Clouds and Precipitation

Earth Science Ms. Kurtzweil

Unit Objectives

 6. Describe what happens when the temperature of air drops to or below the dew point.

 7. Identify the conditions that must exist for a cloud to form.  8. Describe the various types of liquid and solid precipitation.

Basics

 Evaporation is the changing of water from a liquid to a gas   Liquid water on Earth entering the atmosphere as water vapor (when energy is added aka sunlight) Condensation is the changing of water from gas to a liquid  Water vapor in the atmosphere condensing into liquid water droplets in a cloud (when energy is released = cooled)

* When water reaches the dew point, or saturation point, it will begin to condense into water droplets that form a cloud. Four ways that air may cool enough below the dew point to begin condensation: 1. Contacting a colder surface (blade of grass, cold glass) 2. Radiating heat 3. Mixing with colder air 4. Expanding when it rises “Expanding when it rises” is KEY to cloud formation!

Frost

 If the temperature is below the freezing, the dew point is called the frost point.

 If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals

Before a true cloud can form, water vapor still needs something to condense upon!

Condensation nuclei:

condenses. tiny particles on which water vapor -The foundations of water droplets in a cloud Examples of possible condensation nuclei in our atmosphere: 1. Salt 2. Dust 3. Ice 4. Smoke 5. Sulfate/nitrate particles Cloud Seeding during droughts

How Do Clouds Form?

1. Water leaves the Earth’s surface via one of 2 ways: 2. Warm, moist air is less dense; it rises high up into the atmosphere 3. At high altitudes, lower air pressure allows the air mass to expand and cool… eventually to the dew point 4. Water condenses on condensation nuclei in the atmosphere, and…

Voila!!!

A cloud is born!

I’ve heard that fog is just a cloud near to the ground. Is this true?

 For the most part, yes. There are different types of fogs,

radiation

or

advection

, but overall, fog is formed nearer to the ground than a cloud. This occurs when moist air close to the ground cools to its dew point.  But clouds are thicker than fog, you say? As you fly through a cloud in a jet, you will see that clouds only appear as thick as cotton balls from far away!

Radiation Fog

     Forms under similar conditions as dew On calm, clear nights, the ground will quickly lose heat via radiation Cold air sinks and cools A whole layer of air is cooled below the dew point and fog forms Common in humid valleys, near rivers or lakes

OK, back to clouds…

Cloud Types

 The shapes of clouds show how air is moving through them.  Rising? Horizontally? Stable?

 3 main cloud types, based on their location in the sky    Cirrus “curly” • Highest family of clouds in the sky Stratus “sheetlike” or “layered” • Mid-level clouds Cumulus “piled” or “heaped” • Lowest clouds, closest to Earth’s surface

Cloud Type Description Cirrus What kind of weather?

Thin, feathery, ice crystals Just before snowfall or rainfall Picture Stratus Low sheets or layers Little precipitation to heavy rains or snowfall Cumulus Thick, puffy masses, “cotton balls” Fair weather, maybe leading to abrupt storms

 Video: Storm Clouds

Precipitation

 The falling of any form of water from the air to Earth’s surface  When will water leave a cloud and fall to the ground as precipitation?

 Basically, when gravity tells it to. Like thick beads of condensation on a glass, when cloud droplets grow into drops heavy enough to fall to Earth, precipitation occurs.

4 major types of precipitation

  Rain  Large droplets (up to 0.25 cm in diameter) that are far apart and fall rapidly Snow   Clumps of 6-sided crystals that grow by collision The most common type of solid ppt

4 major types of precipitation

 Sleet   Ice pellets that form when rain falls through a layer of freezing air What about “black ice” or “glare ice”?

Sometimes in an ice storm, supercooled rain drops will freeze instantly as they come in contact with a cold surface such as roads, roofs, and power lines

4 major types of precipitation

 Hail    Solid ppt in the form of lumps of ice Begins as raindrops falling from a cumulonimbus cloud Convection currents toss the droplet high up into the cloud where it freezes. It then falls to a lower level and water condenses on it as a liquid. The tossing and freezing process repeats itself and the hailstone keeps adding layers and growing larger.

How Does Hail Form

 http://www.ehow.com/video_4872174_what causes-hail.html

Experience a Hailstorm

 http://www.komonews.com/weather/blog/3 1359739.html