By John Dodge

CHICAGO (CBS) — The Chicago area has set a record for the most tornadoes in a single year, according to National Weather Service data.

Four tornadoes on Aug. 18 brought the total for 2015 to 29, breaking the 2004 mark of 27.

The NWS data goes back to 1950.

One key factor in the increase in the number is due to better detection of weaker (EF 0, EF 1) storms through the use of more trained spotters and improved weather radar technology.

“That being said, 2015 has been an exceptionally active year for tornadoes in the Chicago forecast area,” the NWS said in a statement.

One other curious bit of data: All of the tornadoes in 2014 (18) occurred on a single day, June 30.

The Top 10 List

Rank Year Number
1 2015 29
2 2004 27
3 1974 22
3 2010 22
4 1965 21
5 2003 18
5 2008 18
5 2011 18
5 2014 18*
10 1967 17
10 1972 17
10 2013 17

*All On June 30, 2014

The average annual number of tornadoes since 1950 is nine, the NWS said.

Since the NWS added an enhanced Doppler radar in 1992, the annual average has increased to 11.

This week also marks the 25th anniversary of one of the deadliest tornadoes in Chicago. The EF-5 that hit Plainfield killed 29 people and left widespread destruction.

The reporting area includes: Boone, Cook, DeKalb, DuPage, Ford, Grundy, Iroquois, Kane, Kankakee, Kendall, Lake, LaSalle, Lee, Livingston, McHenry, Ogle, Will and Winnebago counties in Illinois. Also, it includes, Lake, Porter, Newton, Benton and Jasper counties in Indiana.