Severe Weather Graphics
 
Tornadoes
U.S. tornado frequency map 48K
This map displays the average number of tornadoes per year in the continental United States. The total number of U.S. tornadoes is probably higher than reported in the sparsely populated western states.  
U.S. tornadoes per month 4K
This bar graph depicts the average number of tornadoes per month across the United States. In southern states, peak tornado occurrence is in March through May, while peak months in northern states are during the summer. In some states, a secondary tornado maximum occurs in the fall.  
U.S. tonadoes per hour 4K
This bar graph depicts the average number of tornadoes each hour of the day across the United States. Tornadoes are most likely to occur between 3 pm and 9 pm but have been known to occur at all hours of the day or night.  
Number of U.S. tornadoes and tornado fatalities by year 9K
This line graph displays the total number of U.S. tornadoes and the associated deaths each year, from 1916 to 1996. Prior to 1953, statistics of tornadoes and tornado fatalities are not considered to be official by the federal government. The increase in the number of tornadoes with time reflects the facts that the government began to keep tornado records, the general population increased, and the broadcast media heightened public awareness of tornadoes.  
Tornado development, stage 1: Developing (funnel) stage 9K
The rotation of the tornado develops at mid-levels (about 20,000 feet) in the storm. The circulation gradually builds downward. At about the same time, a downdraft, called a rear flank downdraft (RFD), descends to the ground. The tornado and RFD reach the ground within a few minutes of each other.  
Tornado development, stage 2: Mature stage 10K
After the tornado and rear flank downdraft reach the ground, the RFD begins to wrap around the tornado circulation. As it does, it begins to cut off the flow of warm, moist air into the tornado.  
Tornado development, stage 3: Rope stage 9K
When the RFD completely wraps itself around the tornado, the inflow is totally cut off, causing the tornado to lose intensity. The funnel reduces in size, and the tornado tilts and becomes rope-like in appearance. Eventually, it dissipates completely.
  
 
  Precipitation
 
U.S. thunderstorm days frequency map 20K
This map displays the average number of thunderstorm days per year in the continental United States.  
Stages of ordinary thunderstorm development 17K
This diagram shows the life cycle of an ordinary thunderstorm. (The vertical scale is exaggerated so that details of the cloud structure can be shown.) Pink arrows indicate the winds at various heights in the atmosphere. Red arrows are updrafts; blue arrows are downdrafts. Note that because of little or no change in wind with height, rain falls through the updraft, cutting off the fuel for the storm within 30 to 45 minutes.
 
Mature supercell thunderstorm 24K
This diagram shows the structure of a mature "supercell" thunderstorm. (The vertical scale is exaggerated so that details of the cloud structure can be shown.) Pink arrows indicate the winds at various heights in the atmosphere. Red arrows are updrafts; blue arrows are downdrafts. Note that because wind speeds increase with height, rain falls away from the updraft, allowing the storm to be maintained for hours. Supercell thunderstorms are significantly more destructive than ordinary thunderstorms.
  
Surface conditions common with a supercell thunderstorm 20K
This schematic looks down from above and displays typical surface conditions associated with a mature supercell thunderstorm. Note that the updraft and downdraft (rain area) regions are separated physically. In this manner, the supercell can exist in a quasi-steady-state for a few hours.
  
  Lightning, Hail, Hurricanes, etc.
Relative sizes of weather phenomena 5K
Horizontal slices of typical weather phenomena are displayed so as to compare their relative sizes. From smallest to largest, these phenomena include a tornado, thunderstorm, hurricane, and mid-latitude low pressure system.
  
Location of lightning fatalities 13K
This pie chart shows the worst places to be when lightning occurs. In recent years, lightning causes an average of 93 deaths and 300 injuries per year. Most casualties occur during outdoor activities of the summer months, primarily in the afternoon and early evening.  
Life cycle of a hailstone 14K
This schematic depicts the life cycle of a typical hailstone. The numbers on the picture are as follows: (1) A raindrop forms and is lifted by the thunderstorm's updraft above the freezing level (about 15,000 feet up). (2) The raindrop freezes and forms the core of a hailstone. (3) The hailstone enters turbulence and drops below the freeze level. Raindrops collide with the hailstone and form a sphere of water outside the frozen core. (4) The stone enters the updraft again and the liquid shell of water freezes. (5) The hailstone becomes heavier and falls through part of the storm. Raindrops again form a liquid shell around the hailstone. (6) The strong updraft again lifts the hail above the freeze level. Another layer of water is frozen around the hailstone. (7) The hailstone becomes too heavy for the updraft to support. It falls to the ground, colliding with rain along the way.
  
   
 
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