Low Pressure System
• Warm air has low density and it rises (forced
upwards by surrounding denser air)
• As the air rises it cools
• As the air cools the relative humidity increases –
eventually reaching the dew point
– At the dew point condensation takes place and clouds form
• Low pressure systems usually form along fronts
where warm air and cold air meet
• Low pressure systems cause most of the weather in
• A low pressure system leads to precipitation in the
form of rain, snow, hail, sleet.
• When we witness a change in the
weather from day to day it is due to the
movement of air masses. It is the
movement and collision of air masses
that causes the weather to change.
What is Severe Weather?
• Severe weather are events that fall outside
of the ordinary weather patterns.
• Severe weather can include the following:
• Tornadoes are violent funnel-shaped storms with whirling
winds that move in narrow paths over land.
• Tornadoes form from severe thunderstorms
• Tornado producing thunderstorms involve the rapid upward
movement of warm moist air
• The upward moving air begins to rotate (why the rotation starts
is still not understood)
• As the speed of the rotation increases more warm moist air is
drawn into the low pressure at the center
• A funnel shaped cloud extends from the bottom of the cloud
sometimes touching ground
– As it touches ground it picks up dirt and debris that give it
the characteristic dark gray, black color.
• Tornadoes are one of the most destructive types of storms.
Strength of Tornadoes:Fujita Scale
Light Damage: broken branches,
72 – 112
Moderate Damage: mobile homes
113 – 157
Considerable Damage: roofs off,
trees snapped, uprooted.
158 – 206
Severe Damage: trains overturned,
207 – 260
Devastating Damage: houses
leveled, large missles.
Incredible Damage: houses lifted,
trees debarked, auto missles.