The final moments before landing at Reagan National Airport show off a beautiful scene of a snowy D.C. In this image, The Washington Post is slightly to the right of the towering Washington Monument, about halfway up its length. (Joseph Gruber via Flickr)

A debate has long raged about how representative Reagan National Airport’s temperatures are of downtown Washington. Even though the airport is located in Virginia, it is Washington’s “official” weather station — which bothers some people. How can a weather station in Virginia represent D.C.?

We decided to settle the score.

During the summer of 2016, the Capital Weather Gang installed its very own weather station downtown on The Washington Post’s roof at 1301 K St.

It has now been about a year and a half since the weather information began pouring in at Capital Weather Gang headquarters at The Post building — long enough to compare the data between the airport and our spot downtown.

The short story is that National and The Post station downtown run very similar to each other. There is a noticeable difference with low temperatures, although the colder of the two locations may surprise you.

Note, before getting started: This is a tough or impossible task if looking for a perfect comparison. That comparison doesn’t exist. The stations are not sited the same. National (KDCA) is set up per regulations per the Federal Aviation Administration (FAA) and National Oceanic and Atmospheric Administration (NOAA), which include a temperature sensor at two meters above ground, among other standardized items. The Post’s station — while excellent at reading what it reads — is on the top of a 12-story building. It’s not a perfect representation of ground temperatures.

This all said, it’s good enough . . .

The shortest story

Here we compare data from when the Post station went online June 23, 2016, through Nov. 30 of this year. The two stations are located 3.8 miles apart, The Post roughly north-northeast of DCA in a straight line. You can see the airport from The Post’s roof.

At National, the average high during that period was 72.8 degrees, compared to 72.7 degrees at The Post — remarkably close. When it comes to low temperatures, the gap was considerably larger. National’s average low was 55.9 degrees, compared to 57.3 degrees at The Post. This is perhaps a mostly expected result, as research has shown that rooftop stations tend to run warmer than their ground-based counterparts.

There should also be some consideration of the urban heat island here. The Post is arguably more in the middle of the concentration of the heat-retaining asphalt and concrete than the airport, however slightly so.

When it comes to precipitation, there are plenty of instances of disparate totals on a day-to-day basis, especially during the warm months of the year. But in the end, the precipitation total for the period was 49.54 inches at National and 45.83 at The Post. Basically within the margin of error you might expect given hit-and-miss thunderstorms and such.

Temperature ranges for highs and lows

As noted, the high temperature readings at the two locations have been extremely close. Bucketing them by 10-degree ranges, as in the chart below, the similarities are much more apparent than the differences.

High temperatures at The Washington Post and the official station at Reagan National Airport, compared. (Ian Livingston/The Washington Post)

One difference that draws the eye is the lack of 100-degree readings at the Post.

100-degrees might seem common these days, but they still are somewhat rare. Conditions need to be just about perfect. Since the Post station was installed, the first instance of triple digit heat in the region was on July 25, 2016. The Post “only” hit 99 when DCA hit 100. During the exceptional spell of three 100+ in a row in 2016, from the 13th to the 15th of August, the Post was only able to manage highs of 98 and 99. Almost chilly. Perhaps those runways and the Crystal City wind help a bit.

When it comes to lows, there is again similarity, although not at all ranges. There are at least two patterns in the chart below. One for the warm months, the other for the cool.

Low temperatures at The Washington Post and the official station at Reagan National Airport, compared. (Ian Livingston/The Washington Post)

During summer, when the D.C. area gets into stretches of persistent ultra-muggy lows in the 70s or higher, the two stations tend to run fairly close to each other. Disparity increases notably in the cold months. I’ll get to that in greater detail below, but look at the 30s. National had 16 more lows in that range than The Post did.

And it doesn’t stop there. National is consistently chillier than The Post on the coldest and the very coldest nights.

More on the divergence in low temperatures: It’s a cold season thing

Another way to visualize the difference in low temperatures between The Post and DCA is to look at how much warmer or cooler one station is compared to the other. In this case, subtracting DCA from The Post. For example, on a morning where the low is 72 at National and 71 at The Post, the graph will have a red bar extending upward one degree from zero (zero is if the stations have the same low).

The main area of divergence in the locations when it comes to temperature is at night during the cold season. (Ian Livingston/The Washington Post)

You don’t have to look at this one very long to see a pattern. In fact, during last winter, The Post didn’t have a colder low than DCA even one time.

Although we only have one full winter of data, the “National chill factor” peaked in the heart of winter, as you might expect. The biggest range between the two stations was nine degrees on Feb. 6, 2017. It was 33 that morning at National, and 42 at The Post. Other studies of this phenomenon point to the fact that radiational cooling, where heat evacuates the surface, is faster near the ground than well above it. Further, low-level inversions — where warm air sits above cool — can be quite strong in the winter.

We must qualify this analysis by noting we could only examine a short period of data, so this analysis could be extended in the future. As we get additional months and years of information, we can also begin to build a reliable baseline and examine other facets of the relationship between the measuring sites.

If nothing else, this analysis should help give some confidence to the readings obtained at the airport as relatively representative of downtown, especially when it comes to high temperatures.

However, this analysis in no way indicates National’s temperatures are representative of the surrounding Washington region or even the colder areas of the District itself, including northwest Washington. The airport is consistently one of the warmest spots not only in the D.C. area but also the Mid-Atlantic on clear, calm mornings.

Also, this analysis does not address the long-standing issues with snowfall measurement at National and the fact it has often not been reflective of surrounding measurements.

Read more

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