Just realised that I said entirely the wrong thing in my last e-mail - the exciting thing that our research using observed data back 100 years in the Karakoram (Upper Indus) region Pakistan shows is an *increase* in DTR throughout the year, and particularly in the summer season. This is similar to trends in northern India - but I have not found evidence of an increase anywhere else - and the observations are certainly contrary to projections of GCMs for the region (projecting decreases in DTR, as has been observed elsewhere on the globe). We are trying to find an explanation for this. In summer months both max and min temps are falling, but min more than max - hence increasing DTR. Decreasing max may be explained by increased cloudiness (increasing trend in summer precip in last 40 years) but cannot also explain large decrease in min temps? In winter, we are seeing a more regular pattern with warming in max (large) but different changes in min dependent on location - generally mean temps show significant increases in winter though.
The Decreasing Diurnal Temperature Range (DTR)
Daytime temperatures are higher than nighttime temperatures. The difference is defined as the
diurnal temperature range (DTR). Observations show that in most places the DTR is decreasing
in recent years. It is caused by temperatures increasing faster at night than during the day.
Thomas Karl (1993) states: "Since 1950 all of the increase of temperature across the U.S.A. is
due to an increase in the minimum temperature (about 0.75 degrees C/ Century or 1.5 degrees
F/Century ) with no change in the daily maximum temperature. This caused a decrease in the
diurnal temperature range." Subsequently, this type of behavior has been observed at other
locations and is stronger as one goes towards the polar regions. It now appears most of the
observed global surface warming of recent decades is occurring mostly at night.
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