A storm shelter or storm cellar is a type of underground bunker

Bunker

A military bunker is a hardened shelter, often buried partly or fully underground, designed to protect the inhabitants from falling bombs or other attacks...

 designed to protect the occupants from violent severe weather

Severe weather

Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...

, particularly tornadoes. They are most commonly seen in the US Midwest ("Tornado Alley

Tornado Alley

Tornado Alley is a colloquial and popular media term that most often refers to the area of the United States where tornadoes are most frequent. Although an official location is not defined, the area between the Rocky Mountains and Appalachian Mountains is usually associated with it.The areas...

") and Southeast where tornadoes are particularly frequent and the low water table

Water table

The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...

 permits underground structures.

Typical storm shelter

A typical storm cellar for a single family would be built close enough to the home to allow quick access in an emergency, but not so close that the house could collapse on the door during a storm, trapping the occupants. This is also why the main door on most storm cellars is mounted at an angle rather than flush with the ground; an angled door allows for debris to blow up and over the door without blocking it and also decreases the force necessary to open the door if rubble has settled on top. It might have a floor area of eight by twelve feet (2.5 × 3.5 m) and an arched roof like that of a Quonset hut

Quonset hut

A Quonset hut is a lightweight prefabricated structure of corrugated galvanized steel having a semicircular cross section. The design was based on the Nissen hut developed by the British during World War I...

—but it would be entirely underground. In most cases the entire structure would be built of blocks

Brick

A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

 faced with cement and rebar

Rebar

A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...

 through the bricks for protection from the storm. Doing this makes it almost impossible for the bricks to fall apart. New ones sometimes are made of septic tank

Septic tank

A septic tank is a key component of the septic system, a small-scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations...

s that have been modified with a steel door and vents. Most storm cellars would be reached by a covered stairwell, and at the opposite end of the structure there would be conduits for air that would reach the surface, and perhaps a small window to serve as an emergency exit and also to provide some light. Storm cellars, when connected to the house, may potentially compromise security.

Fully enclosed underground storm shelters offer superior tornado protection to a traditional basement

Basement

__FORCETOC__A basement is one or more floors of a building that are either completely or partially below the ground floor. Basements are typically used as a utility space for a building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system...

 or cellar because they provide overhead cover without the risk of being trapped or killed by collapsing rubble from above. For this reason they also provide the only reliable form of shelter against "violent" (EF4 and EF5) tornadoes which tend to blow the house off the foundation, removing the overhead cover protecting the occupant.

Additional uses

Since it is functionally just an underground bunker, storm cellars can also be used as improvised bomb shelter

Bomb shelter

A bomb shelter is any kind of a civil defense structure designed to provide protection against the effects of a bomb.-Types of shelter:Different kinds of bomb shelters are configured to protect against different kinds of attack and strengths of hostile explosives. For example, an Air-raid shelter...

s or fallout shelter

Fallout shelter

A fallout shelter is an enclosed space specially designed to protect occupants from radioactive debris or fallout resulting from a nuclear explosion. Many such shelters were constructed as civil defense measures during the Cold War....

s (although they are not usually dug as deeply or equipped with filtered ventilation). Since the underground construction makes them cool and dark, storm cellars on farmsteads in the Midwest are traditionally used as root cellar

Root cellar

A root cellar is a structure built underground or partially underground and used to store vegetables, fruits, and nuts or other foods.-Construction:Common construction methods are:...

s to store seasonal canned goods

Canning

Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canning provides a typical shelf life ranging from one to five years, although under specific circumstances a freeze-dried canned product, such as canned, dried lentils, can last as...

for consumption during the winter.