temperature or increase the amount of water vapor

Vapor pressure

the atmospheric pressure due to water

Saturation vapor pressure

atmospheric pressure if the air was saturated

What is the relationship between temperature and relative humidity?

They are “anti-correlated” so as temperature increases, relative humidity decreases and vice versa

What is the relationship between temperature and saturation vapor pressure?

As temperature increases, saturation vapor pressure increases

What happens to vapor pressure when relative humidity reaches 100%?

It starts to decrease

What happens to dew point when it is below freezing?

It becomes frost and a bunch of latent heat is released into the environment

Fog

Air comes in contact with the ground which cools enough for water to condense and form tiny water droplets

Radiation fog

The surface cools as infrared radiation is given off and the water in the air near the surface condensates

Tule fog

Radiation fog that occurs in the central valley of California in late Fall to early Spring

Advection fog

Warm, moist air blows over a cold surface and cools enough for water droplets to form

Upslope fog

Winds push moist air uphill to an altitude where temperatures are cold enough to form water droplets

What happens as an air parcel rises?

It expands into lower pressure air and all the energy inside is spread out over a larger volume so the temperature drops

Adiabatic

No interaction between the air inside the parcel and the outside environment

Adiabatic cooling

drop in temperature

Lapse rate

Temperature decreases with height

Dry Adiabatic Lapse Rate

For every kilometer increase, the temperature inside the parcel cools by 10 degrees celsius

Environmental lapse rate

6.5 degrees Celsius per km

What is the relationship between saturation vapor pressure, temperature, and relative humidity inside an air parcel?

The saturation vapor pressure inside the parcel decreases as temperature decreases, so the relative humidity increases INSIDE the parcel.

What happens when the air parcel subsides/falls to the surface?

The parcel begins to shrink because of higher pressure. The saturation vapor pressure increases as the temperature increases as well, so the relative humidity decreases and the parcel dries out

Saturated Adiabatic Lapse Rate

The parcel cools at 6 degrees Celsius per km

What happens when air rises and falls down a mountain?

As air rises up the mountain side, it cools at a dry adiabatic lapse rate until it reaches 100% relative humidity and becomes saturated. After that it cools at the saturated adiabatic lapse rate. Once it reaches the top and falls down the mountain, it no longer has any water vapor, so it warms at the dry adiabatic lapse rate.

What happens when an air parcel reaches 100% relative humidity?

It becomes saturated

Rain shadow

The windward side of a mountain will be lush and tropical while the leeward side is dry

Chinook Winds

Air rushing down the downward side of a mountain warms and dries as it sinks leading to warm surface winds

What can happen between the boundary of warm and cold air?

It can move quickly and slosh back and forth, meaning there is no mix of temperatures but a sudden large increase or decrease in temperature

Gravity Waves

Air parcel travels too far up the mountain and rather than sinking back down, it continues up and moves down and up like a wave.

What are the different types of lifting mechanisms?

Orographic lifting, convergence, frontal lifting, convection

Orographic lifting

The ground forces the air up the side of a mountain and cools as it rises, forming clouds

Convergence

Wind meets in different directions and forces the air to go up and form clouds, OR the leading edge of the wind slows down and the wind behind piles upwards

Frontal lifting

Warm air rides up over cold air. Warm fronts create clouds and cold fronts are below it

Convection

The surface is heated by the sun, becomes less dense, and rises.

Stable

In a stable atmosphere, the air won’t rise up unless something forces it up

Unstable

In an unstable atmosphere, air can rise on its own through convection

Conditionally unstable

In a conditionally unstable atmosphere, the air will rise on its own but needs a push first

Example of unstable atmosphere

An air parcel that is warmer than the area around it will rise by itself and continues to rise until it reaches the same temperature as the area around it

In an unstable atmosphere, what is the difference between the environmental lapse rate and the adiabatic lapse rate?

The environmental lapse rate cools faster than the adiabatic lapse rate

Example of stable atmosphere

An air parcel is colder than the area around and wants to stay in place. It may move up by lifting mechanisms but will go back down.

In a stable atmosphere, what is the difference between the environmental lapse rate and the adiabatic lapse rate?

The environmental lapse rate cools slower than the adiabatic lapse rate

Example of conditionally unstable atmosphere

The air parcel is originally the same temperature as the air around it, but something lifts it up, and it becomes warmer than the air around it, so it continues to rise upwards

In a conditionally unstable atmosphere, what is the difference between the environmental lapse rate and the adiabatic lapse rate?

The environmental lapse rate cools slower than the dry adiabatic lapse rate until it reaches saturation, and then the environmental lapse rate cools faster than the dry adiabatic lapse rate

Stratus

Layer

Cumulus

Heap/pile/puffy

Nimbus

Rain

Alto

Middle

Cirrus

High

High clouds

Cirrus (Ci), Cirrostratus (Cs), Cirrocumulus (Cc)

Middle clouds

Altostratus (As), Altocumulus (Ac)

Low clouds

Stratus (St), stratuscumulus (Sc), nimbostratus (ns)

Clouds with vertical development

Cumulus (Cu), Cumulonimbus (Cb)

What’s the difference between high, middle, and low clouds?

Low clouds are much thicker and harder to see through while middle and high clouds are wispier and easier to see the sun through

Why are clouds grey on the bottom and white on the top?

Clouds have a high albedo, so they reflect the light off of them and have scattered sunlight

Cumulonimbus

Cumulus puffy clouds with rain

Stratus

A uniform layer of clouds

Contrails

Line-shaped clouds made by planes when the air is very humid

Where are clouds in relation to a warm front?

Clouds are ahead of a warm front

How does precipitation form in the summer/in warm areas?

Tiny droplets ride updrafts and collect more water as they rise. Eventually, the droplets get so big that gravity takes over and the drops begin to fall

Collision and coalescence

When large drops fall, they hit small drops, and the big drops continue to grow in size. Some small drops move upward when they are hit while others get trapped in the big drop

What are the water phases within a cloud?

Near the surface, the cloud has liquid droplets. As you go up and the temperature decreases, the middle part of the cloud has a mix of liquid droplets and ice crystals. The very top of the cloud has completely frozen ice crystals.

Accretion

Falling ice crystals come in contact with supercooled droplets, freezing them and making larger ice particles

Aggregation

Falling ice particles collide and break into tinier ice particles. Then, the ice crystals collide and stick to other ice crystals, forming a snowflake

Graupel

Snow pellet

How does hail form?

Strong updrafts from convection carry water droplets into atmosphere where they freeze. These frozen droplets cycle up and down, collecting layers of ice until it gets too heavy and falls to the ground

Freezing rain

Frozen precipitation melts in warm air; rain falls and freezes on cold surface as a sheet of ice.

Sleet

Frozen precipitation melts and refreezes into sleet before hitting the ground

What are the 5 major forces that control wind?

Gravity, pressure gradient, Coriolis, centrifugal, friction

Pressure Gradient Force

Pushes from high pressure to low pressure

What happens to air in a high-pressure system? 

Air flows out and diverges

What happens to air in a low-pressure system? 

Air flows in and converges

If there is a weak PGF, what happens to winds?

The winds are weak

If there is a strong PGF, what happens to winds?

The winds are strong

What is the relationship between isobars and PGF?

The closer the isobars, the stronger the PGF (and the stronger the winds!)

During the day on the beach, where are the high vs. low pressures?

The land heats up and air rises, so there is high pressure above land and low pressure above sea.

During the night on the beach, where are the high vs. low pressures?

The land is cooler than the sea, so the sea has higher pressure above and the land has lower pressure

Coriolis force

A “fake” force that makes it look like something is moving on its own when its actually moving in a straight line but spinning

In the Northern Hemisphere, Coriolis Force always deflects in what direction?

To the right

What does Coriolis force depend on?

Latitude

If there is Coriolis force on the equator…

There is zero Coriolis force

Geostrophic Wind/Balance

When the pressure gradient force and the Coriolis forces balance and the direction of the wind becomes straight

What region of the planet is warmest?

The equator

Why is it colder over the land in the Northern Hemisphere winter?

During the winter, the land has a lower specific heat, so the land is colder than the water

What are the regions of the world and their temperatures?

Equatorial/tropical is the hottest, subtropics are warm, midlatitudes are colder, subpolar and polar regions are coldest

What happens to the air that moves poleward?

The air sinks and creates high pressure at the surface of the poles

What doesn’t the Hadley cell explain?

The mid-latitude westerlies

What is the three-cell model?

Model of global atmospheric circulation with the Hadley cell, the Ferrel cell, and the Polar cell

Hadley Cell

Tropical cell where warm air rises at the equator and moves poleward before sinking at around 30 degrees latitude, some of the air sinking back to the equator

Ferrel Cell

Mid-latitude cell is a “westerlies cell” where air sinks at the subtropical high and rises at the subpolar low; this is where Coriolis force takes place

Polar Cell

Cell found at high latitudes with cold air sinking at the poles and rising again around 60 degrees latitudes, where it moves towards the poles

Jet Streams

Fast-moving, high-altitude band of wind in the atmosphere that flows from west to east, driven by temperature difference between the poles and the equator

Why are there deserts?

Sinking air creates high-pressure zones that inhibit cloud formation and rainfall

Intertropical Convergence Zone

The region where trade winds meet near the Equator

What causes monsoons?

The Intertropical Convergence Zone causes monsoons

Monsoons

Shift in winds that causes very rainy or dry seasons

Summer monsoons

In the Northern Hemisphere’s summer, the sun is more directly overhead the northern tropics. The land is warmer than the oceans, causing ITCZ to shift northward. The resulting low-pressure area pulls in moist air leading to heavy rainfall

Winter monsoons

In the Northern Hemisphere’s winter, the land cools down while the ocean remains warm. The ITCZ shifts back towards the equator and winds blow from dry land to the ocean, resulting in a dry season