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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). CONDENSATION. A surface for water vapor to condense on (condensation nuclei). How does water vapor get into the air?. T. E. E. T.

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slide2

Ingredients Required for Clouds:

Water vapor (water as a gas)

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

CONDENSATION

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

slide4

T

E

E

T

E

T

By evaporation and transpiration

slide5
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.

Moisture Capacity

Temperature

how do you get the water vapor out of the air
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.
how do you get the water vapor out of the air1
How do you get the water vapor out of the air?
  • 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
slide9

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

The Ocean

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

Pollution

(First three pictures are not by the author)

slide10

The best condensation nuclei are hygroscopic, or water absorbent

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

Water vapor molecules

Condensation Nucleus

slide11

The best condensation nuclei are hygroscopic, or water absorbent

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

slide12

The best condensation nuclei are hygroscopic, or water absorbent

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

slide13

The best condensation nuclei are hygroscopic, or water absorbent

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

slide14

The best condensation nuclei are hygroscopic, or water absorbent

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

slide15

The best condensation nuclei are hygroscopic, or water absorbent

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

slide16

The best condensation nuclei are hygroscopic, or water absorbent

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

Liquid water (drops coalesced together)

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

slide17

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

slide18

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!

slide19

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!

slide20

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!

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

slide21

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!

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

slide22

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!)

slide24
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
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
slide26

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

slide27

WINDWARD SIDE

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

LEEWARD SIDE

DRY 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
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.
slide30

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 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 masses1
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 masses2

Open your ESRT to page 13!

Air Masses

Air Mass ClassificationSource 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);
slide34
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
What happens when air masses meet?
  • The boundary between two air masses is called a FRONT.
fronts boundary that separates 2 different air masses
Fronts – boundary that separates 2 different air masses
  • Either a warm front is advancing or a cold front is advancing.
kinds of fronts
Kinds of Fronts
  • Cold Fronts
  • Warm Fronts
  • Stationary Fronts
  • Occluded Fronts
cold front
Cold Front
  • A cold front occurs when a more dense air mass pushes under a less dense air mass – short–lived thunderstorms
warm front
Warm Front
  • A warm front occurs when a less dense air mass rides up over a more dense air mass – longer, steady precipitation
stationary front
Stationary Front
  • A stationary front occurs when the air masses on either side of the front are not moving toward each other.
slide45

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

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

slide46

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.

slide47

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

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

the same process occurs in the atmosphere
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.

slide51

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

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

slide52

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.

in summary
In Summary

What is needed for clouds to form?

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

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

What is an air mass?

What is frontal uplift?

How does convection create clouds?