Scroll or click the on the section links on the right for resources on severe weather.
severe weather
Severe Weather Notes
Thunderstorms
Thunderstorms are created the same way all other precipitation forms. Warm humid air is forced up into the atmosphere. The more rapid the rise, the more severe the storm will be.
Air Mass Thunderstorms
Air mass thunderstorms are short lived and are associated with rising warm air caused be local surface heating (similar to a sea breeze). As water vapor condenses into liquid it releases energy into the atmosphere intensifying the updraft.
These storms die quickly as the downdraft created by the precipitation kills the updraft that creates the storm.
Severe Thunderstorms
In the case of severe thunderstorms, the rapid rising of the air along fronts creates an updraft. A strong downdraft is created by the pressure of the falling precipitation.
​
Because the air is being forced up by a moving front, the downdraft does not kill the updraft as the storm moves along with the front.
Lightning
The updraft and downdraft moving past each other create a static electric charge to build up. The discharge is lightning. Lightning will normally occur from cloud to cloud, but does strike the ground often. Lightning carries thousands of volts of with it and superheats the air as it passes through the atmosphere up to 50,000 degrees Fahrenheit. This causes the bright flash we see, but as the air is heated it also expands rapidly and creates a shockwave through the air that we hear as thunder.
​
​
Lightning is believed to be caused by wind shear. As updrafts and downdrafts flow past each other at high speeds it creates a static charge that builds up until it is discharged.
​
When a sufficient charge has built up, a leader will begin to travel through the air searching for a connection to discharge along. When a leader makes contact, the ther leaders will be abandoned and the static charge will transfer. All of this happens within a fraction of a second.
Super slow motion cameras catch leaders traveling from cloud to ground and then discharging.
Thunderstorm Safety
If caught in a thunderstorm outside, avoid being in open area where you are the tallest object or standing next to solitary trees. Get to the lowest point you can.
 |  |  |  |  |  |
|---|
Lightning
Thunderstorm Diagrams and Images
​
 |  |  |  |  |  |  |
|---|---|---|---|---|---|---|
 |
Tornadoes
Tornadoes are formed in supercell thunderstorms, which are often the result of mid-latitude cyclones.
This is when a warm and moist air mass collides (mT) a cold and dry air mass (cP).
Wind traveling at different speeds or different directions at different altitudes create a tumbling tunnel of air between them (a). Updrafts from forming storms can lift these tumbling tunnels of air (b). This can give the storm rotation (c) and creates what is classified as a supercell.
Tornadoes are common in the midwest and southwest of the United States during the spring and late spring.
​
Tornadoes have extremely high wind speeds but only last for a very short period of time, usually around 15 minutes but up to 1 hour. Tornados normally move NE with the storm.
Tornado Damage
Tornadoes are rated on the Fujita Scale or Enhanced Fujita Scale:
Hurricane
Hurricanes are giant spinning collections of thunderstorms that have merged together. They are the most powerful and destructive storms on Earth.
Hurricanes develop through several stages. Hurricanes live for as long as 3 weeks. They begin as a cluster of thunderstorms near the equator called a tropical disturbance. Once this disturbance begins to rotate as a result of the Coriolis Effect and the surface winds are fast enough it becomes a tropical depression. If the depression continues to grow it will form a tropical storm and eventually a hurricane. To classify a tropical storm as a hurricane it must reach a sustained wind speed of 75 mph. In the northern hemisphere, hurricanes rotate counterclockwise.
To classify a tropical storm as a hurricane it must reach a sustained wind speed of 75 mph. In the northern hemisphere, hurricanes rotate counterclockwise.
To live, hurricanes need 2 main things as fuel:
-
Heat
-
Humidity
Hurricane Path
​
Hurricanes will follow the prevailing winds and ocean currents. They form at low latitudes but not directly on the equator. The will move west towards the east coast of a continent. In the Atlantic they will eventually turn north and then north east.
Hurricanes die when they lose their fuel source, warm ocean water. Either the storm makes landfall, or turns into colder ocean water.
Hurricanes and Positive Feedback
​
Positive feedback describes a cycle where a process continues to strengthen itself under the right conditions. Warm and Humid air over the ocean create the local Low Pressure that forms the first thunderstorm.
In hurricanes, as the wind moves rapidly towards the eye it speeds up evaporation of the ocean, adding more humidity. When the humid air rises it condenses into clouds and rain. Condensation is a warming process which intensifies the low pressure. This in turn speeds up the wind as the pressure gradient steepens. Faster winds evaporates more water creating a positive feedback loop of infinite growth.
Click Me
Saffir Simpson Scale
Hurricanes are rated based on windspeed:
HURRICANE DAMAGE
Hurricanes will have a diameter of 400 miles and have 3 primary dangers:
​
-
Inland flooding from heavy rain
-
Strong winds
-
Storm Surge
Storm Surge is the most dangerous. The most life and property destruction is the result of storm surge.
Hurricane Damage
Video provided by The Weather Channel simulating the damage and rating system for hurricanes.
Click Me
Storm Surge Damage
Wind Damage
Eye Wall
The eye wall is a thick band of clouds moving with the fastest wind speeds around a calm center.
The most dangerous wind speeds in the northern hemisphere will typically be found on the east or north east side of the eye.
This is because the speed of the storm must be added to the wind speed. (see diagram) The stronger the wind the greater the storm surge will be.
Hurricane Safety
​
Hurricanes are the most powerful and destructive storms on earth. The best course of action is to evacuate days before the storm arrives. Hurricanes are long lived and can be forecast days in advance.
​
If evacuation is not an option, make sure to have 3 weeks of water and food safely stored. After a major hurricane infrastructure may be seriously damaged or completely broken down. There will be no water, no food and no power for days or even weeks.
Blizzard
Blizzards are simply severe storms occurring in the winter. A storm must meet 3 requirements to be considered a blizzard:
​
-
56 kilometer per hour winds
-
Visibility less than 400 meters due to snow
-
Lasting for more than 3 hours
It is important to remember that it DOES NOT need to be snowing for there to be blizzard conditions. If snow is on the ground and is picked up by strong winds and blown around, it will still meet the visibility requirement.
​
The dangers from a blizzard include: Low temperatures, wind damage and reduced visibility. Preparation for blizzards include: Stocking up on food and water and having alternative fuel sources for heat in case of power outages.