What is weather?

What is Weather?

• Weather - referring to the present state of the atmosphere and describing current conditions– Important factors of weather :

• Air pressure• Wind• Temperature • Amount of moisture in the air

(humidity)

– Factors of weather determine state of atmosphere

Factors Of Weather

• Water cycle forms the basis of our weather

• Sun is also very important– Evaporates water– Heats air

• This causes wind motion by the interaction of air, water and sun

Humidity

• Humidity - amount of water vapor in air

• The air is like a sponge– Water vapor molecules fit in spaces

between air molecules

• Humidity changes daily because the change in air temperature– Temperature determines the amount of

water vapor that the air can hold• Cooler temperatures: air moves slower

– Allows water molecules to join and condense

• Warmer temperatures: air moves faster– No time for water to condense, stays a vapor

» Hotter, damper, and holds more water vapor

Relative Humidity• Relative Humidity - measure of the amount of water

vapor air is holding in comparison with the amount it can hold at a certain temperature

• Saturated – air containing all the moisture possible for its temperature– Ex. If weather reporter says the relative humidity is 70%, then it

needs 30% more to be saturated

Dew Point• Dew point – temperature at which air is

saturated and condensation begins– The dew point will change with the amount of

moisture in the air• Ex. A cold glass of milk will cool surrounding air to its

dew point causing water droplets from surrounding air to form onto cup

Clouds

• Formed when warm air travels up, expands and cools– When air cools, the amount of water vapor needed for

saturation goes down and relative humidity increases– When air is saturated (100% relative humidity) then

water vapor condenses around nuclei• Small particles of dust, salt and smoke in the atmosphere• Water molecules are so small they remain suspended within

the air– Millions of these water molecules form a cloud

Cloud Classification

• Many different types, all classified by shape and height– 3 main types:

• Stratus• Cumulus• Cirrus

Stratus

• Forms layers and appears as smooth sheets in the sky

• Created when layers cool below dew point– In low altitudes

• Associated with fair weather and precipitation• Sometimes seems like a dull gray blanket

covering the sky– Brings drizzle

• When cooled near ground it is called fog

Cumulus

• Appears as puffy white clouds– Often flat bases– Referred to as

“cauliflower” clouds– May be very high in length

• Formed when air current rises

• Associated with fair weather and thunderstorms

Cirrus

• Appears as curly tendrils and fibers– High, thin white feathery

clouds

• Contains ice crystals• Associated with fair

weather, may indicate approaching storm

Cloud Height

• Prefixes are used to describe the height of a clouds base– Cirro: high elevation,

above 6000m– Alto: middle elevation,

between 2000-6000m– Strato: low elevation,

below 2000m

Rain Capacity

• Nimbus~ dark clouds– So full of water, no

sunlight penetrates it– Nimbus that is also a

cumulus cloud is called a cumulonimbus

• These grow huge• Can be up to 18000m• May produce violent

thunderstorms• When water droplets

become .2mm they fall– too heavy

Precipitation• Precipitation – water falling from clouds• Temperature determines whether water

falls as :– Rain; drops of water falling above freezing– Snow; drops below freezing, turn directly to

solid– Sleet; when snow passes through a warm

layer and melts, then refreezes near the ground

– Hail; form of ice, they grow larger when they fall up and down (because of convection current)

• Most smaller than 25cm• Produced the most damage

Section Wrap Up

1) When does water vapor in air condense?

2) How do clouds form?

3) How can the same cumulonimbus cloud produce both rain and hail?