1. Nature of Storms
Chapter 13

2. Thunderstorms For a thunderstorm to form, three conditions must exist.
1. There must be an abundant source of moisture in the lower levels of the atmosphere.
2. Some mechanism must lift the air so that the moisture can condense and release latent heat.
3. The portion of the atmosphere through which the cloud grows must be unstable.

3. The air in a thunderstorm will keep rising until:
1. It meets a layer of stable air that it cannot overcome
2. The rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air

5. Thunderstorms are often classified according to the mechanism that caused the air to rise.
An air-mass thunderstorm is a thunderstorm that results from the air rising because of unequal heating of Earth’s surface within one air mass.
Mountain and Sea-breeze

7. Frontal thunderstorms are thunderstorms that are produced by advancing cold fronts and, more rarely, warm fronts.
Cold-front thunderstorms get their initial lift from the push of the cold air which can produce a line of thunderstorms along the leading edge of the cold front.

8. Stages of T-storm development
Cumulus Stage
In the cumulus stage, air starts to rise nearly vertically upward.
Transported moisture condenses into a visible cloud and releases latent heat.
As the cloud droplets coalesce, they form larger droplets, which eventually fall to Earth as precipitation.

9. Mature Stage
As precipitation falls, it cools the air around it which becomes more dense than the surrounding air, so it sinks creating downdrafts.
The updrafts and downdrafts form a convection cell.
In the mature stage, nearly equal amounts of updrafts and downdrafts exist side by side in the cumulonimbus cloud.

10. Dissipation Stage
The supply of warm, moist air runs out because the cool downdrafts cool the area from which the storm draws energy.
Without the warm air, the updrafts cease and precipitation can no longer form.
The dissipation stage is characterized primarily by lingering downdrafts.

11. Severe Thunderstorms
Supercells are self-sustaining, extremely powerful severe thunderstorms, which are characterized by intense, rotating updrafts.

12. Lightning
Lightning is an electrical discharge caused by the friction of falling and rising ice crystals within strong drafts of a cumulonimbus cloud.

14. Lightning Facts
A lightning bolt heats the surrounding air to about °C.
Thunder is the sound produced as this superheated air rapidly expands and contracts.
Each year in the United States, lightning accounts for about 7500 forest fires, which result in the loss of millions of acres of forest.
Lightning strikes in the United States cause a yearly average of 300 injuries and 93 deaths to humans.

16. The Fury of the Wind
Downbursts are violent downdrafts that are concentrated in a local area and can contain wind speeds of more than 160 km/h.

17. Hail
Hail is precipitation in the form of balls or lumps of ice that can do tremendous damage.
Hail forms because of two characteristics common to thunderstorms.
Water droplets exist in the liquid state in the parts of a cumulonimbus cloud where the temperature is actually below freezing.
The abundance of strong updrafts and downdrafts existing side by side within a cloud.

18. The supercooled water droplets in the cloud freeze on contact with other ice pellets and are caught alternately in the updrafts and downdrafts.
The ice pellets are constantly encountering more supercooled water droplets and growing.
Eventually they become too heavy for the updrafts to keep aloft and fall to Earth as hail.

19. Floods
When there are weak wind currents in the upper atmosphere, weather systems and resulting storms move slowly.
Flooding can occur when a storm dumps its rain over a limited location.
Floods are the main cause of thunderstorm-related deaths in the United States each year.

20. Tornadoes
A tornado is a violent, whirling column of air in contact with the ground.
Before a tornado reaches the ground, it is called a funnel cloud.
Tornadoes are often associated with supercells.
The air in a tornado is made visible by dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud.

21. A tornado forms when wind speed and direction change suddenly with height, a phenomenon known as wind shear.
Under the right conditions, this can produce a horizontal rotation near Earth’s surface.
A thunderstorm’s updrafts can tilt the twisting column of wind from a horizontal to a vertical position.
Air pressure in the center drops as the rotation accelerates.
The extreme pressure gradient between the center and the outer portion of the tornado produces the violent winds associated with tornadoes.

23. Tropical Storms
Tropical cyclones are large, rotating, low-pressure storms that form over water during summer and fall in the tropics.
Tropical cyclones thrive on the tremendous amount of energy in warm, tropical oceans.
Rising air creates an area of low pressure at the ocean surface.

26. Hurricane Hazards
Hurricanes can cause a lot of damage, particularly along coastal areas.
Much of this damage is associated with violent winds of the eyewall, the band about 40 to 80 km wide that surrounds the calm eye.
A storm surge occurs when hurricane-force winds drive a mound of ocean water, sometimes as high as 6 m above normal sea level, toward coastal areas where it washes over the land.