Could cities be linked to pesky summer afternoon downpours? - A Richmond case study
Written by Aasma Acharya
Have you ever stepped outside on a summer afternoon and felt the pavement radiating heat like an oven? Then, by early evening, the forecast mentions “isolated storms”?
Many Virginia summer days follow that pattern: sweltering afternoons followed by sudden, sometimes intense, late-day downpours. But what if these aren’t just passing weather quirks? What if the heat and the rain are connected parts of a larger, climate signal hiding in plain sight?
I set out to explore this pattern through the lens of compound extremes, as part of my master’s research at George Mason University. Focusing on Richmond, a midsized city with a dense urban core and growing suburbs, we asked how the Urban Heat Island (UHI) might shape not just how hot cities feel, but where and how intense rain falls.
We examined 11 summers of data from 2011-2021 to see how Surface Urban Heat Island (SUHI) links to extreme rainfall rates) across the Richmond region. SUHI can be thought of as “how much hotter the city’s surfaces are than surrounding areas.”
Quantifying Urban Heat: How Hot Does Richmond Get?
On typical summer days, Richmond’s human-built surfaces run about 1–2°C (1.8–3.6°F) warmer than surrounding areas by late afternoon. On the hottest days, urban areas can spike to approximately 4–5°C (7.2–9°F) hotter than their rural counterparts during the afternoon. The warmest zone centers on city centers and often leans downwind in the afternoon. The mean map (Figure 1) shows the typical urban-rural contrast, while the diurnal boxplot (Figure 2) reveals the daily temperature climb by hour and occasional spikes.
Figure 1: Mean Surface Urban Heat Island (SUHI) intensity (°C) on clear-sky summer afternoons (11 years). The dot marks Downtown Richmond, which consistently shows the strongest heat signal. The dashed polygon outlines the urban core (Global Human Settlement Layer built-up surface). The 53 km circular domain includes the core city plus surrounding suburban and rural buffers to enable urban–non-urban comparison.
Beyond uncomfortable heat and higher energy bills, these afternoon heat peaks can give storms extra fuel, raising the odds of short, intense downpours. This is because hot surfaces warm the air just above them causing the air to rise and surrounding air to replace it, what we call convergence. This leads to strong updrafts that help to build thunderstorms and thus heavier rain, often downwind of the city’s hottest area.
Figure 2: This boxplot shows the hourly Surface Urban Heat Island (SUHI) intensity (°C) during clear-sky summer afternoons over 11 years. The analysis is limited to the urban core of the Greater Richmond Metropolitan Area, as defined by the built-up surface extent in Figure 1.
A Closer Look at Urban Heat’s Signature on Extreme Rain
To understand how strong urban heating affects local extreme precipitation, we identified the hottest urban afternoons, hours when SUHI exceeded the seasonal average, and examined the hours immediately after those heat spikes. We then compared those “hot-city” hours to typical summer hours (hours with below-average SUHI for that season).
A consistent pattern emerged: when the city runs hot, the heaviest rain tends to hit harder southeast of the city, which is also downwind (Figure 3). Downwind of downtown Richmond, the extreme-rain anomaly is higher than on typical hours, with local peaks up to ~30%, while upwind areas change little or even dip slightly. This aligns with the idea that heat and moisture are carried downwind from the city before storms develop.
Figure 3: Extreme-rain intensity (% anomaly) on hot-city hours compared to typical clear-sky summer hours (computed using RI95 = top 5% of hourly rain rates from 2011–2021). The anomaly is the percent difference between the hot-city composite and the typical composite. Blue tones indicate heavier extremes than typical (up to ~30% higher), while red tones indicate lighter extremes. The dashed polygon outlines Richmond’s urban core (GHSL built-up surface), the arrow shows the average mid-level wind (~700 hPa), and the grey circle (~53-km radius) defines the analysis domain. The strongest positive anomalies appear downwind of the urban core, whereas upwind sectors hover near zero or negative.
Why It Matters: Small Shifts, Big Impacts
A ~30% bump in the intensity of the heaviest hours downwind may not sound dramatic at first, but that’s a substantial local shift, especially when it repeats over the same neighborhoods. Over time, those increments add up and raise the risk of pluvial flooding, particularly where drainage is already stressed. Concentrated rain in impervious zones can overwhelm stormwater systems in fast-growing suburbs just downwind of downtown.
Hotter afternoons plus sudden, localized rain are a recipe for street flooding where paved surfaces prevent infiltration and strain storm drains. Knowing where and when that combination occurs helps target tree planting, cool roofs, and stormwater upgrades where they’ll pay off most.
Richmond creates a powerful case study. It's not as large as NYC or DC, but it has the right mix of dense urban infrastructure, sprawling suburbs, and increasing summer heat. More importantly, this research shows that you don’t need a massive metropolis to affect local weather.
Our work raises practical questions that cities across the U.S. should be asking: Are local heat islands creating new flood risks? Can cooling strategies and smarter drainage reduce that risk? As compound extremes like “heat followed by rain” become more common, understanding the local urban imprint now will help to design more resilient neighborhoods across Virginia in the future.
This research was conducted as part of Aasma Acharya’s master’s thesis in Climate Science at George Mason University’s Department of Atmospheric, Oceanic, and Earth Sciences: “Assessing the Surface Urban Heat Island and Urban Extreme Precipitation Anomalies: An Analysis of Richmond, Virginia” under the supervision of Dr. Zafer Boybeyi and co-advised by Dr. James Kinter and Dr. Luis E. Ortiz. View Aasma’s data sources and references here.
Author
Aasma Acharya
Aasma is a Summer 2025 graduate of George Mason University’s MS Climate Science program.
