MOISTURE , CLOUD AND PRECIPITATION

MOISTURE

refers to the presence of a liquid, especially water, often in trace amounts

Moisture also refers to the amount of water vapor present in the air.

WATER VAPOR Odorless, colorless gas, changes from one state of

matter to another at the temperatures and pressures experienced near earth’s surface

Always present in the air around us  is the source of all condensation and precipitation,

which is any form of water that falls from a cloud. When understanding atmospheric processes , water

vapor is the most important gas in the atmosphere.

LATENT HEAT

o Stored or hidden heat 

o Important in atmospheric processes

General Properties of Water Vaporization

Evaporation = Liquid is changed to gas 600 calories per gram of water are added - called latent heat of vaporizationCondensation = Water vapor (gas) is changed to a liquid Heat energy is released - called latent heat of condensation 600 calories releasedMelting = Solid is changed to a liquid 80 calories per gram of water are added - called latent heat of melting

Freezing = Liquid is changed to a solid Heat is released (80 calories)- called latent heat of fusion

Sublimation = Solid is changed directly to a gas (e.g., ice cubes shrinking in a freezer) 680 calories per gram of water are added

Deposition = Water vapor (gas) changed to a solid (e.g., frost in a freezer compartment) Heat is released 680 calories

HUMIDITY

• Amount of water vapor in the air 

• Saturated air is air that is filled with water vapor to capacity 

Relative Humidity• Ratio of the air's actual water vapor content compared with the

amount of water vapor required for saturation at that temperature and pressure

• is when you compare how much water vapor is in the air to how much it could hold

• Relative humidity can be changed in two ways  Changing the air temperature  Add or subtract moisture to the air

TRANSPIRATIONS

• watery vapor passed through a membrane or pores in a plant

Dew Point Temperature

Temperature to which a parcel of air would need to be cooled to reach saturation

Cooling the air below the dew point causes condensation 

e.g., dew, fog, or cloud formation  Water vapor requires a surface to

condense on

Measuring Humidity 

A hygrometer is an instrument to measure relative humidity

A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity.

CLOUDS

A cloud is a large collection of very tiny droplets of water or ice crystals.

How are clouds formed?

• All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor.

• When warm air rises, it expands and cools.• Cool air can't hold as much water vapor as warm air, so

some of the vapor condenses ( onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle.

• When billions of these droplets come together they become a visible cloud. 

THREE WAYS CLOUD FORM

• CONVECTION CURRENT

 In cold air the molecules are closer together and the air is therefore, more dense and heavier than warm air. Because it is heavier, gravity pulls harder on cold air which then forces the lighter warm air up and out of its way. As a result, the warmer air is pushed up (lifted) into cooler zones above the surface.

As the warm air goes up it gets cooled. Cool air cannot hold as much water vapor as warm air. As the warm moist air is cooled, the vapor changes to tiny droplets of liquid, the stuff of clouds.

• MOUNTAIN LIFTING When warm moist air meets a mountain, it is pushed upward into

cooler temperatures by the slope of the mountain itself. As this warm, moist air moves up the mountain, it is cooled and

cannot hold as much water vapor.  Once again, the vapor changes into tiny droplets; exactly what is

needed to make a cloud. As wind or air mass movement pushes this warm moist air toward

the mountain, the slope of the mountain forces it up into higher cooler air . In this cooler environment, the air just can’t hold as much water vapor, and clouds are formed as the vapor changes to liquid droplets.

• AIR MASS MOVEMENT Clouds formed because of air mass lifting are pretty much like

clouds formed by mountains, except that the lifting is forced by another mass of air instead of the mountain. Air mass lifting can occur with both warm and cold front movement but it is always the warm air that gets lifted.

When a cool air mass moves into a mass of warm moist air, the mass of warm air is quickly forced up and thunder clouds develop. Thunderstorms often occur.

When a warm air mass moves into a mass of cold air, it slides over top of the cold air, is cooled and stratus clouds form. A gentler longer rain often follows.

Processes that lift air

Orographic liftingFrontal wedgingConvergence -Localized convective lifting

Orographic Lifting

Elevated terrains act as barriers

Frontal wedging

Cool air acts as a barrier to warm air  Fronts are part of the storm systems called

middle-latitude cyclones

Convergence 

where the air is flowing together and rising (low pressure)

Localized convective lifting

Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy

WHY ARE CLOUDS WHITE?

Clouds are white because they reflect the light of the sun. Light is made up of colors of the rainbow and when you add them all together you get white.

The sun appears a yellow color because it sends out more yellow light than any other color. Clouds reflect all the colors the exact same amount so they look white.

WHY DO CLOUDS TURN GRAY?

Clouds are made up of tiny water droplets or ice crystals, usually a mixture of both. 

The water and ice scatter all light, making clouds appear white. If the clouds get thick enough or high enough all the light above does not make it through, hence the gray or dark look. 

Also, if there are lots of other clouds around, their shadow can add to the gray or multicolored gray appearance.

WHY DO CLOUDS FLOAT

A cloud is made up of liquid water droplets. A cloud forms when air is heated by the sun.

As it rises, it slowly cools it reaches the saturation point and water condenses, forming a cloud.

As long as the cloud and the air that its made of is warmer than the outside air around it, it floats!

Classifying Different Types of Clouds

• Classified by height above ground and shape

High clouds-Bases above 20,000 feet Middle Clouds-occupy heights from 6,500-20,000 feet Low Clouds-form below 6,500 feet

High Clouds

• Bases above 20,000 feet

• Prefix cirro is used to nameCirrus Cirrostratus Cirrocumulus

Cirrus Clouds

• Thin and Wispy

• Feathery Appearance

• Have a net heating effect

• Could be sign of approaching frontal system

• Maybe remnants of a thunderstorm

Cirrostratus Clouds

Thin, Ice crystal cloudsOften form halos around moon or sunCan be signs of a warm frontCan signal precipitation

Cirrocumulus Clouds

• Composed of ice crystals

• Usually in ripples or waves as in the picture

• Very uncommon 

• Usually turn into cirrostratus clouds

Middle Clouds

Occupy heights between 6,500 feet and 20,000 feet

Prefix alto is used in nameAltocumulusAltostratus

Altocumulus Clouds

Can be anywhere form white to grayUsually appear as layersOften precede a cold frontCan start the development of a

thunderstorm

Altostratus Clouds

o Made of Ice Crystalso Can produce very light precipitationo Sun often shines through themo No halos are ever produced

Low Clouds

Form below 6,500 feetStratusStratocumulusNimbostratus

Stratus Clouds

Flat and hazy Can be form dark gray to nearly white May produce light drizzle or snow Can cause fog

Stratocumulus Clouds

Soft, Gray cloudsUsually occur in patchesMost of the time no precipitationUsually give fair weather without much

happening

Nimbostratus Clouds

Low cloud that produces rain Usually have thickness of 2000 meters Smooth, gray appearance Commonly found in the midlatitudes

Clouds of Vertical Development

Extend vertically through more than one height range

Cumuluscumulonimbus

Cumulus Clouds

Globular individual cloud massesCauliflower-like structureDenseCan extend up to 39,000 ft. Usually white

Cumulonimbus Clouds

Very tall and denseOften involved in thunderstormsHeavy rainfall TornadoesResult of Atmospheric instability

OTHER CLOUDS

A few clouds can be found above the troposphere;

noctilucent polar stratospheric clouds. Some clouds form as a consequence of

interactions with specific geographical features.

Morning Glory.

Noctilucent

Night clouds or noctilucent clouds are tenuous cloud-like phenomena that are the "ragged edge" of a much brighter and pervasive polar cloud layer called polar mesospheric cloud  in the upper atmosphere, visible in a deep twilight

They are made of crystals of water ice Noctilucent roughly means night shining in Latin.

Polar stratospheric cloud

Polar stratospheric clouds or PSCs, also known as nacreous clouds, are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters

Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures

Morning Glory cloud

is a rare meteorological phenomenon occasionally observed in different locations around the world. 

Fog • is really a low-lying cloud that is near or touching

the surface of the Earth.• Fog formation requires the presence of moisture,

a gentle breeze, and a combination of warm and cold temperatures.

• Fog is hazardous to aviation because it limits both ceiling (height above ground level of lowest layer of clouds that cover more than half the sky) and visibility (distance an object can be Seen with the unaided eye).

Fog at sea

is a continual hazard to safe navigation. frequently formed through a process known as

advection. Fog is likely to develop when warm air that has passed over warm water moves to an area of colder water.

Advection fog 

is the name given to air-mass fog produced by air in motion, or to fog formed in one place and transported by wind to another. It will usually dissipate when the Sun rises.

warm, moist air moves over a cool surface

Radiation fog

Earth's surface cools rapidlyForms during cool, clear, calm nights

The heat that the Earth radiates causes radiation fog. It forms only at night, over a land surface. The Sun usually burns the fog away.

Fog produced by the nocturnal cooling of the surface boundary layer to a temperature at which its content of water vapor condenses

Upslope fog

Humid air moves up a slopeAdiabatic cooling occurs

Steam fog

Cool air moves over warm water and moisture is added to the airWater has a steaming appearance

Steam fog is a type of advection fog formed by air saturation.

Saturation = A condition in the atmosphere corresponding to 100 percent relative humidity 

Frontal fog, or precipitation fog

Forms during frontal wedging when warm air lifted over colder air Rain evaporates to form fog.

A fog caused by the movement of two dissimilar air masses A fog caused by the movement of two dissimilar air masses 

Precipitation

is any form of water that falls from a cloud. When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

The Water Cycle

The water cycle begins with evaporation, then water condenses, precipitation occurs, and finally water either becomes runoff or ground water.

If all the water in the atmosphere were to fall as precipitation it would only be 2mm deep.

How Precipitation Forms

For precipitation to form, cloud droplets must grow in volume by roughly one million times. 

Bergeron process

The Bergeron process is a theory that relates the formation of precipitation to supe rcooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

Temperature in the cloud is below freezing 

Ice crystals collect water vapor 

Collision-coalescence process

Warm Cloud Precipitation The collision-coalescence process is a

theory of raindrop formation in warm clouds (above 0 o C) in which large cloud droplets collide and join together with smaller droplets to form a raindrop.

Large droplets form 

Forms of Precipitation

Rain  = is the most common type of precipitation in our atmosphere. Rain takes place when drops of liquid water fall all the way to the surface of the Earth.

droplets have at least a 0.5 mm diameter Rain often takes one of two main forms.  Showers  drizzles Showers lasts just a brief period of time, and usually are made up

of large heavy drops. Drizzles generally last much longer, and are made up of smaller

finer droplets of water.

Snow

the second most common precipitation in the North East.

Snow forms when water vapor turns directly into ice without ever passing through a liquid state. This happens as water condenses around an ice crystal.

Hail

Hail is ice crystals that begin to fall towards the Earth’s surface.

Then the strong winds pick up the ice crystals and push them back up into the clouds.

When they start to fall again they grow in size.This process may be repeated several times or

until the hail becomes too heavy for the wind to carry.

Sleet

Sleet refers to a mixture of snow and rain, as well as raindrops that freeze on their way down.

Mist

Mist is a bunch of small droplets of water which are in the air. This occurs with cold air when it is above a warm surface,for example water.

Fog and mist are very similar, the only difference is their visibility.

Glaze

Glaze is the ice coating, generally clear and smooth, formed on exposed surfaces by the freezing of super cooled water deposited by rain or drizzle.

Rime

Rime is the white opaque deposit of ice granules more or less separated by trapped air and formed by rapid freezing of super cooled water drops impinging on exposed objects.

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