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A discussion on precipitation...the effects of precipitation...air masses and the different types of storms they can create.

Julie Southwood

on 17 February 2015

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Transcript of Weather

Chapter 7.3
Any form of water that falls from clouds and reaches Earth's surface
Freezing Rain
Most common
Size: 0.5-5mm
Drizzle: 0.05-.5mm
Mist: 0.005-0.05mm
Occurs when
Air temperature is above 0ºC
Ground temperature is above 0ºC
Measuring Rain
Rain Gauge: An instrument used to measure the amount of precipitation over a set period of time.
Freezing Rain
Occurs when
air temperature is above 0ºC
ground temperature is below 0ºC
Raindrops fall as liquid
Freezes when they come in contact with a cold surface
Occurs when:
Air temperatures is below 0ºC
Raindrops fall through a layer of freezing air
Smaller than 5mm
Ice pellets
Forms when water vapor freezes into ice crystals
Air temperature is below 0ºC
Different size and shapes
Many have six sides or branches
Measuring Snowfall
Usually measured in two ways:
measuring stick
melting collected snow and measuring depth of the water it produces
10 cm of snow = 1 cm of rain
Round pellet of ice larger than 5mm
Layers of ice
Only forms in cumulonimbus clouds
Occurs when air temperature is below 0ºC
Strong updrafts carry ice pellets up into cold regions
Each time causes it to add a new layer of ice
Eventually gets heavy enough to fall to the ground
How Can Precipitation Effect Us?
What is a flood?
An overflowing of water in a normally dry area.
Cause & Effects
Spring Thaw

Heavy Rains
When ice and snow starts melting due to warm srping weather
Land is still frozen preventing water to seep into the ground
Spring storms also add more water to the melting snow and ice
When there is an excessive amount of rain fall
Can lead to flash flooding
A rapid flooding in low-lying areas
Happens in less than six hours
Rivers overflow
Trees are uprooted
Pluck boulders from the ground
Damage property
What is a drought?
Causes & Effects
A long period of time of dry weather with very little to no rainfall.
Dry weather systems that remain in one place
Can last for weeks or even months
Crop failure
Streams and ponds dry up
Types of Precipitation
Air Masses
Chapter 7.4
Chapter 7.5
A huge body of air in the lower atmosphere that has similar temperature, humidity, and air pressure at any given height.
Maritime Tropical
Continental Tropical
Maritime Polar
Continental Polar
Air masses that form over land
Air masses that form over the ocean
Warm air masses that come from the tropics.

Low air pressure
Cold air mass that forms 50ºN or 50ºS (latitude)

High pressure
When one air mass moves and interacts with another air mass, it causes the weather to change.
How Air Masses Move
Prevailing Westerlies
Jet Streams
Major wind belt over the continental United States
Pushes air masses from west to east
The boundary where the air masses meet
Air masses collide but do not mix
Storms and change in weather often develop along fronts
Bands of high-speed winds
About 10km above Earth's surface
Within the prevailing westerlies
Major Air Masses
What Are the Main Types of Fronts?
Cold Fronts
Warm Fronts
Occluded Fronts
Stationary Fronts
Cold Front Symbol
When faster cold air mass runs into a slower air mass
Cold air mass pushes the warm air mass up along the leading edge
Warm air rises, expands and cools and reaches the dew point
Heavy rain or snow may fall
Leading edge runs along the ground
Can cause weather changes such as storms
Brings in cold dry air, lower temperatures, clear skies
Warm Front Symbol
Fast-moving warm air mass runs into a slow moving cold air mass
Warm air moves over the cold air
Humid air = Light rain or Snow
Dry air = Scattered clouds
Can last for a few days
Brings warm, humid weather


Occluded = Warm air mass is cut off
Most complex weather situation
Warm air masses is caught between two cooler air masses
The two cold air mass meet and mix
Temperature near the ground is cooler
Clouds form and possible rain or snow
When a cold and warm air mass meets but neither one moves.
Where both masses meet there will be either rain, snow, fog or clouds.
Could last for a few days
When air masses meet the fronts can become distorted
Can be caused by:
Surface features (mountains)
Strong winds (the jet stream)
Low-pressure centers can form from the swirling air
When Two Air Masses Meet
A swirling center of low air pressure
Circled "L" on a weather map = Low Pressure
As warm air rises, pressure decreases
Cooler air from near by high pressure areas moves inward
Coriolis effect causes the air to move counterclockwise
Associated with clouds, wind and precipitation
Opposite of a cyclone
High-pressure centers of dry air.
Shown as an "H" on weather maps
Winds spiral outwards away from the center
Moves towards areas of lower pressure
Coriolis effect makes wind spin clockwise
Causes dry, clear weather
A violent disturbance in the atmosphere.
Winter Storms
A large amount of snow falls over a short period of time.
Blizzards : a snowstorm with prolonged winds around 56km/hr or more and a considerable of falling or blowing snow.
Visibility is reduced
Block roads
Cold temperatures and bust water pipes
A storm often accompanied by heavy precipitation and thunder and lightening.
Formed in cumulonimbus clouds
Build up of positive and negative charges
Heats the air to about 30,000 degrees Celsius
Sun's surface: 5778K
Air expands rapidly
Thunder is the sound of the explosion
Tropical cyclone with winds up to 19km/hr or higher
They can reach about 600km across
Hurricane formation:
Warm air over water causes a low-pressure system to form.
The warm, humid air begins to rise and form clouds
Air begins to spiral in towards the area of lower pressure from surrounding cool, high pressure areas
Rapidly spinning funnel-shape cloud
On land: tornadoes
Over water: waterspouts
Duration: brief (15min or less)
Power: Varies
Cumulonimbus clouds
Can from from in any situation involving sever weather where warm humid, air interacts with cold, dry air
When and where:
Spring to early summer
Late afternoon - ground is warm
Tornado Alley
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