Cape Cod Weather Patterns and Statistics

Type I: Low pressure coming from the midwest.

This weather pattern happens when a low pressure system from the midwest tracks along the great lakes and passes over northern New England or the St Lawrence valley. These low pressure systems are usually weak in the summer, which means that Type I conditions are typically encountered from mid-september to early June.

Usually the approach of the system is marked by an increase in SW flow. This SW wind is typically at its best within the 12 hours prior to the passage of the cold front associated with the perturbation. The nice thing about this SW flow is that it can be very warm, sometimes resulting in > 50 F temperatures in the middle of winter. The SW flow can result in winds in the 3.5 range in late fall and early spring. More typically one is likely to encounter 4.0 - 5.5 weather. The South Shore (i.e. Kalmus, Ned's Point and Dennis) is best at this point.

After the cold front the wind shifts to the NW. If the perturbation is strong enough the NW wind can be quite good (although always very gusty) for about 24 hours after the frontal passage, otherwise the sailing is either marginal or good for just a few hours. The North Shore (i.e. Corporation, Mayflower, and Chapin) often gets the best wind in those conditions.

A high pressure system typically follows the perturbation. At that point it is advisable to go back to work as the wind will be dead for a few days.

Type II: High pressure off the Atlantic coast.

A high pressure system locks in place off the Atlantic Coast. The center of that system is often close to Bermuda, hence its name Bermuda High. That type of system can bring good wind from early June to late September exclusively on the South facing part of Cape Cod (i.e. Kalmus to Chatham). The rest of the coasts usually shuts off. The Bermuda High, which can last many consecutive days, brings us the only sailable weather in the summer. If a cold front is approaching at the same time it can really crank, otherwise expect anything from 4.5 to 6.5.

Type III: Nor'easter

Low pressure starts forming offshore from Cape Hatteras and rapidly tracks along the East Coast. Since the storm is usually offshore the most powerful winds come from the northeast (hence its local name Nor'easter). Nor'easters bring us our most powerful winds (excluding hurricanes) and can sometimes reach hurricane strength! Nor'easter season starts late September and lasts until late May, the best months being October, November, april, and May. 3.0 to 4.0 winds are the norm for those storms. A good drysuit is essential, as the temperature is often in the 40's and lower 50's. The best beaches to sail are the North Shore Beaches (i.e. Corporation, Chapin, and Mayflower).

Type IV: Thermals

The beaches of the South Shore of the cape are famous for the thermal winds. These winds are usually accompanied by a southerly flow which enhances the wind gradient. The signs to a thermal wind are very warm days, cloudless skies, and southerly flow. Usually the wind does not turn on till the afternoon around 12:00 which can start up out of no where blow like crazy and then shut off around 5pm and then startup again around 5:30 and blow till 8:00pm. Although this wind is not the most reliable it is the best that the summer has to offer, the best months being June, July, and August. Otherwise the land does not heat up enough to get much of a thermal.

The season on Cape Cod usually begins around the beginning of April and usually raps up around the end of November. If you have cold weather gear your season may extend beyond this range, however, most people start and end their season with a trip down to Hatteras for April and November. In June, July, and August the thermal winds of Kalmus Beach (Type IV) kick in and account for 99% of the wind statistics for these months in the season. The rest of the cape may have shut off by this time.

The graph below contains average wind data. The first column contains number of days it blew over 25 mph, the second column contains the number of days it blew over 20 mph, and the third column contains the number of days it blew over 15 mph. Below the graph lists average air and water temperatures for each month as well.

	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov
Water (F)	55	60	65	75	85	70	60	55
Air (F)	60	65	70	80	75	65	60	50

This graph contains historical wind averages for each year listed. The first column is winds over 25 mph and the second is winds over 20 mph and the third is for winds over 15 mph. Once again these are averages from various sailing sites throughout the cape during the season which runs about 245 days (April - November).