For anyone who lives in a tornado-prone area, the issuance of an actual tornado warning can be a call to arms. Underground or lower level storm shelters are opened, children may don bicycle or football helmets, and adults may make a last-minute check on pets and other vulnerable possessions. Televisions are turned to maximum volume as the meteorologists provide vital details, and battery-powered devices such as radios and flashlights are brought to the most protected areas of houses or apartment. Tornado warnings may appear suddenly or follow an established pattern of stormy conditions, tornado watches, sightings in other counties and finally an official warning. Not all tornadoes are seen on radar soon enough to issue an official warning, but meteorologists continue to improve their technology.
So what exactly is a tornado warning? In general, certain weather conditions have to be in place before meteorologists feel comfortable issuing watches or warnings. Areas of low pressure move towards other areas with high pressure, causing the air between them to circulate. If the air is already saturated with water, these heavy clouds may form very unstable 'supercells' ahead of the main weather fronts. By this point, most meteorologists would issue at least a tornado WATCH, meaning the conditions are right for possible tornado formation. If the rogue supercells are strong enough, they may also issue severe thunderstorm warnings for the areas directly under and ahead of the storm cloud formations. The weather fronts themselves generally form a line of storm cells, accompanied by high winds. If a tornado forms, it's usually in the supercells ahead of the front, not in the front itself.
As these supercells move ahead of the main fronts, the circulation may intensify- a condition which can be measured by a Doppler radar system. Winds either move towards or away from a fixed central point, which in this case is the radar tower itself. If a strong wind is moving towards the radar and an equally strong wind is moving away at the same time in the same area, this means the air is twisting around itself. Circulation doesn't always mean a tornado, but it does set off signals at the weather monitoring stations. The storm clouds continue to build up, and hail is formed as the rain is caught up in the circulation and sent up to the top of the cell. Large hail is also an indicator of a potential tornado formation, which can also be measured by calculating the amount of liquid in a vertical measurement of the cloud's height.
There are also law enforcement officers and trained civilians who move to assigned areas and watch the sky for signs of a funnel cloud or 'wall cloud', a low-hanging cloud bank which usually forms right ahead of a tornado. If any of these weather spotters notice severe weather, they report their findings directly to the weather monitoring service. Comparing the field data to the radar readings, professional meteorologists will then decide if a tornado has actually formed. If the answer is yes, the weather service notifies a network of local broadcast stations that a tornado warning has been issued for a specific county or counties. This information is immediately repeated by the local television weathermen, who may or may not be meteorologists themselves, and citizens in the path of the tornado take shelter. Whenever a tornado moves through the area, an 'all clear' is usually given and the warning is moved to the next affected county or state. In general, once the actual front has moved in, the weather behind it is much milder.
An official tornado warning must meet specific criteria before the National Weather Service issues it. Distinctive radar readings can indicate a tornado has formed, but it may or may not actually touch the ground. Spotters are usually the first people to confirm an actual funnel cloud and any subsequent damage it causes. Trained meteorologists working as broadcast weathermen can also issue their own warnings based on radar readings, but they must be very careful not to endanger too many people with false alarms. Many public institutions will not consider a tornado warning to be official until the National Weather Service has issued one, but certain local weather reporters may have better views than the NWS. Any tornado warning should be taken very seriously, regardless if it was issued by an official government agency or a trained broadcast meteorologist.
