The Little City May Be Home to a Little Different Climate

By William Henneberg
Special to the Falls Church Times
February 6, 2016

Falls Church City is small, a virtual micro-city compared to the neighboring large counties. But within its narrow confines, our micro-city also may house a small and somewhat rare weather feature known as a “microclimate.”

The National Weather Service defines a “microclimate” as “the climate of a small area…that may be different from that in the general region.” In other words, a microclimate is present when weather patterns exist that are unique to a given area.

The most well-known microclimate type is the “urban microclimate,” which is caused by pavement, buildings, and dense development, and leads to what is known as the “urban heat island effect.” In the Washington area, this explains why the weather station at Reagan National Airport consistently reports warmer observations than the more vegetated suburbs only a few miles away (particularly at night). In Falls Church, too, the climate of the more urbanized areas (warmer) differs from that of the green spaces (cooler) and that is where this analysis begins: the downtown corridor.

There is no historical temperature record for downtown Falls Church, but evidence for a warmer downtown climate came two summers ago when a thunderstorm popped up over the city. When an area is warmer than its surroundings, the hot air rises and, if the rising parcel of air can sustain itself (with enough moisture present), a “heat island thunderstorm” can develop. This seems to have been the case on August 2nd, 2014, as a storm initiated over the most urban (and likely warmest) part of Falls Church, the intersection of Broad and Washington streets.

Radar time sequence of a thunderstorm over Falls Church City, August 2, 2014.

This makes the case for an urban microclimate over the downtown area. But microclimates are not limited to anthropogenic (human) causes. Small elevation changes, which are common throughout the Virginia Piedmont, of which Falls Church is a part, also can influence precipitation and temperature patterns. And there is some temperature data that supports this idea. But first we must take a look at the topography of Falls Church.

The general topography of Northern Virginia is higher elevation to the west (Appalachians and Blue Ridge), sloping downward to the southeast to the Chesapeake Bay and Atlantic Ocean (below, left). Falls Church lies more or less on the “fall line” — where the hilly Piedmont region transitions to the flat Coastal Plain. Far western areas of Falls Church tend to be higher than central and eastern areas (the elevation of Fort Taylor Park, the Koons Ford car dealership, and the Eden Center in East Falls Church is higher but is a small blip compared to the overall downward slope). This helps explain why the August 2nd storm propagated to the west, as the slight upslope tendency may have encouraged continued development towards the northwest (and most of East Falls Church never saw a drop of rain).

Topography of Northern Virginia and Falls Church. Note the elevation decrease from northwest to southeast. Red dots on the Falls Church image indicate the location of two weather stations. The East Falls Church station is at 317 feet while the West Falls Church station is at 387 feet.

During the August 2nd event, western Falls Church would have ended up much cooler than the east due to the storms, and not necessarily because of a microclimate, making the analysis tricky. But can small elevation changes alone influence the temperature, independent of rain?

Shenandoah National Park, 65 miles away, is filled with examples of natural, topographically induced microclimates. A 2007 climate summary identified temperature changes between even small hills and valleys in the park. On clear, calm nights, cold and denser air flows to lower elevations in the valleys. This “cold air pooling” can shorten the growing season for lower elevations (and explains why Thomas Jefferson was able to grow artichokes at Monticello, on top of a hill).

At other times, however, adiabatic cooling (decreasing temperatures with increasing elevation) dominates, and higher elevations are cooler than the surrounding valleys. In other words, pressure and temperature are proportional — so the higher surface pressure at lower elevations increases temperatures. That is known as “compressional warming.”

Here in Falls Church, data from two privately-owned weather stations reproduce these natural microclimate phenomena at a smaller scale. The table below shows monthly data from a station in far western Falls Church (elevation of 387 feet) and one in East Falls Church (elevation of 317 feet). Note that during the winter and summer months (January and July), the East Falls Church station (EFC) is warmer.

During these months, prevailing westerly winds dominate, the air is well-mixed and adiabatic cooling/compressional warming keep the higher elevations cooler and the lower elevations warmer. During the “transition seasons” of fall (October) and spring (March), however, the wind direction is much more variable and the westerlies do not always dominate. The compressional warming is limited and instead, the colder, denser air masses can find their way to lower elevations in the middle and eastern parts of the city, similar to the “cold pooling” that occurs in the Shenandoah Valley. Higher elevations of West Falls Church can actually be warmer than lower elevations.

Temperature data from two station sites depicted as AvgTemp./Monthly High/Monthly Low (°F). Red text indicates the warmer station and * denotes incomplete data.

With no official threshold, there is no set standard for how “different” the climate must be from the surrounding region to constitute a true microclimate. In the situations mentioned here, it ranges from a few tenths of a degree to several degrees on any given day (keeping in mind the data is too sparse spatially and temporally to make a statistical significance analysis).

Nonetheless, there appear to be three microclimates within the Little City: the downtown corridor, which is likely the warmest spot year round; the lower elevations in central and east Falls Church, which seem to be distinct from the higher elevations in west Falls Church (and Mount Taylor in East Falls Church) depending on the prevailing wind conditions (below).

Interestingly enough, the Washington & Old Dominion Trail goes through all three areas, so, the next time you’re riding, running, or walking through the City, you may just be able to notice a little bit of each.

Microclimates of Falls Church. Blue – the higher elevations of Falls Church, Black – lower elevations of central and eastern Falls Church, Red – downtown urban corridor.

William Henneberg is a lifelong resident of Falls Church City. He holds a Master’s degree in Meteorology from Plymouth State University in Plymouth, N.H. He currently works as an operational meteorologist for Commodity Weather Group, LLC, in Bethesda.