A rummage around any weather forecasting organisation's offices will, among the computer screens displaying the latest computer model forecasts, usually reveal a book on climatology.

Long-term weather patterns cannot be ignored when forecasting on a day-to-day basis. You need to be a very bold forecaster to predict showers tomorrow in a part of the world which is in the grips of its historical dry season.

Similarly, a temperature prediction should, unless there is a very good reason, be close to the long-term average observed in that location.

Al Jazeera weather team's favoured book used to be World Weather Guide by Pearce and Smith. The last edition was back in 2000. But these are data sets built up over many decades. The data on temperature and rainfall should change over centuries, but not decades, surely?

Unfortunately, that no longer seems to be the case. Global rainfall patterns are changing. A good example is the Atacama Desert of northern Chile and southern Peru. Once famed as being the driest region on Earth, until recently some places had recorded no measurable rain in a century.

That has all changed. Deadly floods hit the region in April 2015, and another flooding event occurred as recently as May this year.

It is temperatures, however, where the climatology books are having to be re-written. Heatwaves across North America, Europe, the Levant and the Middle East seem to have skewed the statistics in such a way that average maximum temperatures no longer reflect reality.

Let's take Kuwait as an example. According to the World Weather Guide, the average maximum in August is 40 degrees Celsius. In reality, the temperature in the city has reached at least 44C every day since late June.

Heatwaves are becoming a more regular occurrence, across the northern hemisphere.

Analysis from James Hansen, former NASA climate scientist and Professor at Columbia University, has revealed a marked change in summer temperatures across the northern hemisphere.

Between 1951 and 1980 he defined approximately one-third of summers as "cold", one-third "average" and one-third "hot". Since then there has been a marked shift towards warmer summers. Between 2005 and 2015 two-thirds of summers were "hot" and one-sixth were "extremely hot".

The current heatwave across southern Europe may be slowly easing, and it does not compare with the more severe and prolonged heatwaves of 2003 and 2006. But the warming trend is entirely consistent with the increase in anthropogenic carbon dioxide and other greenhouse gases.

Fortunately for us, the World Meteorological Organisation, updates its climatological data bases on a regular basis, giving a more up-to-date indicator to typical weather patterns for most of the world's major cities.

Our World Weather Guide may now be redundant, but it is likely to remain on our bookshelf, as a reminder of what the weather was like in the "good old days".

Source: Al Jazeera and news agencies