Some people claim that we are experiencing more extreme weather now than in the past: more heat waves, cold spells, floods and droughts. To examine this allegation scientists Karl et al studied rainfall and temperature data from 187 places in the USA, China, the former Soviet Union (FSU) and Australia (1). It turns out that day-to-day differences (as well as differences between the means of longer sequential periods) have generally decreased during the past century, both for precipitation and temperature. Inter-annual (year-to-year) differences of temperature have increased over the past few decades in Australia and the FSU, but not in China and the USA.
The rainfall intensity also yields a mixed signal. A larger fraction of the total precipitation derives from heavy rain events, at least in the USA. The fraction of summertime precipitation of more than 50 mm/d has significantly increased in the US and Australia since 1910, while the fraction of 12.7 - 25.4 mm/d rainfall fell appreciably. The data came mainly from rural weather stations to avoid urban effects. No similar shift of rainfall intensity emerged from the data from FSU or China.
(1) Karl, T.R., R.W. Knight and N. Plummer 1995. Trends in high frequency climate variability in the twentieth century. Nature, 377, 217-20.