Saturday, December 28, 2013

Sea Smoke

Note: This program first aired on December 28, 2013.

We’ve had some very cold weather so far this winter, and beyond the obvious experience of stepping out the door on an arctically cold morning, I can tell it is cold when I am driving to work and I see sea smoke on the harbor. Sea smoke is the wispy mist that forms on cold days just above the surface of open water, and when you see that, you know its cold out.

Sea smoke is like fog, it results from the difference in temperature between the air and the water. I wrote about this many years ago on this show, but I think it is time for us to revisit it. The fog we encounter in the summer comes from warm air hitting cold water, or a rapidly cooling landscape. Warm air can hold more water vapor than cold air and when the warm air gets cooled, it can’t hold as much water vapor. The water vapor in the air has to go somewhere, and it condenses out as fog.

What we are really talking about here is relative humidity. Air of any given temperature can hold a certain amount of water molecules in the gaseous phase—water as a true gas. We can’t see water in this phase—its different than steam, or seeing your breath. The maximum amount of water vapor the air can hold at any given temperature is called the saturation point. When air is holding as much water vapor as it can at its given temperature, it is said to be 100% saturated. Its relative humidity is 100%, because, relative to its temperature, it is holding as much water vapor as it can. As air cools down, its relative humidity rises. The amount of water the air may contain doesn’t change, but the ability of the air to hold that water does, when air cools down to a temperature where it is 100% saturated, we call that temperature the dew point. If the air cools below that, liquid water will condense out of the air on any surface it contacts. In the winter when this happens, it forms frost.

Sea smoke is different than the regular summer fog we can see on land and in river valleys as well as on the ocean. In the regular fog scenario, the water vapor is in the warm air mass, and only condenses because the warm air gets cooled when it encounters cold water, or cools as it travels up over a ridge line or pools in a valley. The source of the water vapor in sea smoke is the sea, not the surrounding air mass. Very cold winter air can’t hold very much water. Its relative humidity may be high, but 100% of a tiny amount is still a tiny amount. That is why everything dries out in the winter, your skin, your wood flooring, your shingles, you get the idea. Sea smoke forms when a very cold air mass rests over warmer water, though again, its all relative. The water only has to be warmer than the air. For example, during this recent period of cold, the air temperatures were around zero many mornings; according to real time buoy data water temperatures are in the low 40’s, Fahrenheit. That is much warmer than the air was, but hardly what any of us would call warm.

The warm water warms the air directly above it, on a large scale this is the reason that the immediate coast tends to have milder temperatures in the winter. That slightly warmed up air can hold more water vapor, and water is constantly evaporating from the surface of the ocean. Warm air also rises, so this slightly warmed, more heavily laden with water vapor air rises up into the much colder air around it, where it cools off. When it cools, it can’t hold as much water vapor, and that water vapor must condense out into liquid form, or as often the case with sea smoke, as ice crystals. As the sun rises further and warms the air mass more uniformly, the sea smoke dissipates, so when you see it, you know you are seeing a special and ephemeral combination of elements; here one moment, gone the next. You also know, its cold.

From a class at the University of Illinois, a nice diagram showing the non linear relationship of temperature and the overall amount of water in the air mixture.


Great old article from the Woods Hole Oceanographic Institute:

Here’s a much better use of your time than Facebook, its real time buoy data from the Northwest Atlantic: