Thursday, July 23, 2015

Perspectives on Our Fantastic July

Wonderful July weather continues in Salt Lake City.  Most of the month has simply been fantastic with tolerable temperatures and the occasional showers and thunderstorms to help moisten things up.  The National Weather Service graphic below shows the observed highs and lows (bars) for the month at the Salt Lake City International Airport compared to the 1981–2010 average for each day.  We suffered through some well above average temperatures through the 4th, but since then, we've only had one day with a maximum temperature above average and we've even had a few minima that have been below average.  Simply splendid.

Source: NWS
Let's have a closer look at how this July (so far - keep this in mind for the discussion below) compares to past Julys at the Salt Lake City International Airport since 1928.  In terms of average temperature, this July would actually rank above the long-term average for the entire period, but it is the coolest July since 1999.

Source: NOAA Regional Climate Centers
However, if you look at the graph above, the average temperature this July isn't a heck of a lot lower than it was in 2009, 2010, or 2011.  Why does this year seem so cool.  The answer is provided the graph of the average maximum temperature which shows this July is really quite an anomaly compared to the recent past with an average maximum temperature of 89.2ºF, the lowest since 1993.  In addition, since 1993, there has only been one other July with an average maximum temperature below 90ºF, 1997.  And, considering that the four hottest days of the month were the 1st to the 4th, the past two weeks have been unusually cool in terms of the maximum temperatures compared to what we've seen for about 20 years.

Source: NOAA Regional Climate Centers
Curiously if you look at the minimum temperature, one finds that this year has been fairly warm compared to past Julys at the Salt Lake City International Airport.
Source: NOAA Regional Climate Centers
This is somewhat consistent with the high humidities and frequent cloud cover we've seen this month, which typically reduces the daily temperature range.  However, there is a clear upward trend in minimum temperature that is much stronger than evident in maximum temperatures for the month.  Whether or not this reflects changes in instrumentation, movement of the observing site, changes in the characteristics of the land surface around the observing site, a stronger urban climate influence, or climate change is something that warrants further investigation.

We have 9 more days to go, so the numbers for this July could change.  We'll have to see where we end up when we get to August, but I'm quite thankful for the weather we've experienced the past couple of weeks.

5 comments:

  1. Chris Cline, SLC UTJuly 23, 2015 at 11:33 AM

    My impression is that the increase in daily minimum temperatures is a more reliable "signal" of what is happening with warming. Could you address this?

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    1. PS- I am very thankful, too!

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    2. Yes, a hallmark of global warming is a more rapid increase in minimum temperatures than maximum temperatures and a decrease in the diurnal temperature range. I'm reluctant, however, to attribute all of what is happening at KSLC to that without more investigation given that at any given location, other factors could be contributing.

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    3. At KSLC, winds are typically from the NW during the afternoon (a trajectory relatively unaffected by urban development) and SE at night (greatly influenced by urban landscape). So I think the increase in minimum temperature makes sense from this perspective. Also, changes in urban landscaping in recent years such as lower water landscapes (rockscapes, etc) is probably a factor in raising temperatures. I think the bottom line is that KSLC is very disproportionately affected by the heat island at night due to prevailing wind direction, regardless of whether anything has changed right around the site itself.

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    4. If you're interested in a broader geographical perspective on the diurnal temperature range, check out Dai et al. (1999): http://journals.ametsoc.org/doi/full/10.1175/1520-0442%281999%29012%3C2451:EOCSMP%3E2.0.CO;2 and Rohde et al. (2013): http://berkeleyearth.org/static/papers/Results-Paper-Berkeley-Earth.pdf.

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