We are writing this on Memorial Day weekend in New York City, with a gray sky pressing down on the parks and the ferry lines. You planned outdoor time. Maybe a long walk, a rooftop, time with family. Then the forecast slid toward showers again. If it feels like weekends here steal your sun, the frustration is justified. The pattern shows up in the numbers, not only in group chat complaints.

This article builds on an independent data investigation by Trace Cohen (@Trace_Cohen), who mapped New York rainfall by day of week, traced storm timing across the Midwest and Northeast, and published an interactive dashboard at Value Add VC’s NY Rain Data project. We extend that work with additional analysis on air pollution timing, upwind sources, and what repeated late-week rain means for vitamin D and mood. Full credit to Trace for the core charts and the storm-track framing; see his posts here, here, and here.

Six years of daily data: Friday tops the rain leaderboard

Across 2,192 days of weather records (roughly six years), the share of days with measurable rain in New York City varies sharply by weekday. Monday: 27.8%. Tuesday: 32.9%. Wednesday: 31.8%. Thursday: 34.5%. Friday: 38.0%. Saturday: 35.8%. Sunday: 32.3%. Friday is the rainiest day of the week. You are not imagining a wet end to the workweek.

Chart titled The Weekly Pattern: Friday rain rate 43.6% in Trace Cohen’s visualization; Sunday has the highest average volume when it rains. Credit Trace Cohen.

Trace’s visualization sharpens the same story with a slightly different sample window: Friday rains about 44% of the time in his chart, the highest frequency in the week. Frequency and volume diverge. Sunday has the lowest chance of rain but the highest average rainfall when storms arrive (about 0.170 inches in his graphic vs. near 0.096 on Friday). Thursday often looks like the lightest rain day by volume. For someone trying to protect outdoor vitamin D synthesis, that split matters: a drizzly Friday can still block UVB windows, while a dry-looking Sunday forecast can still deliver a soaking system.

Summer weekends: 27 of 38 got rain

Zoom in to June through August and the weekend sting gets sharper. Trace’s summer grid counts 27 of 38 weekends with rain from 2023 through 2025. In 2023, rain hit 12 of 13 summer weekends. 2024 was wet but slightly less relentless. 2025 looked drier by comparison, yet many of us still remember planning around radar loops. Color intensity on the chart maps to total weekend inches: the darkest cells are the weekends that wash out plans entirely.

Grid of NYC summer weekends 2023–2025 showing 27 of 38 weekends with rain; darker blue means more rainfall. Credit Trace Cohen.

For vitamin D, summer weekends are prime synthesis time when the sun is high and skin is exposed. Losing even one clear Saturday can matter if your weekday routine is indoors. Observational work links low 25-hydroxyvitamin D status to lower mood and fatigue in many populations; a 2013 meta-analysis in the British Journal of Psychiatry reported an association between depression and vitamin D deficiency, though causality is still debated. Rain does not erase vitamin D on its own, but it pushes you back inside, under glass that blocks UVB, and into the gray-light mood state many New Yorkers know well this time of year.

Storms do not start in Manhattan: the 790-mile, 3.5-day track

Trace’s second line of argument is meteorological, not statistical. Mid-latitude low-pressure systems often organize over the Rockies and central Plains early in the week, then ride the prevailing westerlies and jet stream toward the East Coast. His storm-track graphic labels the path: genesis on Monday, Chicago on Monday, Pittsburgh by Wednesday, Philadelphia on Thursday, New York by Friday. Distance from Chicago to NYC is about 790 miles; typical storm motion in that flow is on the order of 20–30 mph, which is roughly 3.5 days of travel. A system that peaks in Chicago on Monday can reasonably peak in New York on Thursday or Friday, right as the weekend starts.

Infographic: storm forms Monday over Rockies, reaches NYC by Friday along jet stream, 790 miles at ~25 mph. Credit Trace Cohen.

When we line up rain rates by city and weekday in the same multi-year window, the eastward march is visible: Chicago peaks Monday (33.2% rain days), Pittsburgh Wednesday (34.1%), Philadelphia Thursday (35.2%), NYC Friday (38.0%). That does not prove every Friday shower started in Colorado on Monday, but it matches how synoptic forecasting textbooks describe mid-latitude cyclone movement in the Northern Hemisphere. Weather is chaotic; individual weeks break the pattern. The multi-year average still leans Friday in NYC.

Midweek pollution and the Friday rain hypothesis

Trace highlights a provocative amplifier: weekday air pollution along the storm’s path. We looked at NYC fine particulate matter (PM2.5) by day of week in parallel with the rain series. Wednesday averages about 8.5 µg/m³, roughly 23% higher than Sunday’s near 6.9 µg/m³ in the same period. Those particles can serve as cloud condensation nuclei (CCN), tiny surfaces where water vapor condenses. Under the right moisture and uplift, more CCN can change cloud droplet numbers and influence precipitation efficiency and intensity. Reviews of aerosol–cloud interactions, including work summarized by the Intergovernmental Panel on Climate Change, treat anthropogenic aerosols as a real forcing on cloud properties, with regional and weather-dependent sign and magnitude.

A simple lag story fits the weekly rain curve if you treat it as a hypothesis, not a law: midweek pollution buildup → late-week cloud formation → elevated Friday and Saturday rain risk. NYC is not an island atmospherically. A large fraction of PM2.5 can arrive from upwind. The I-95 corridor (Philadelphia, Trenton, Newark, and surrounding metros) sits southwest of the city, and prevailing winds often transport pollution northeast toward Long Island and Connecticut. In that frame, New York is near the end of an aerosol pipeline that intensifies as storms ingest air masses that crossed multiple urban cores at weekday traffic levels.

What COVID lockdowns did to the weekly rhythm

One of the cleaner natural experiments is 2020. When commuting collapsed, traffic-related emissions fell sharply in many U.S. cities. Satellite and ground studies documented drops in nitrogen dioxide and related pollutants; a 2021 study in PNAS quantified large, rapid reductions in transportation-linked air pollution during early lockdowns. In our NYC PM2.5 series, the normal seven-day aerosol cycle flattened: the midweek peak weakened when vehicle miles dropped. When traffic returned, the weekly pollution rhythm came back. That does not prove tailpipes alone cause every Friday shower. Weather remains multi-factor. It does suggest human weekly activity is entangled with weekly air chemistry, and that entanglement is worth serious study alongside storm tracks.

We want to be explicit: causality between urban PM2.5 and local weekend rain totals is not settled science. Experts in cloud microphysics, regional modeling, and air quality should stress-test this chain. If you work on aerosol indirect effects or Northeast corridor transport, Trace’s dashboard and this weekly pattern are open invitations to dig in. Contact paths are on his X profile and the NY Rain Data site.

Why this belongs on a vitamin D science page

Rays exists because sun exposure is the main natural source of vitamin D for most people, and because timing that exposure is harder than it sounds. Cloud cover and rain reduce UVB reaching skin. Glass blocks UVB entirely. When Friday and Saturday skew wet, you lose the days many office workers finally have free. When Sunday delivers fewer rainy days but heavier totals, you can still end the weekend with little meaningful synthesis despite “better odds.”

Blood 25(OH)D reflects weeks and months of behavior, not one ruined barbecue. Still, repeated weekend losses push reliance toward diet and supplements. For adults already below 30 ng/mL (75 nmol/L), missing sun blocks makes repletion slower. Seasonal mood matters too: light exposure influences circadian timing and may interact with vitamin D pathways in ways researchers are still mapping. The NIH Office of Dietary Supplements notes that insufficient vitamin D is common and that sun remains a major contributor for many. A rainy Memorial Day weekend is not a clinical emergency. It is a recurring nudge away from the outdoor photons your skin uses to make cholecalciferol.

What you can do when the weekly forecast fights you

Check Thursday when you need a planning edge. Trace’s charts and our aggregates both show Thursday as relatively lighter on rain volume, even when frequency is only middling. Use mid-morning to mid-afternoon windows when the sun is above roughly 35° elevation if the sky breaks. On wet weekends, prioritize food and supplement strategies discussed in our other science posts, and test 25(OH)D at least twice a year if you are optimizing status.

Explore the full interactive view: animated storm tracks, city-by-city charts, and live updates at valueaddvc.com/ny-rain-data. Again, that work is Trace Cohen’s; we are adding interpretation and the vitamin D lens.

Key Takeaways

Six years of NYC daily data put Friday at the top for rain frequency (about 38% of days), with Saturday close behind. Sunday often rains less often but can dump more inches per event. Summer weekends from 2023–2025 were wet on 27 of 38 in Trace Cohen’s count. Storm systems commonly travel east over several days, which can align Midwest peaks with NYC rain on Thursday and Friday. Midweek PM2.5 is higher than weekend levels in NYC, and lockdowns temporarily flattened that weekly pollution cycle. The link from aerosols to local rain is plausible and testable, not proven. Lost sun on free days makes vitamin D and mood harder to manage without a deliberate plan.

What to do next

When the forecast finally opens a clear window, use our vitamin D calculator to estimate how long to be outside for your skin type and location. For ongoing tracking without manual logs, Rays detects time outdoors and models synthesis so you can see whether you are actually recovering from a string of rainy Fridays. Share Trace’s dashboard with anyone who swears the city schedules storms for the weekend; the data are on their side.