Tuesday, July 15, 2014

Near or Above Average Temperatures Are an Easy Bet in July

Bloody hot out there again today.  At 1 PM, it was 95ºF at the Salt Lake City Airport, just 2ºF cooler than yesterday when we hit 103ºF.  Another 100 is likely.  Average high for today is 93ºF.  Wouldn't that feel wonderful.

If you are looking for a month to bet on for near or above average temperatures in Salt Lake City, July is your best option.  July features the smallest amount of year-to-year temperature variability of any month of the year.  This, combined with the gradual increase in global temperatures, means it's pretty hard to have July that is substantially below average unless something really exceptional happens.

Below is the average July temperature for Utah Climate Division 3, which covers the Wasatch Front.  One can see the overall warming trend since 1895, which reflects a surge of warming prior to about 1940 and then after the late 1970s.  These two warming surges are also apparent in globally average temperatures (not shown).  Year-to-year fluctuations are only about 3-4ºF on average.  The grey line is the 1981-2010 climate average and we haven't had a year significantly below it since 1997.  Ah, the good old days.

Source: NCDC
Now lets take a look at January.  January sees considerably more variability from year to year.  So much so that it's hard to see the longer-term warming trend (although it's there) and the two surges noted above.  The year-to-year variability in January is very large compared to July and about 10ºF (note that the scale has changed and covers more than double the range of temperature in the July plot).

Source: NCDC
In July, the storm track is usually to our north and the monsoon usually has a modest influence on the weather of northern Utah.  As a result, we see less variability from year to year.  The signal to noise ratio is small in July, so the long-term warming trend has a more obvious influence on regional temperatures.  In contrast, January is more at the whims of the jet stream and there is tremendous variability from year to year that obscures the large scale warming signal.  You can get a well-below average in January, leading everyone to ask what happened to global warming.  Getting a well-below average July is more difficult.

Of course, there is always the potential for a black swan event.  July 1993 is a good example as it is easily the coldest July in the instrumented record in northern Utah and a full 4ºF colder than any July since 1915.  Several factors came together in 1993 to give us an unusually cold summer.  The first was the eruption of Mt. Pinotubo, which dropped temperatures globally.  The second was a weaker-than normal circumpolar vortex, which enabled cooler air to spread more frequently from the high to the low latitudes.  The third was a strong negative phase of the PNA pattern, which put persistent troughing over the interior west.  See A Tale of Two Summers: 1993 vs. 2013 for more discussion.


  1. Two primary drivers of average global temperature have been identified. A simple equation using them very accurately explains the reported up and down measurements since before 1900. The coefficient of determination, R2>0.9 (correlation coefficient = 0.95). The equation provides credible estimates back to the low temperatures of the Little Ice Age (1610).

    R2 = 0.9049 considering only sunspots and ocean cycles.
    R2 = 0.9061 considering sunspots, ocean cycles and CO2 change.
    The tiny difference in R2, whether considering CO2 or not, demonstrates that CO2 change has no significant effect on climate.
    The coefficients of determination are a measure of how accurately the calculated average global temperatures compare with measured. R2 > 0.9 is excellent.

    The calculations use data since before 1900 which are official, accepted as valid and are publicly available.

    Solar cycle duration or magnitude considered separately fail to correlate but their combination, expressed as the time-integral of solar cycle anomalies, gives excellent correlation. A solar cycle anomaly is the difference between the sunspot number for a year and an average sunspot number for many years.

    Everything not explicitly considered (such as the 0.09 K s.d. random uncertainty in reported annual measured temperature anomalies, aerosols, CO2, other non-condensing ghg, volcanoes, ice change, etc.) must find room in the unexplained 9.51%.

    The method, equation, data sources, history (hind cast to 1610) and predictions are provided at http://agwunveiled.blogspot.com and references

  2. Dr. Jim - Thanks for pointing out the difference in variability between July and January. It makes sense and is very interesting. I wonder how analogous plots for tropical locations look. I would guess that some tropical locations would have very little variability from year to year, while others would show quite a bit due to ocean surface temperature oscillations events like ENSO. Do tropical locations show the signature of climate warming as much as mid-latitude locations?

  3. A rookie question here (from a non-meteorologist), but why is it that in the continental US, January variation is so huge and July variation so nonexistent? Here in Columbia, SC, the July average is a high of 93 and a low of 72. The hottest it's ever been in this month is 108. This is 15 degrees F above normal. In January, the average high is 56, and 15 degrees above that is 71, which happens almost every year, with 80 being a possibility. Why is it that summer weather is so uniform in the lower 48 and winter weather so variable?

  4. Hi Roger - I see no one has replied, so I'll give it a shot. As the original blog article states: January is at the whim of the jet stream. Simplifying the effect of the jet, colder air masses are north of the jet, while warmer air masses are south of the jet. The jet can move south of a location and stay there for long periods of time to cause a cold snap in that location. It can also move to the north of a location for long periods of time and cause a warm period. Occasionally, the jet to the north (ridge) can persist for an entire month, likewise, the jet located to the south (trough) can also persist for an entire month. Thus, you can have high variability in mean January temperature from year to year.