Our Local Heat Island
The Urban Heat Island effect has been widely discussed and documented. A number of factors
are responsible for this phenomena.
1. The concrete and asphalt commonly found in urban areas
have different thermal properties compared to the more widespread vegetation found in nearby
suburban regions. They absorb and store heat during the day and release it slowly during the
nightime hours. This results in relatively elevated overnight temperatures in the city.
2. Urban geometry also causes what is known as the ‘Canyon Effect’. Tall
buildings provide multiple faces that serve to reflect and absorb heat energy. Tall
buildings can also block the free flow of air which will inhibit cooling by convection.
3. Human activity, waste heat from autos, buses and trucks and air conditioning all
contribute a small but significant amount of heat.
4. The lack of vegetation in built-up urban areas inhibits cooling by evapotranspiration.
A comparison of temperature records from Bergenfield, New Jersey and Central Park in New York City provide another Heat Island example. Bergenfield is well within the New York City metropolitan area, only 10 miles from Central Park. However, the daily maximum and minimum temperature profiles of the two sites show marked differences.
Difference Between Maximum and Minimum Temperatures ( Bergenfield vs Central Park, N.Y.C. )
| Jan | Feb | Mar | Apr | May | Jun | July | Aug | Sep | Oct | Nov | Dec | Ann | |
| Maximum | -0.5 | 0.7 | 1.0 | 1.1 | 2.4 | 3.0 | 3.4 | 3.2 | 2.6 | 2.3 | 1.1 | -0.2 | 1.7 |
| Minimum | -3.8 | -4.3 | -3.9 | -3.6 | -3.2 | -2.7 | -3.0 | -3.2 | -4.8 | -4.9 | -5.3 | -4.7 | -4.0 |
The table above is based on temperature data from 2003 - 2008. A close inspection of this record shows some interesting effects. During the winter months the maximum temperatures recorded in Central Park and Bergenfield are quite similar. However as summer approaches, it becomes hotter in Bergenfield. The July maximum in Bergenfield averages a full 3.4ºF. higher than Central Park. Notice how smoothly this temperature difference rises and falls during the course of a year. I suspect that, to some extent, the cooler high temperatures found in the city during the summer are a result of onshore sea breezes that frequently develop during the warm months of the year. The city is close to the ocean and surrounded by water on all sides. Bergenfield is just far enough inland to be out of reach from these cooling breezes.
Bergenfield has colder morning temperatures throughout the year. The difference is smallest during the summer months and greatest during the fall. This temperature anomaly during the autumn months is not what I anticipated. I had assumed that the longer winter nighttime hours would provide more time for temperature contrasts between my site and Central Park to develop. Maybe there is an increase in average cloudiness during the winter months that would inhibit overnight cooling. Also, high pressure systems that stall over the region and provide ideal radiational cooling conditions with clear skies with little or no wind may be more frequent during the fall. These periods are known as "Indian Summer" with brisk morning temperatures and mild afternoons.
During the winter, fall and spring months, minimum temperatures are frequently 5 to 10ºF. cooler in Bergenfield. With the right conditions, this thermal deficit can be extreme. The temperature in my backyard fell to -7ºF. on 22 January 1984. In Central Park, the minimum temperature was 10ºF.; a full 17ºF. higher. This was a good example the urban heat island effect at its strongest. Clear skies, no wind and a snow cover of 7" made for optimal radiational cooling.