Nighttime heat is the star of summer in NYC

As June comes to a close and summer gets into full swing, we're entering the warmest month of the year in New York City. Both our average high and low temperature are at their warmest in July; the high temperature averages 85°, and the 30-year average for low temperature is 70°. At summer's peak, we don't get a lot of relief from the heat during the overnight hours. But it hasn't always been that way. 

The chart below shows the average low temperatures in July at Central Park's official observation site, which has been at Belvedere Castle since 1920.

  

Yes, that incredible building is located inside of the park!
Photo of Belvedere Castle from Central Park Conservancy

Low temperature data from NWS New York City

Other than a brief dip in low temperatures in the early 2000s, the upward trend in overnight lows has been pretty consistent. Follow the trend line, and you'll see an increase of about 5 degrees since 1920. 

The Urban Heat Island effect is one of the reasons for this increase. Central Park may be green this time of year, but it is surrounded by paved roads (there's even a few paved roads within the park) and high-rise buildings. Dark colored pavement absorbs heat during the day and slowly releases it after sunset; asphalt is one of the most efficient substances on Earth for absorbing heat from the sun. When the Belvedere Castle observation site was established, uptown NYC looked like this: 

Images of Broadway at 90th & 96th St in 1920 from the Museum of the City of New York

Most side streets are still unpaved, and even the main thoroughfares are not as busy as they are today. In the 21st century, there are more cars and high rises in Manhattan than ever before. Every automobile and building adds an incremental amount of heat, and in the city that never sleeps, all that infrastructure keeps pumping in the heat well after sunset.

Another warming factor is humidity, which is higher in the Tri-State area when evaporation increases off the waters of the Atlantic Ocean and Long Island Sound. It's a vicious cycle that leads to steamy summer nights: warmer air holds more water vapor, and water vapor molecules hold onto almost double the amount of heat energy of dry air (the fancy term for this in physics is "specific heat capacity"). As temperatures have risen in New York in recent decades, so has the humidity, as this dew point graphic from Climate Central shows:

Data from Oak Ridge National Laboratory, graphic by Climate Central

Higher dew points = higher moisture, which means more heat is being stored in the air, both day and night. 

We've managed to make it through June without a heat wave (a stretch of at least 3 consecutive days with high temperatures in the 90s), but July is the most likely month to produce these hot stretches. If you don't have air conditioning at home, make sure you know the location of your nearest cooling center, and follow the National Weather Service's tips for beating the heat. 

NYC is showing its (warming) stripes

 It’s finally here, the 3rd and final “first day of summer”, otherwise known as the summer solstice. As we discussed a few weeks ago, many people consider Memorial Day to be the unofficial start of the season, and June 1st is the start of meteorological summer. But in climate science, today is also "Show Your Stripes Day". Your favorite TV meteorologist might show a version of the graphic below in their weathercast today, or perhaps it's prominently displayed in one of their social media posts:

The "Warming Stripes" for Central Park.
Graphic courtesy of Climate Central

These stripes represent the changing average temperature, either globally or from a single location, depending on which version is shown. The version above is from Central Park reporting station, which has been active since 1869. It’s a highly visual way to demonstrate a somewhat abstract concept: temperatures in NYC have risen quite a bit since record keeping began here. Each stripe on the graphic above represents a temperature departure from the 20th century average. The deeper blues are much colder than average, whereas the red stripes are much warmer. In this article, we’ll break down the stripes further.

Notice the concentration of red stripes on the right side of the graphic, representing an upward trend in temperatures. In fact, both 2021 and 2022 made Central Park's list of top 10 warmest years on record. Here's the full list:

Warmest Years at Central Park (1869 to present)
	Temperature	Year(s)
1	57.3	2020/2012
2	57.2	2016/1991/1990
3	57.1	1998
4	57.0	1953
5	56.9	1949/2021
6	56.8	2006
7	56.7	2010/2015
8	56.5	1999
9	56.4	2011/2002
10	56.3	2017/2022

These temperatures are computed by averaging the high and low temperature for each day of the calendar year, then computing the average of those numbers. The 21st century dominates the list, even though it's only 2023. Let's compare the warmest list to the list of record coldest years in New York: 

Coldest Years at Central Park (1869 to present)
	Temperature	Year(s)
1	49.4	1888/1875
2	50.2	1883
3	50.6	1885
4	50.7	1917/1893
5	50.8	1873
6	50.9	1904/1887
7	51.2	1926
8	51.3	1886/1872/1871
9	51.4	1874
10	51.7	1882

It's basically the opposite of the record warm list, as most of these records were set in the 19th century! And putting the two datasets on the same line graph looks downright weird. See below: 

But it does bear a resemblance to the "Warming Stripes" from the beginning of this post:

2023 is on track to add another red stripe to this image. We've already had the warmest January on record in Central Park, and April will go down in history as the 2nd-warmest (the charts, last updated in 2021, are available here). If you want more information about the warming stripes, visit climate scientist Ed Hawkins' website- and for much more on global average temperature changes, Hawkins' University of Reading in England has a wealth of information on their site. 

Summer's here! Here's the weather we're expecting, and why

Depending on which summer you observe (meteorological, astronomical, or holiday), summer has either already begun or it starts next week. Meteorological summer started on June 1st; astronomical summer begins with the Summer Solstice, which this year is on June 21st; Memorial Day kicks off the summer season for beach towns in the Tri-State. This is the time of year when we see the most thunderstorms and everything that comes with them, like lightning, hail, and damaging winds. Here's a breakdown of common summer weather phenomena in the Tri-State, including thunderstorms, and why they happen more often this time of year.

Seabreezes
Seabreezes are great for cooling us off on hot days at the beach. They're most common on sunny days early in the summer, when ocean temperatures are in the 50s and 60s. This is because seabreezes are dependent on a temperature difference between ocean and land. The seabreeze often doesn't "kick in" until the afternoon for this same reason.
As land near the ocean heats up after sunrise, the air rises. Remember that warm air is less dense than cold air. So as the warming air rises over land. cooler denser air above the ocean rushes in to replace it. And, since that air is denser than the rising warm air, it stays close to the surface (or, in this case, the sand). This is what we feel as a seabreeze. 

Along with a seabreeze, you'll often see puffy clouds form near the water's edge 

when ocean temperatures are cooler than they are inland. (photo taken by me)

Rainbows
Of course, rainbows are not exclusive to the summer months, but they're a lot more common in the Tri-State this time of year because heat and humidity is higher in the summer. This combo is conducive to the formation of convective showers and thunderstorms in the late afternoon and early evening, when the sun angle is best for rainbows. 
Rainbows are optical illusions, created by sunlight passing through water droplets, which act as tiny prisms to separate the sun's rays into all the colors of the rainbow. The rainbow is then projected in the opposite direction of the setting sun, and it's much easier for us to see it when it's cloudy in that direction. This allows the clouds to act as a white or gray projector screen. 
Since most thunderstorms in the Tri-State move from west to east, and the sun sets in the west, your best bet to see a rainbow is to look eastward after a rain shower or storm. That's what I did when I captured the beautiful image below: 

Did you know that a double rainbow is just a reflection of the main rainbow?
Notice that the top rainbow's colors are in reverse order of the bottom one.

(photo taken by me)

Rainbows can happen early in the day, of course, but we don't see them as often because all the elements described above don't come together. It's pretty uncommon in the Northeast for a rain shower to move from east to west as the sun is rising. And we can forget about a mid-day rainbow; with the sun's rays directly overhead, there's nowhere for the prism's image to go. 
So, why don't we see more rainbows in the winter, when the sun is low in the sky for a longer percentage of the day? The main reasons are:

- Snowflakes don't refract the sun's rays like a prism. Instead, snowflakes appear white because they scatter all the wavelength colors that come from the sun. 

- When it rains in the winter, it usually comes from stratus clouds, which blanket the whole sky. The pop-up convective clouds we see more often in the summer allow the sun to poke through. 

Severe thunderstorms
As mentioned above, convective clouds are more common in the summer than in the winter. That's due to an increase in lapse rates in the Northern Hemisphere summer. In the winter, the temperature difference from the surface to a few thousand feet aloft is smaller, on average, than it is in the summer. When the temperature drops quickly with height, it causes instability. Remember from the seabreeze example that colder air sinks and warm air rises? Higher lapse rates cause this type of vertical instability more frequently in the summer. And when there's enough humidity present, you can get towering cumulonimbus clouds to form, like the one in the photo below. 

Severe thunderstorms often have an anvil shape, like this one. 

This cloud can only form if it's much warmer at the surface than it is aloft. 

Photo taken by 2022 AMS Weather Band Photo Contest runner-up, Jim Conley.

As the word "instability" may indicate, this process and the clouds that result are more conducive to severe thunderstorms. When the clouds grow tall enough, they can displace cold air aloft and cause it to sink rapidly to the ground. This is called a downburst. Hail also forms in clouds like the one above; as raindrops get tossed upward and freeze, they form hailstones, which are eventually carried back to the surface by gravity or surface-bound winds called downdrafts. The rotating columns of air that cause tornadoes are more complicated, but are also a result of an unstable atmosphere. (You can find an awesome webinar about tornadogenesis here.)  

Heat lightning
Before we go any further, let's start with the fact that heat does NOT create lightning, so "heat lightning" is not an actual thing! But I think this misleading moniker comes from its frequent summer occurrence. 

Intra-cloud lightning is sometimes seen, but not heard as thunder.
Photo by 2022 AMS Weather Band Photo Contest runner-up, Brad Nelson

The phenomenon we've come to know as heat lightning is really just lightning that is so far away that we can't hear the thunder. It's more common in the summer, not just because we get more storms at this time of year, but also because it's when storm clouds tend to grow taller. Some of that lightning is happening miles above the surface, inside of the cloud and never reaching the ground. Since this intra-cloud lightning is happening so high up, there's an increased chance it'll be too far away for us to hear. And, water in the cloud muffles the sound of thunder more than air near the surface. The old rule "When Thunder Roars, Head Indoors" applies here too. If you see lightning strikes inside a cloud but never hear thunder, it's likely that the storm is farther away than it looks. 

Why the red sky? Tri State's dangerous haze, explained

 It's been the talk of the town this week. Thick smoke from wildfires in Canada created the stunning headlines on Wednesday, proclaiming (correctly) that, for a time, New York City had the worst air quality in the world. 

Unaltered images of an orange sky above the Queensbridge Houses on Wednesday.
Photos courtesy of Joseph Kurtek

The bad air quality didn't just impact New York. In addition to the Yankees game, the Phillies also postponed their matchup at Citizens Bank Park due to the hazardous air quality, and in my hometown of Bethlehem, PA, the Air Quality Index briefly topped out at 340. For context, "Good" air quality has an AQI of 50 or lower. 

Hazardous air over Bethlehem, PA on Wednesday.

Thanks for the photo, Mom!

Over 8 million acres have burned so far in Canada, and over a half-million acres have burned in Quebec, the province directly to our north. Most of the active fires are at least 500 miles away from the Tri-State area. So, why is so much of the smoke ending up here? Not surprisingly, it's our current weather pattern that's to blame. The satellite image below shows yesterday's smoky plume curved alongside a deck of cloud cover in New England: 

Image from GOES-East of the smoke enveloping much of upstate NY and eastern PA on Wednesday.
Image credit: NASA

Those clouds are from an area of low pressure sitting over the Canadian Maritimes. Meteorologists call setups like these "quasi-stationary" because little to no movement of these systems is happening in the short term. At the same time, an area of high pressure to the north of the Great Lakes was exacerbating the scenario, concentrating the smoke due south and southeast, causing it to beeline to the Tri-State. You can see the setup on the surface map below: 

Notice the location of the isobars, or lines of constant pressure, in eastern Canada.  They are close together and oriented north to south. When isobars are close together, it indicates a stronger wind than when they're spaced farther apart. And, with high pressure to the west and low pressure to the east, it's like a superhighway for Quebec's wildfire smoke to cruise into our area. 

The smoke doesn't even have time to loft high into the atmosphere; instead, those winds carry the hazy plume near the surface for hundreds of miles. This persistent pattern is finally expected to break by Saturday, as the storm over the Canadian Maritimes moves eastward and the smoky superhighway from Quebec comes to an end. 

If you want to monitor the wildfires in Canada, including hundreds more active fires burning in the western half of the country, go to the Canadian Interagency Forest Fire site. And there are many resources for monitoring air quality; AirNow is a comprehensive resource from the EPA.