temperature

Temperature changes at SeaTac...

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Weather data collected at Seattle Tacoma International Airport (SeaTac) is generally considered the main data of record for the Seattle metropolitan area, though there are several other sites in the region where meteorologist and climatologists gather their information. Data has been collected at SeaTac continuously since 1948, though some data is missing from the records for the period of October 1, 1996 through October 31, 2005. This missing data is generally the snow and sky cover observations. Temperature, the focus of this page, and precipitation data was collected during this period and is available from the National Oceanic and Atmospheric Administration (NOAA) and their publicly accessible National Centers for Environmental Information (NCEI) Climate Data Online (CDO) tool.

So, having temperature data from 1948 through yesterday (Feb 22, 2023), how much has the average daily temperature changed at SeaTac over the past 75 years (inclusive) if at all? And are there any trends? If so, one would assume that the temperatures have slowly warmed over the years based on all the science and reporting published over the past decade or longer.

 

An undulating plot

The National Weather Service generally reports the daily minimum temperature (Tmin) and daily maximum temperature (Tmax) at the weather stations. The average of these two values is calculated and recorded as the average daily temperature (Tavg). Often when looking at or downloading past data from a weather station, the user receives only the Tmin and Tmax values for a given date. This is common on older records. Sometimes the data includes the calculated Tavg. Regardless, knowing that Tavg is the average between the Tmin and Tmax values, the Tavg shown in the following charts has either been calculated by the NWS and reported, or this author has performed the calculation in the spreadsheet I’ve imported data from the NWS into.

If you plot the TAVG value for every date from January 1, 1948 through today as shown in Figure 1 you get an undulating or wavy curve form. Obviously, the average daily temperature of a summer day is much higher than the same on a winter day in Seattle. It is difficult to determine if there has been a fall or rise in the average daily temperature and, if so, by how much by just looking at this wave form.

Figure 1. Average daily temperatures trace from 1948 to 2022, Seattle Tacoma International Airport.

I’ve added a linear trend line in red in Figure 2. This trend line shows a modest rise in daily temperatures over the 75-year period. The average daily temperature recorded at Seattle Tacoma Airport has risen from roughly 50˚F to about 54˚F during this period, a rise of roughly 4-4.5 ˚F, or 2.2-2.5 ˚C.

Figure 2. Average daily temperatures trace from 1948 to 2022 with red trendline, Seattle Tacoma International Airport.

Annual Cumulative Degree-Day Index (ACDD)

Another method of looking at temperature changes over time and comparing these to other years over a long period is to plot the cumulative total of daily average temperatures and plot that accumulation over the course of a year. Then plot a similar curve for each year in the period of study. This is shown in Figure 3.

A simple way to check this value is described here. Say the average annual daily temperature at SeaTac is 50˚F. This is the average daily temperature over the course of a year. Some days will be cooler and some warmer. If you multiply (50F DEGREES) X (365 DAYS) you’ll get a value of  18,250 DEGREE-DAYS, or 18,250 DD. I’ll use the term Annual Cumulative Degree Day (ACDD) to indicate the degree-days accumulated over an annual period at a specific weather station.

Looking at the y-scales on Figure 3 shows that summing the average daily temperatures for each day of the year will get you to approximately 18,250 ACDD.

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Note: My use of an index I call degree-day is not the equivalent of Heating Degree Days (HDD) or Cooling Degree Day (CDD), both indexes used in the building systems world to calculate required heating or cooling loads for designing heating and air-conditioning equipment and systems and both reported on in daily summary statistics from NOAA/NWS data repositories. But it is similar in the sense of understanding the total amount of heat received over the course of a year (as defined by the sum of the daily average temperatures) in any given year at SeaTac airport. Each gray line or trace shown in Figure 3 represents one year in the 1948-2023 period.

Figure 3. Annual Cumulative Degree Days (ACDD) plots for each year, 1948-2022, Seattle Tacoma International Airport (KSEA).

 

Comparing individual years

Figure 4 shows ACDD traces of four individual years (highlighted in red) plotted against similar ACDD traces all years. Of the four selected, 1948 and 1985 started off relatively cool through the end of June and then each had a cool autumn and early winter. The plot for 1966 shows it too started cool through spring and into early summer before a late summer surge gave it a relatively average ACDD Index value for the entire year. The plot for 2015 shows the year had an average winter and spring temperature-wise for this region, followed by a warm period extending through the remainder of the year. For the 74 full years between 1948-2022, this year (2015) was the warmest on record in Seattle.

Figure 4. Individual ACDD trace samples for four select years, Seattle Tacoma International Airport.

 

Small Multiples

Figure 5 is a series of plots grouped as a whole. Each individual plot can be clicked on to expand it. This form of data graphic is collectively called small multiple. It allows one to compare one year to other years easily. It also allows for a deeper dive into any individual year.

Detecting trends requires a little more study, but you can see that, in generally, the ten years from 1948-57 were cooler than average at SeaTac. One can also see that the 10-year period from 2013 through recently completed 2022 have been warmer than average. There was another five-year period running from 2003-07 which were warmer than average. Most of the other years hovered near average, though it is common over any ten year stretch to see back-to-back years running much cooler or warmer than average.

Figure 5. A small-multiples data graphic comparing the Annual Cumulative Degree Days (ACDD) tracing for each of 74 consecutive years at Seattle Tacoma International Airport.

Another interesting thing that shows up when plotting the Degree-Day number for each year is best seen going back to Figure 3. Notice how in the winter and early spring months, Seattle tends toward either warmer or cooler springs. Very few annual periods are “average”. Notice the “gap” that exists between early February through May. This period in the calendar tends towards two groups or strands of tracings which braid together in midsummer. By the end of each calendar period, the tracings have advanced towards split ends.

I don’t know why. I wonder if La Nina years tend towards one strand and El Nino years towards the other. Mathematicians, meteorologists, or atmospheric scientists might understand this “strand” phenomenon or “strand theory” better than I.

An Animated Chart

Finally, Figure 6 is an animated chart. It is an alternative to the small multiples chart shown in Figure 5. It displays each year’s trace above the gray traces of all years sequentially. You can imagine it looking like a dog’s tail wagging. This means of presenting data highlights the year-to-year variability. It also can show trends if over a course of periods of years, the “tail” wags high or lower. You can witness by watching the video the early years of recorded temperatures tend towards cooler days and as time marches forward, the wagging tail inches up the chart. Year-to-year the tail wags randomly (annual variability), but over time a pattern emerges of the tail drifting upwards into warmer cumulative temperatures (periodic trending).

Figure 6. ACDD traces for Seattle Tacoma International Airport, 1948-2023. The animated GIF should loop through once, probably upon initial loading of the web page. Notice that the wagging red line slowly drifts higher as the years progress. Drifting high indicates the total daily temperature load is increasing (e.g. days are getting warmer on average).

HINT: Refresh (Command+R) the web page to cycle the GIF in sequence. And remember to scroll back down to the bottom of the page after refreshing.

That’s all for this post, just some interesting new ways at looking at annual temperature profiles for data from selected NOAA / NWS collection sites. I used SeaTac Airport as an example as I live in Seattle.

1993 vs. 2022

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Precipitation data updated on June 30. This is the final update to this post.

The spring and early summer of 1993 was memorably wet for some of us who have called Seattle home for many years. That was the one year where I belonged in a van pool to go to and from work. each day. One dreary morning following a series of similar days in June or July, several people in the van were complaining about the weather. I can’t recall if I was one of them or just in my mind agreeing with that sentiment. I do remember a fellow vanpooler and native of Seattle chastising the complainers and suggesting this weather was normal for Seattle and telling us if we didn’t care for it, we should move. I had mixed feelings but did not completely disagree with her.  Nevertheless, that was my 10th summer in the city, and I hadn’t recalled any of the others being so miserable weatherwise.

We appear to be experiencing a similar wet and cool spring and early summer in 2022. It has brought up memories of that gray early summer of 1993. I was just speaking to a neighbor yesterday about the very thing though she hadn’t lived in Seattle in 1993. What triggered my discussion was looking at the daily rain total from the day before and seeing we had set a record for that June day. The previous record was in place since 1993.

So how does this wet spring and early summer compare to a similar period from 1993? In all honesty, I didn’t know if 1993 was our wettest spring since 1984, my first year in Seattle, let alone for the recorded history of spring weather at Seattle-Tacoma International Airport (KSEA) going back to the 1940s. But it was, for me, a memorable wet period, as is this spring, so let’s compare.

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Precipitation

Figure 1 shows the cumulative precipitation traces for calendar periods starting on October 1 of one year through September 30 of the following year recorded at SeaTac airport. This period is often considered the water year on the U.S. west coast due to our seasonal weather patterns of a strong wet season usually followed by a strong dry season. The midpoint of this multi-calendar year span is April 1 which roughly represents the transition period from wet-to-dry seasons.

 

Click to enlarge.

Figure 1. Cumulative precipitation traces for water years 1948-49 through 2021-22 at Seatlle-Tacoma International Airport (KSEA).

 

I’ve highlighted two specific annual traces: the 1992-93 and the 2021-22 water years. The first obvious thing one notices is that since October 1 of last year, this has been one of Seattle’s wettest periods. It is in the top five wettest water years so far. Ironically, two of the other 74 traces in the top five category are also recent water years, 2015-16 (#3) and 2016-17 (#1).

A second obvious feature is that in the first six months of water years 1992-93 and 2021-22 leading up to April, it has been far wetter this water year (2021-22) than 1992-93. On April 1, SeaTac Airport had already seen 35.8 inches of rain during the previous six months. On April 1, 1993, SeaTac Airport had seen 21.35 inches of rain in the previous six months. This is a difference of 14.45 inches, or more than two average November’s worth of rain. Memory plays tricks on us, but I’d argue that up to June 10, this water year has been much drearier than 1993.

There are two boxes on Figure 1. These boxes encapsulate the period between April1 – June 10 for each trace being compared, 1993 (red) and 2022 (blue). The total amount of precipitation for each period is similar, 8.94 inches and 8.83 inches respectively. The typical Seattle summer dry spell in 1993 began on or around July 30. It remains to be seen when this will occur in 2022.

Figure 2 shows the same data as Figure 1, cumulative precipitation at SeaTac for the years 1948-2022 except the it shows precipitation for the months of April through August. The values have been zeroed to March 31 for comparison purposes. Plotting this shows just how wet this spring has been. Aside from 1948, 1993 and 2022 have been two of the wettest springs recorded at SeaTac Airport. The spring of 1993 ended up being the second wettest spring recorded to-date at SeaTac. The spring of 2022 ended up being the fifth wettest.

Click to enlarge.

Figure 2. Cumulative precipitation since April 1 at Seattle-Tacoma International Airport (KSEA) for the years 1948 through 2022.

Figure 3 is a dot plot which ranks the precipitation periods from April 1 to Jun 17 and is analogous to Figure 2 showing the values for all traces between 1948 to 2022. In mid-June, the springs of 1993 and 2022 were virtually tied for 2nd place. But the latter half of June 2022 was relatively dry and the final tallly of April-June precipitation for 2022 dropped the year to fifth place on Figure 3. June 1993 ended up having .68” inches more of precipitation than June 2022.

Five of the top ten wettest years for this April-Jun period have occurred since 2010. At mid-month it was six. The 2017 period dropped to #12 on the list by the end of the month.

Figure 3. Ranking of spring-summer precipitation at Seattle-Tacoma International Airport (KSEA).

Figure 4 shows the same data displayed in Figure 3 but lays the data out in chronological order to see if any trends over time are evident. Any trending appears to be mild, but there appears to be a slight increase in spring and early summer precipitation in recent years.

Click to enlarge.

Figure 4. Cumulative spring-summer precipitation trends, April 1 through June 30, KSEA.

Temperatures

This spring has been much cooler than normal since April. How do the temperatures of Spring 2022 compare to Spring 1993?

Taking a look back at April through June daily average temperature departures from normal (30Y average for 1961-90) shows that spring and early summer was normal to slightly warmer than normal. The average April daily temperature was about 1˚F warmer than 30Y normals. The average May daily temperature was about 4˚F warmer than long-term normals. And the average June daily temperature was just about normal.

Contrast this with this spring and early summer (to-date). This year at SeaTac airport, the average daily temperature in April was -4˚F, the average May daily temperature was -4.6˚F, and the average June daily temperature (to June 17) is -2.4˚F. These are averages across an entire month. Of course there were nice days during these periods, even some warm one, but in general it has been a very cool spring and early summer. These differences are shown in Table 1.

Table 1. Monthly Average Temperature Deviation from 30-Year Normals

 

Figure 5 shows the daily temperature departures for the period from April 1 to June 30 for 1993. A 7-day average line plot is included.

Figure 6 shows the same data for this year through the most recent data point.

Click to enlarge.

Figure 5. Daily temperature departures for April through June 30, 1993.

 

Click to enlarge.

Figure 6. Daily temperature departures for April through June 18, 2022.

Looking back, spring and early summer of both 1993 and 2022 were wet and gloomy. Some people like the rain and clouds. Come spring and summer I’m ready for light, increased sun, sunrises and sunsets, and warmer temperatures to separate from our normal 6 month wet and cool winterspring climate.

But this spring and early summer has been much cooler than those of 1993. However, the current spring / early summer did get a boost of warmth in the last two weeks of June.

It’s also been notably darker in terms of solar radiation compared to recent years (I don’t have similar data for 1993).

Based on this, barring any sudden change in weather for the last two weeks of June, I’d have to say this year takes the cake for weather gloominess.

 

On summer's maximum daily temperatures...

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I looked at the extreme departures from 30-year normals of the daily average temperatures in my last blog post in late June, 2021. This was right after Seattle and the Pacific Northwest endured record high temperatures for several days.

Today, on the National Weather Service (NWS) site there is a warning of warm temperatures ahead for the next few days. They don’t expect the temperatures in Seattle to exceed the 100˚F mark as it did in late June. But they expect a couple of the days will reach above 90˚F. Other areas in the West to the south and east of Seattle are expected to have temperatures exceed 100˚F over the next few days.

In my book, anything above about 85˚F is hot. Maybe not uncomfortably hot, but it’s certainly starting to hit the unpleasantly hot range for me. I grew up in the Great Lakes Midwest, so hot summers are nothing new. But my most recent 38 summers have mostly been spent in Seattle, a city with generally cooler and more pleasant summers than those in the Midwest. I enjoy those and have become accustomed to those temperatures. I’ve become a heat wimp.

However, though not common, temperatures at 85˚F or greater are not rare in Seattle summers. Here’s a look at several charts depicting days – over a very limited 21 year time scale – where maximum temperatures match or exceed the following thresholds: 85F, 90F, 95F, 100F, and 105F at Seattle-Tacoma International Airport, Seattle’s official NWS site of record.

Click on any image below to expand it. Hint: once you expand the first chart, you can navigate to the others using your “left” and “right” arrow keys on your keyboard.