Thunderstorms

• Small area storms formed by the strong upward movement of warm, unstable air
• Always accompanied by lightning and thunder
• Almost always produce rain
• Can also produce high winds, hail, or tornadoes
• Most are 10-20 km (6-13 miles) around
• Divided into two groups:

1. Air-mass thunderstorms (local)
2. Frontal thunderstorms (organized)

Air-mass Thunderstorms

• Form in a warm, moist air mass
• Occur mostly in spring and summer
• Last less than an hour
• Form mostly over land; are widely scattered
• Not an easy storm to predict

Frontal Thunderstorms

• Form in a warm, moist air mass in front of cold fronts
• Occur in lines along the frontal surface
• Also occur in lines ahead of the front called Squall Lines
• Lines of thunderstorms can be 100's km long
• Can produce heavy rains and regional flooding
• Occur most often in spring and summer
• May last for several hours
• Easier to predict because of their large size and movement of the fronts that carry them

Lightning

• All thunderstorms produce lightning
• Discharge of electricity from:

1. Cloud to cloud
2. Ground to a cloud
3. Cloud to the ground

• Lightning is believed to be over 28,000 deg C
• This causes the air to expand in an explosion known as Thunder
• Lightning is seen almost instantly because light travels at 300,000 km per second!
• Thunder however, travels about 1 mile/second
• Since lightning strokes are usually long, the sound produced is more of a rumble than a crack
• A "crack" tells you the lightning is close by
• Thunder usually never travels more than 16 km (10 miles) away…if you see lightning but don't hear thunder, you are too far away from the lightning stroke

Danger of Lightning

• 200 people per year get struck by lightning
• 1000's of fires are caused by lightning
• Lightning rods are used to conduct the electricity from a high point to the ground to help prevent fires in buildings, churches, etc.
• Take shelter in a building…the lightning can hit it, but not hurt you
• Don't be near any electrical appliances!
• Cars are very safe because you can be insulated from the stroke if you are in a car
• Never hide under a tree or any tall objects!
• If in the open, crouch on the ground or lie on the ground with your butt in the air!

Tornadoes

• A narrow, funnel-shaped column of spiral winds that extend downward from the cloud base and touch the ground
• Much more frequent in the US than anywhere?
• Most occur in the Mississippi River valley and the Great Plains
• Strong thunderstorms are often associated with the formation of tornadoes
• When a cold air mass and a warm air mass collide, this may produce a tornado
• Tornadoes can have winds between 360-500 km/hour!
• The funnel is usually less than 500 m across
• Travel across the ground at about 40-65 km/hour
• Never on the ground for more than 25 km
• Often exist for no more than an hour and are associated with heavy rain, lightning, and hail
• Tornadoes over water are called Waterspouts; they are usually weaker than tornadoes and diminish when they reach the shoreline

• Safety tips:

1. Plan safety rules…know where you need to go and what things to bring with you!
2. Stay away from outside walls…basements are the best place to be!
3. Get out of mobile homes and vehicles
4. If outside, jump into a ditch or ravine!

Warnings and Watches

• Local weather stations issue severe weather warnings and watches
• A WATCH is the time in which severe weather is likely!
• A WARNING is the time in which severe weather has been spotted and likely coming to you!
• Severe storms are statistically most likely to occur in the late afternoon
• 54 people in the US each year die from tornadoes; one in 1925 killed 689!
• Because of improved warnings and technology, few people are injured by severe storms or tornadoes
• Note: All storm warnings are issued by the National Weather Service

Cyclonic Storms

• Hurricane - an intense tropical low-pressure area with winds of 120 km/hr or more!
• Produce major damage to coastal towns
• Happen only when water temperatures reach above 78 deg. F
• Hurricanes swirl in a circular pattern and also produce heavy rains
• The swirl creates a void in the center of the hurricane called the Eye
• Winds increase toward the center of the hurricane and are most violent just outside the eye - may be greater than 240km/hr!
• Area of destructive winds may be more than 800 km across!
• Rainfall increases toward the center of the hurricane - heaviest just outside the eye
• Hurricanes, since they form far out into the ocean, are very predictable and therefore allow people a chance to evacuate and prepare
• In some 3^rd world countries, deaths are more common since evacuation is not easy
• 1991 - Bangladesh - 139,000 died
• 1992 - Florida - 38 died
• Hurricanes weaken when they pass over land
• Most hurricanes are found to only travel westward
• Most common in the late summer (northern hemisphere) because that brings about the warmest seas
• 1953 - we gave female names to hurricanes
• 1979 - we now give male and female names
• Names are made up on a list that lasts for 6 years, after that, the list is repeated
• If hurricanes are severe or deadly, that name used is retired and never used again
• National Hurricane Center in Miami issues all hurricane warnings and watches for the US

Winter Storms

• Strong, low-pressure systems bring the winter's major snowstorms
• You must have two things: 1-must have enough moisture, 2-must be cold enough
• A Blizzard forms when a snowstorm forms with high winds and low temperatures
• The National Weather Service issues all winter storm warnings as seen on TV
• Heavy snow, mixed rain, freezing rain, and rain are all the same but controlled simply by the temperature of the air at the time of falling
• Because of this, true predictions of amounts and contents are very difficult!

Forecasting and Weather Mapping

• Weather forecasts are based on computer models of the current atmosphere
• Weather Data System (computer data) is sent around the world by weather stations at noon and midnight (Note: 7am and 7pm EST)
• Satellites and radar also gather local weather all the time
• Weather Service Forecast offices issue their weather reports to TV stations, etc. four times each day (10am, 4pm, 9pm, 4am) to allow time for the stations to compile a weather forecast for YOU.

Radar and Satellites

• Because the Weather Data System is located sporatically around the world, it may miss many smaller frontal activity - like rain showers, snow fall, or small thunderstorms
• We have developed satellite imagery to allow us to cover all the surface of earth
• Satellites provide continuous pictures of the cloud cover
• Radar is also used to see the current images of the area
• Radar is sent out from a transmitter and it bounces off the precipitation, etc. in the air

Making a Weather Map

• Data for weather maps are taken from stations all over the world
• In the US, over 700 stations contribute data to the maps
• Official US weather maps are drawn up every three hours at the National Meteorological Center in Camp Springs, Maryland
• Up to 20 different weather observations (data) may be plotted at each of the weather stations on a map - this is called a Station Model
• Station models and data codes are able to be read by any meteorologist around the world
• See the sample station model and code on page 568 - it is very unique!