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WEATHER
What is weather?
• Weather is the day to day conditions of a particular place.
• Ex. Sunny, rainy, hot, cold
• These conditions are caused by atmospheric pressure,
humidity, and temperature.
Atmospheric Pressure
• Pressure is the weight of the atmosphere on the Earths surface.
• Pressure is lower on the top of mountains and highest at sea level
• Drops in air pressure mean unstable conditions (storms)
• Atmospheric pressure is measures with barometers
Humidity
• Humidity is how much water vapor is in the atmosphere
• Air is always collecting evaporated water from rivers, lakes
and oceans.
• In tropical parts, warm air hold more water vapor and
humidity is higher
Relative Humidity
• Relative humidity is the measure of how much
water vapor is actually in the air compared to
how much water vapor the air can hold.
• This is measured using a wet and dry
hygrometer
Temperature
• Temperature is the measure of how cold or hot somewhere is
• Temperature is measured using a thermometer, with units in degrees on
Fahrenheit (ºF) or Celcius (ºC) scales.
• Temperature is usually higher during the day than at night. Rural areas (the
countryside) are often cooler than towns and cities.
• This is because there are more buildings and factories ( known as 'heat islands')
which absorb heat during the day, releasing it slowly at night and warming the
surrounding air.
Sunshine
• The amount of sunshine we have depends on latitude and how much cloud there
is in the sky.
• In the Eastern Sahara desert, the sun is covered by clouds for less than 100 hours
a year.
• In Britain we have from 1,850 hours in Southern England to 1,200 hours in North
Scotland.
• There is usually more sunshine where atmospheric pressure is higher.
Sunshine
• Hours of sunshine are usually recorded on a simple machine called a
pyrheliometer also known as a Campbell-Stokes recorder.
• It works by using a glass ball to focus the sunlight and rays onto a strip of card.
• As the sun moves round during the day, the card is scorched creating a record of
how many sunshine hours there were.
Pyrheliometer
WIND
• On the surface of the Earth, wind consists of the bulk
movement of air.
• Wind is caused by differences in atmospheric pressure.
• When a difference in atmospheric pressure exists, air moves
from the higher to the lower pressure area, resulting in winds
of various speeds.
• Extreme wind can cause cyclones and tornadoes
TORNADOES
• A tornado is a violently rotating column of air that is in contact with
both the surface of the earth and a cloud
• Most tornadoes have wind speeds less than 110 miles per hour
(177 km/h), are about 250 feet (76 m) across, and travel a few miles
(several kilometers) before dissipating.
• The most extreme tornadoes can attain wind speeds of more than
300 miles per hour (483 km/h), stretch more than two miles (3.2 km)
across, and stay on the ground for dozens of miles (more than
100 km)
TORNADOES
• Tornadoes have been observed on every continent except
Antarctica. However, the vast majority of tornadoes occur in
the Tornado Alley region of the U.S.
• They also occasionally occur in south-central and eastern
Asia, northern and east-central South America, Southern
Africa, northwestern and southeast Europe, western and
southeastern Australia, and New Zealand
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