The Weather of New York City

I’m writing this blog from a hotel in Greenwich Village in New York City! I flew in here on Wednesday afternoon to spend a few days in the city before going to the Hamptons Jazz Festival on Long Island, a summer-long jazz festival hosting some of the biggest names in jazz. Unfortunately, I was feeling sick Saturday morning and in a cruel twist of fate, I had contracted COVID! So I’m cooped up in my hotel room for the time being and have had to delay my trip back home, but thankfully I was able to see a lot of cool things during my time here before contracting the disease.

While I’m isolating, I figured I’d write a blog about the weather of New York City. I’ll start with a “synoptic-scale” (weather-speak for large-scale) discussion of the climate of New York City and the east coasts of midlatitude regions in general, then I’ll move into some of the terrain near New York and how that impacts New York’s weather. Finally, I’ll end with a discussion on New York City’s “urban heat island” and the microclimates of New York City’s boroughs.

Climate Classification

Using the Köppen climate classification system, New York City is right on the border between a “Humid Subtropical Climate” and a “Humid Continental Climate,” with hot, humid summers, cool, wet winters, and a moderate amount of rainfall evenly spaced throughout the year. During the summer, southwesterly winds bring humid, unstable air from the Gulf of Mexico and Atlantic Seaboard north into the area, with occasional thunderstorms and brief but very heavy periods of rain. During the winter, prevailing winds are from the west, bringing alternating periods of mild, wet weather when storm systems move through the area and dry, cold, northwesterly winds in the wake of strong cold fronts. On occasion, the East Coast sees “Nor’Easters,” which are rapidly-intensifying storms that rush north along the Eastern Seaboard and can bring torrential rain, gusty winds, and blizzard conditions from the Mid-Atlantic all the way into Labrador and Newfoundland.

The stark difference in summertime humidity between coasts is due to ocean temperatures along the East Coast being much warmer than those along the West Coast. As the below image of current sea-surface temperatures (SST) shows, warm water tends to pile up in the western portions of ocean basins due to easterly direction of the trade winds, and this water then moves northward along the western boundaries of these ocean basins (or east coasts of continents) in strong, warm currents, bringing warm, muggy, subtropical air into the midlatitudes. These “Western Boundary Currents” – such as the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific, are some of the strongest currents in the world and are one of the principal ways in which heat gets transferred from the tropics to the higher latitudes.

As a side note, the reasoning behind the film “The Day After Tomorrow” was that global warming would change the salinity of the ocean via melting ice, and this would halt these currents entirely, resulting in very little heat transport to the polar latitudes and the dawn of a new “mini” ice age. Such a phenomenon may have actually occurred at the end of the last ice age, with meltwater from North American ice sheets disrupting the Gulf Stream (read: The Younger Dryas). A slowing of the Gulf Stream is predicted to occur as a result of climate change, resulting in comparatively less warming for the North Atlantic than other locations. But make no mistake – the Earth is still expected to see extraordinarily abrupt and disruptive warming over the next 100 years, especially over the arctic regions.

Additionally, the East Coast of the US a much more active storm formation region than the West Coast. This is because mid-latitude cyclones derive their strength from horizontal temperature gradients at all levels of the atmosphere, and, as the image above shows, the horizontal temperature gradients along the East Coast are consistently much stronger than the West Coast due to the presence of the Gulf Stream offshore and cold, continental air inland. The same can be said for Eastern Asia, where the combination of the warm Kuroshio Current and frigid air coming off Siberia results in frequent cyclogenesis. Meanwhile, the temperature contrasts are generally much weaker in eastern ocean basins in the midlatitudes, where this cold, continental air has moderated and the warm currents have dissipated. We still can see intense, “Bomb Cyclones” along the West Coast, but they are nowhere near as frequent as those impacting the Northeast US and far eastern Canada.

Topography

Compared to cities like Pittsburgh or Syracuse, New York is partially shielded from brutally cold air from the Appalachian mountains. However, this shielding is nothing like the protection that Seattle sees due to the Rockies and Cascades, and since the weather – like that in Seattle – typically arrives from the west, it is not moderated by the ocean before it gets to the city. As a result, New York City – and most places from the Mid-Atlantic Coast northward – can see much colder weather than Seattle on a much more frequent basis. New York City’s lowest temperature ever recorded was -15F on 2/9/1934, and freezing temperatures have been recorded in Central Park as late as 5/6.

Northeast US Topographic Map
Credit: Nations Online Project

During arctic outbreaks, the very coldest air will move into NYC via the Hudson River Valley, which is near sea-level all the way to Lake Champlain in Upstate New York. Outside of NYC, Long Island can see bouts of “sea effect snow” when arctic air moves over the comparatively mild ocean, picks up moisture, and forms snowbands.

Urban Heat Island

The “Urban Heat Island” is the phenomenon where cities are warmer than the surrounding areas due to differences in land surfaces. For example, asphalt absorbs much of the heat that it receives from the sun, and the lack of vegetation and water means there is little cooling due to evapotranspiration. Additionally, skyscrapers provide additional surface area for heating (this is known as the “urban canyon effect”), and those same skyscrapers help reduce winds at the surface, preventing cooler air aloft from mixing down to the surface. Though the urban heat island helps increase daytime highs, it has a much larger impact on overnight lows, as all of the excess heat absorbed during the day slowly radiates back out to space, keeping the area warmer than its surroundings.

In a 2006 study, NASA created high-resolution images of New York City’s urban heat island and vegetation density using their LANDSAT satellite. As you can see, green areas, such as Central Park, were cooler than the surrounding regions, while heavily paved surfaces, such as JFK Airport in Southern Queens and LaGuardia Airport in Northern Queens seeing locally higher temperatures.

However, I was very surprised to see that Midtown and Lower Manhattan were slightly cooler than many other parts of the metropolitan area. Anecdotally – both from walking around Brooklyn and Lower Manhattan at night and talking to lifelong residents, Manhattan seemed far “stuffier,” with noticeably less wind due to drag from the skyscrapers. I took a quick look at weather stations across New York’s boroughs for the last week and did not see a consistent difference in temperatures across the boroughs, so the reason for Manhattan’s higher stuffiness is likely due to lesser wind speeds there. I’m sure there is a bit of a psychological aspect as well.

Summary

On a synoptic-scale, New York sees much more dramatic seasonal swings than the Pacific Northwest, with cold continental air in the winter, hot, humid air in the summer. Additionally, the combination of the warm Gulf Stream offshore and the cold, continental westerlies makes the East Coast of the US a much more active extratropical storm formation region than the West Coast, as extratropical cyclones derive their strength from strong horizontal temperature gradients. The mountains to the west of New York help prevent the very coldest air during arctic outbreaks from making it into the city, but since the Appalachians are relatively low and are bisected by the Hudson/Mohawk River Valleys, New York still sees relatively little protection from arctic blasts and can experience subzero temperatures.

Finally, New York City experiences a significant urban heat island, but contrary to what you might expect, the strongest heating is not over the heavily urbanized areas of downtown Manhattan but over the more urbanized areas away from downtown. However, most long-time residents (and myself, as a tourist) have noticed that Manhattan feels “stuffier” at night, and this may be due to the skyscrapers dramatically reducing wind speeds at ground level, and/or due to the psychological aspect of being surrounded by these buildings in the first place.

One thing New York City does have plenty of is air conditioning, so if you need relief during the heat of the day, it’s generally not too hard to find!

Charlie