What is weather

1. What is weather?

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Cloud Classification
•Many different types, all classified by
shape and height
–3 main types:
•Stratus
•Cumulus
•Cirrus

9. 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

10. 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

11. Cirrus
•Appears as curly tendrils
and fibers
–High, thin white feathery
clouds
•Contains ice crystals
•Associated with fair
weather, may indicate
approaching storm

12. 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

13. 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

14. 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

15. Section Wrap Up
1)When does water vapor in air condense?
3)How do clouds form?
5)How can the same cumulonimbus cloud
produce both rain and hail?