Today was another one of those classic days where inland areas soared into the upper 70s, and coastal areas had trouble exceeding 60 degrees, with plenty of wind.
The explanation for this is the sea breeze. Whenever we have a warm, springtime airmass, the temperatures over the land warm up quickly. However, the sea is still relatively cold this time of year, especially considering how cold of a winter we had. The ocean has a high specific heat capacity, which means that it can absorb the same amount of heat as the land does, but will not warm up nearly as much as the land. Thus, during the day, the land will often be much warmer than the ocean.
Once the land heats up, thermodynamics dictate that the warm air must rise, since it is less dense. However, the relatively cold ocean does not have this same heat, as it is more dense, so the air does not rise. The rising air over the land creates a slightly lower surface pressure, since the air is escaping the surface, rather than exerting a force on it. The lack of rising air over the sea thus creates a slightly higher surface pressure, relative to the land. The atmosphere always wants to balance itself out, so to compensate for the air over the land escaping the surface, air from higher pressure has to flow towards it. This air happens to be coming from the colder ocean, which helps to create colder temperatures and windier conditions along the coast.
The circulation gets reinforced because when the air over the land rises, it creates an area of higher pressure at the upper levels of the atmosphere, since lots of air is rushing towards the upper levels. However, over the sea, there is no air rising into the upper levels, so the pressure is relatively lower there. The air with higher pressure aloft on the land thus flows towards the area with lower pressure above the sea. Now that this air is above the sea — where air at the surface is already heading towards the land — the air has to sink to replace the air that is leaving the sea. This sinking air towards the sea surface reinforces the high pressure, which reinforces the source of air that is blowing towards the land, which reinforces the sea breeze.
Of course, sea breezes do not occur every time it warms up during a spring or summer day. Or at the very least, they do not always penetrate further than the immediate beaches. This is where the general weather pattern kicks in. In the example above, from April 14, we had a low pressure system to the north which was already bringing southerly winds to begin with. This helped to combine with the sea breeze to increase the winds along the south-facing shores, and make temperatures much colder. At 5:00pm that day, JFK had a temperature of 61 degrees, with southerly winds at 35mph, gusting to 47mph! That’s equivalent to a weak Tropical Storm. And as we can see, the sea breeze was not just relegated to the immediate coast — most of Long Island also had stronger winds and temperatures stuck in the low to mid 60s — the sea breeze was strong enough to penetrate well inland. Meanwhile, it was in the upper 70s in most of New Jersey, with winds generally less than half that of JFK.
However, when the general weather pattern is favoring westerly winds or northerly winds, either behind a cold front, or when a large dome of high pressure is situated to our south and west, those winds can actually “collide” with the sea breeze and stop it from from occurring. This is one reason why the most widespread warmth throughout the entire area actually occurs on westerly winds: in the right weather pattern, westerly winds can still bring in the warm air that southerly winds can, but the westerly winds can “push” the sea breeze away. Westerly winds also tend to downslope off the Appalachian Mountains, meaning the air descends, which allows it to compress and warm. During some of our heat waves last summer with westerly or even west-northwest winds, the coastal areas often recorded the warmest temperatures, as Westhampton Beach, NY exceeded NYC’s temperature on a few occasions.
Sometimes, the general wind flow will only be able to stop the sea breeze to a limited extent, if the sea breeze is strong enough, or the general wind pattern is too weak. Where the general wind is flowing “collides” with the sea breeze is often where a small-scale cold front, called a sea breeze front, forms. This was illustrated in a classic fashion today along Ocean and Monmouth counties. Relatively light southwesterly winds in a warm airmass allowed temperatures to warm up into the upper 70s to around 80, right up to the Garden State Parkway. However, the sea breeze circulation was still strong enough to have the cooler temperatures penetrate right up to the Parkway — notice how temperatures in the 50s and 60s were common right along the coast, along with a wind direction that tended to either be east or southeast. This created a sea breeze front just east of the Garden State Parkway. If the general wind pattern was out of the southeast to begin with, the sea breeze would have been able to penetrate much further inland. Conversely, if the winds out of the southwest were much stronger, the sea breeze would have been pushed away and kept at bay, and even the immediate coast would have warmed up into the upper 70s. Today was largely a “compromise” day, so to speak, along the Jersey Shore.
Not surprisingly, of course, at 3:00pm this afternoon, JFK had a temperature of 60 degrees, with southerly winds at 23mph, gusting to 29mph — a classic indicator that the general southerly and southwesterly winds “combined” with the sea breeze circulation to make winds stronger and help to further cool the temperature, as the stronger winds are able to more efficiently generate cold air from the cold ocean. At the same time, Newark airport had a temperature of 76 degrees, with a southwest wind at 10mph.
The exact placement and strength of a sea breeze can often change the forecast dramatically, as a 5-mile shift can lead to a 20-degree difference in temperature. The general takeaway is that along the coast, if you do not want a sea breeze to cool off your temperature, root for westerly or northerly winds. Also, if the general wind pattern happens to be in the same direction as the sea breeze, then the cooling effects can penetrate well inland — the sea breeze is not always just an effect along the immediate coast. Even today along the Jersey Shore, the sea breeze was still able to penetrate a few miles inland, despite the fact that the general wind pattern was in a direction that slightly opposed it.