By: Loan Budy
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on September. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
The average hourly temperature, color coded into bands. The shaded overlays indicate night and civil twilight.
The month of September in New York City experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 37% throughout the month. The clearest day of the month is September 23, with clear, mostly clear, or partly cloudy conditions 64% of the time. For reference, on January 3, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 53%, while on August 29, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 64%.
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In New York City, the chance of a wet day over the course of September is essentially constant, remaining around 26% throughout. For reference, the year's highest daily chance of a wet day is 35% on August 1, and its lowest chance is 22% on January 28.
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day. The average sliding 31-day rainfall during September in New York City is gradually increasing, starting the month at 3.4 inches, when it rarely exceeds 5.7 inches or falls below 1.3 inches, and ending the month at 3.7 inches, when it rarely exceeds 6.5 inches or falls below 1.4 inches.
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night. The chance that a given day will be muggy in New York City is very rapidly decreasing during September, falling from 33% to 11% over the course of the month.
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages. The average hourly wind speed in New York City is gradually increasing during September, increasing from 6.8 miles per hour to 7.8 miles per hour over the course of the month.
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation. The average daily incident shortwave solar energy in New York City is decreasing during September, falling by 1.2 kWh, from 5.6 kWh to 4.3 kWh, over the course of the month.
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere). The growing season in New York City typically lasts for 7.8 months (238 days), from around March 29 to around November 22, rarely starting before March 12 or after April 14, and rarely ending before November 4 or after December 13. The month of September in New York City is reliably fully within the growing season.
New York City is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water. The average surface water temperature in New York City is decreasing during September, falling by 5°F, from 72°F to 67°F, over the course of the month.