Oceanic summer cooling effect

I’m playing around with data today and was surprised when I plotted historic January high temperatures against historic July high temperatures. I expected them to plot along an upward-sloping line. Instead, I saw a curve going up (as I expected) but then bending downward:

Historic high temperatures in January and July

The same effect shows up when I plot average January high temperatures against average July high temperatures:

Average high temperatures in January and July

I initially assumed a data problem. But I quickly discovered that most of the outliers — i.e., observations with much cooler July high temperatures than predicted by their January high temperatures — are islands in the Pacific Ocean or Atlantic Ocean coastal cities:

                                      JAN       JULY
EUREKA, CA.                           55.0      63.3
LONG BEACH, CA                        68.0      82.9
LOS ANGELES AP, CA                    65.6      75.3
SAN DIEGO, CA                         65.8      75.8
SAN FRANCISCO C.O., CA                58.1      68.2
SANTA BARBARA, CA                     65.4      76.7
SANTA MARIA, CA                       63.9      73.5
KEY WEST, FL                          75.3      89.4
MIAMI, FL                             76.5      90.9
HILO, HI                              79.2      82.5
HONOLULU,HI                           80.4      87.8
KAHULUI, HI                           80.3      86.9
LIHUE, HI                             77.9      83.9
GUAM, PC                              84.0      86.8
JOHNSTON ISLAND, PC                   81.9      86.5
KOROR, PC                             87.6      87.5
KWAJALEIN, MARSHALL IS., PC           85.6      86.6
MAJURO, MARSHALL IS, PC               85.2      85.9
PAGO PAGO, AMER SAMOA, PC             86.8      83.8
POHNPEI, CAROLINE IS., PC             86.8      88.2
CHUUK, E. CAROLINE IS., PC            87.0      87.7
WAKE ISLAND, PC                       82.4      88.8
YAP, W CAROLINE IS., PC               86.5      87.2
SAN JUAN, PR                          82.4      87.4

Oceans moderate air temperature fluctuations because oceans are colder than than the air in summer and warmer in winter. Consequently, the linear shift in land temperatures we observe over much of the continental United States (i.e., significantly higher temperatures in summer than winter) does not hold for locations on or surrounded by oceans. (This is obvious to anyone who has fallen in love with Hawaii or Silicon Valley for their moderate winters.)

Posted by James on Monday, June 13, 2011