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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