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After longer periods without especially large tornadoes, we sometimes forget about them. °ÄÃÅÁùºÏ²Êƽ̨ tornado that hit Marietta, Oklahoma the night of April 27 was rated an EF-4, the strongest tornado in the United States in more than a year, and there has not been an EF-5 since the in 2013.

On average, there are a year in the U.S., but over the last three decades, their annual number has ranged from 886 (2014) to 1,813 (2004). Detection has improved since the early 1990s with the development of Doppler radar, which is able to sample wind movement within a storm.

Tornadoes can and do happen at any time of year, but they are most frequent in the middle to late spring. This is when the upper levels of the atmosphere remain relatively cold from the winter, while the increasingly high sun angle warms the ground quickly during the day. Together, this makes the atmosphere unstable, meaning when air in touch with the ground warms and rises, it rises even more rapidly as it encounters much colder air aloft.

A trigger is needed to give storm formation a start, and this is where the geography of the U.S. matters. Canada provides a large source of cold, dry air moving south, which meets warm and humid air moving northward from the Gulf of Mexico. °ÄÃÅÁùºÏ²Êƽ̨se intersecting boundaries are the focus of storm development.

°ÄÃÅÁùºÏ²Êƽ̨re are few other places around the world where this is common. Tornadoes occur in Europe and Australia, as well as small sections of South America, southern Africa and eastern Asia.

But the United States leads the world in tornadoes, and the contrast between cold and warm air masses is only one part of the puzzle. One extra bit of geographical help is the higher terrain of the Rockies and the Mexican plateau. °ÄÃÅÁùºÏ²Êƽ̨se introduce some warm and dry air a mile or so above sea level. That warm air drifts high over the Plains, acting like a lid to thunderstorm development — at least at first.

Initially, rising air from the ground remains warmer than its surroundings as it climbs upward through the atmosphere. But if it encounters a warmer layer of air a mile or two up in the sky — the lid — it becomes less buoyant, effectively running out of gas.

However, as more and more of these little parcels of warm air rise toward the lid, it erodes away, allowing a sudden and explosive development of thunderstorms.

But even more is needed to get tornadoes to form. In this case, getting the necessary spin to produce tornadoes requires the winds in the atmosphere to change direction with height. In most cases, the winds near the ground are from the south, but gradually change direction with increasing altitude, turning from the west around three miles up in the sky. This change in direction induces a horizontal spinning motion, like a pencil rolling across a table.

When upward racing air encounters that horizontal spinning, it is tilted into the vertical, and that air — known as an updraft — begins to rotate. This is the initial step toward producing the larger area of rotation in the thunderstorm called the mesocyclone, from which the tornado descends to the ground.

Meteorologists are very good at forecasting the conditions leading to the storms that produce tornadoes, but there is not much skill in precisely forecasting where and when a tornado will strike more than a few hours in advance. This is why it is still important to respect the warnings when they are issued. Communication technology has improved, so we no longer need to rely on outdated sirens, as warnings can now come to your mobile phone in addition to the traditional weather radio.

False alarms still occur, but as our understanding of tornadoes, the warnings are getting more accurate. Most of the time, the full area under an individual tornado warning will be larger than the actual tornado, but there remains enough uncertainty in the precise nature of tornadoes that the warnings should be respected.

A basement is always the best shelter, but if a tornado warning is issued and you cannot get underground, get to the most interior room in whatever building you are in, with the idea of putting as many walls between you and outside as possible. Wait until the warning expires — most of the warnings are 30 to 45 minutes long — before leaving your safe space.

You may do it a few times and the tornado misses your house – perhaps by a few hundred yards or a couple of miles — but it will be excellent practice for that one time that could save your life.