cloud formation & fronts

Cloud Formation & Fronts

Ingredients Required for Clouds:

Water vapor (water as a gas)

Conditions favoring the change of state (from gas to liquid or ice)

A surface for water vapor to condense on (condensation nuclei)

CONDENSATION

How does water vapor get into the

air?

By evaporation and transpiration

T T

T

E E E

• Temperature controls the amount of moisture that COULD go into the air

• The amount of moisture that COULD get into the air is called its CAPACITY.

• The greater the temperature the greater the moisture capacity of the air.

Temperature

Moi

stur

e C

apac

ity

How do you get the water vapor out of the air?

• The vapor in the air must CONDENSE!

• This happens when the air is “Filled to its capacity” or saturated (100% rel humidity).

• These terms above are not really accurate, but will serve as a good starting point.

• A surface is also needed.

• Morning dew is a perfect example of moisture getting out of the air.

• Dew forms on cool surfaces only.

• Dust, smoke, ash, and salt particles in the air serve as the surface for water vapor to condense on.

• When the vapor hits these surfaces, it clings to them and forms super tiny water droplets around them

How do you get the water vapor out of the air?

Cloud droplets can survive by latching onto microscopic solid particles, or condensation nuclei in our atmosphere. These solid particles can be dust, smoke, and salt particles.

From volcanoes From Forest

Fires

Pollution

The Ocean

Salt water droplets from the ocean are carried by updrafts into the atmosphere. When the water evaporates, the salt is left behind.

(First three pictures are not by the author)

The best condensation nuclei are hygroscopic, or water absorbent

We can think of them as water-droplet “magnets”

Water vapor molecules

Condensation Nucleus



Condensation nuclei allow a water droplet to grow to a size large enough that can now avoid being dried out by evaporation.

Liquid water (drops coalesced together)

Condensation nuclei hold the liquid droplets long enough so another vapor molecule can condense on it.


They increase the probability that more water molecules will “hit” the growing drop rather than leave it!


They increase the probability that more water molecules will “hit” the growing drop rather than leave it!

Due to condensation nuclei, clouds can form even at relative humidities that are below 100%! (Even as low as 75%!)

If the condensation nuclei is soluble (such as salt), they are even more effective at keeping the growing liquid droplet together. The reason for this is that dissolving anything in water lowers the vapor pressure of the water (lowers the evaporation rate!)

What factors result in cloud formation?

•The air must rise•As the air rises, it encounters less pressure

•The air molecules expand and cool (Adiabatic Cooling)

•When the air is cooled to the dew point temperature condensation occurs and clouds begin to form

What causes the air to rise?• Forced up over a mountain range• A mass of relatively low density is forced

up and over a mass of cooler, more dense air

• Carried by a convection current

Air rising and cooling to the dew point by expansion (adiabatic cooling)

By forced lifting—such as when air is forced over a mountain:

Pictures from the National Audubon Society Field Guide to Weather

WINDWARD SIDEWET Air rises, expands, cools, condensation occurs, clouds form and it rains.

LEEWARD SIDEDRY Air descends, is compressed, and heats up.

The second way to get air to rise is for less dense air to be force

up and over more dense air.

• This happens at the boundaries between AIR MASSES

• AIR MASS: A region of air that has similar temperature and moisture.

What is an Air Mass?

• Air masses are large bodies of air which have similar temperature and moisture characteristics.

• Air masses form when air stays over a region (called the source region) for several days.

• Air masses that form over water will be moist.

• Air masses that form over land will be dry.

AIR MASSES• Air takes the characteristics of the surface

over which it formed.

• An air mass that forms over the ocean will be moist. These air masses are called MARITIME air masses

• An air mass that forms over land will be relatively dry. These air masses are called CONTINENTAL air masses

Air Masses• An air mass that forms over a cold

portion of the Earth is called a POLAR air mass.

• An air mass that forms over a warm portion of the Earth is called a TROPICAL air mass.

• An air mass that forms over an extraordinarily cold portion of the Earth is called an ARTIC air mass.

Air MassesAir Mass Classification

Source Region Identifiers:

• A for Arctic, • P for Polar, • T for Tropical;

Moisture Content Identifiers:

• c for continental (meaning the air is relatively dry),

• m for maritime (meaning the air is relatively moist);

Open your ESRT to page 13!

• cT Continental Tropical – dry & warm• cP Continental Polar – dry & cold• mT Maritime Tropical – humid & warm• mP Maritime Polar – humid & cold

• cA Continental Artic – very dry & very cold

What happens when air masses meet?

• The boundary between two air masses is called a FRONT.

Fronts – boundary that separates 2 different air masses

• Either a warm front is advancing or a cold front is advancing.

Kinds of Fronts

• Cold Fronts• Warm Fronts• Stationary Fronts• Occluded Fronts

Go to page 13 of your ESRT!

Cold Front

• A cold front occurs when a more dense air mass pushes under a less dense air mass – short–lived thunderstorms

Cold air advancing toward warmer air

Warm Front

• A warm front occurs when a less dense air mass rides up over a more dense air mass – longer, steady precipitation

Warm air advancing toward cold air

Stationary Front

• A stationary front occurs when the air masses on either side of the front are not moving toward each other.

Occluded Front


By forced lifting—such as when less dense warm air is forced above more dense cold air (when two air masses meet)

A continental polar air mass comes down from Canada and meets a

maritime tropical air mass from the Gulf of Mexico. Watch the cloulds

form.


By forced lifting—such as when less dense warm air is forced above more dense cold air (when two air masses meet)

The third way in which air rises to form clouds is by riding a rising

convection current

Remember convection cells in the Mantle driving plate tectonics?

The same process occurs in the atmosphere!

CONVECTION: Energy Transfer due to differences in density of a fluid.

The rocks in the mantle can flow and so can the air.


By convection: The Sun heating the ground (by radiation), which then heats the air above (by conduction), which then rises due to convection (is less dense than the cooler air surrounding it.)

Picture from the National Audubon Society Field Guide to Weather

Air will rise when it is heated, because it will become less dense. Air moves

in to replace the air that rose up. The converging air will further

rise. As it rises, it cools and water

vapor will condense. This

will release latent heat, and further

heat the column of air.

What caused this band of clouds to form over the

equator?

In SummaryWhat is needed for clouds to form?

How is air forced to rise and cool (3 ways)?

What is an air mass?

What is frontal uplift?

What is the difference between the windward and the leeward sides of mountains?

How does convection create clouds?

cloud formation & fronts

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