Last week’s outbreak of 16 tornadoes across the upper Midwest of the US have left two people dead.

Although the tornado season has started very slowly, this is a reminder that the deadliest twisters often happen during the month of April.

Tornadoes occur throughout the year in the US, but activity is greatest during the spring months.

This is because the temperature contrast between warm air from the Gulf of Mexico and cold air over the Canadian Prairies is at its greatest.

Tornadoes tend to form along frontal systems which are defined by these temperature differences.

Although tornado frequency is greatest during May, it is April which usually sees the highest number of tornado fatalities. There are several reason why this is the case.

Firstly, shorter days during early spring mean that there is a greater chance that a tornado will occur at night.

This makes spotting an approaching twister much less likely, and a nighttime ‘hit’ must be a particularly terrifying and chaotic experience with less time to take shelter.

Secondly, early season tornadoes are more likely to hit the southern states. Here, building construction is much less robust than further north.

This is partly due to the more temperate climate, and partly due to differences in affluence.

A high water table in parts of the South means there are few basements to serve as tornado shelters.

This weekend marks the 50th anniversary of the Palm Sunday tornado outbreak. 47 tornadoes hit the Midwest, killing 271 people and injuring another 1,500.

More recently, a series of tornadoes hit the South, Midwest and Northeast between 25 and 28 April 2011; 348 people were killed and damage was estimated at $11b.

In the coming days, heavy rain is likely to be a greater threat for parts of the south, from Texas to Alabama.