Weather radar is a vital forecasting tool. By depicting precipitation and its intensity as a color-coded image, it allows forecasters and weather novices alike, to keep up with rain, snow, and frozen precipitation that may be approaching an area. As a general rule, the more vivid the radar color, the more severe the weather associated with it. Because of this, yellows, oranges, and reds make severe storms easy to detect at a glance.
In the same way radar colors make it easy to spot an existing storm, shapes make it easy to classify a storm into its severity type. Some of the most recognizable thunderstorm types are shown here as they appear on reflectivity radar images.
Single Cell Thunderstorm
The term "single cell" is commonly used to describe an individual spot of thunderstorm activity. However, it more accurately describes a thunderstorm that goes through its life cycle only once.
Most single cells are non-severe, but if conditions are unstable enough, these storms can produce periods of brief severe weather. Such storms are called "pulse thunderstorms."
Multicell thunderstorms appear as clusters of at least 2-4 single cells moving together as one group. They often evolve from merging pulse thunderstorms, and are the most common thunderstorm type.
If watched on a radar loop, the number of storms within a multicell group grows exponentially; this is because each cell interacts with its neighbor cell, which in turn grows new cells. This process repeats fairly rapidly (about every 5-15 minutes).
When grouped in a line, multicell thunderstorms are referred to as squall lines.
Squall lines stretch over a hundred miles in length. On radar, they can appear as a single continuous line, or as a segmented line of storms.
Sometimes a squall line slightly curves outward, resembling an archer's bow. When this happens, the line of thunderstorms is referred to as a bow echo.
The bow shape is produced from the rush of cool air that descends from a thunderstorm downdraft. When it reaches the earth's surface, it is forced horizontally outward. This is why bow echoes are associated with damaging straight-line winds, especially at their center or "crest." Circulations can sometimes occur at a bow echo's ends, with the left (northern) end being the most favored for tornadoes, due to the fact that air flows cyclonically there.
Along the leading edge of a bow echo, thunderstorms may produce downbursts or microbursts. If the bow echo squall is particularly strong and long-lived--that is, if it travels farther than 250 miles (400 km) and has winds of 58+ mph (93 km/h)--it is classified as a derecho.
When storm chasers see this pattern on radar, they can expect to have a successful chase day. That's because a hook echo is an "x marks the spot" indication of favorable locations for tornado development. It appears on radar as a clockwise, hook-shaped extension that branches off from the right rear of a supercell thunderstorm. (While supercells can't be distinguished from other thunderstorms on base reflectivity images, the presence of a hook means the storm depicted is in fact a supercell).
The hook signature is produced from precipitation that gets wrapped into the counterclockwise-rotating winds (mesocyclone) within a supercell storm.
Because of its size and solid structure, hail is exceptionally good at reflecting energy. As a result, its radar return values are quite high, usually 60+ decibels (dBZ). (These values are denoted by reds, pinks, purples, and whites centrally located within the storm.)
Quite often, a long line extending outward from the thunderstorm can be seen (as pictured at left). This occurrence is what's called a hail spike; it almost always indicates that very large hail is associated with the storm.