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. http://butane.chem.uiuc.edu/pshapley/GenChem1/L13/1.html

References:

Great old article from the Woods Hole Oceanographic Institute: http://www.icess.ucsb.edu/~norm/steamfog.pdf

Here’s a much better use of your time than Facebook, its real time buoy data from the Northwest Atlantic: http://neracoos.org/realtime_map

Fall Equinox

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

It’s fall in Maine. We’ve just passed the autumnal equinox, that point in the Earth’s orbit where, mathematically we can think of the spin axis of the Earth as a tangent line on the orbital ellipse. What this means in practice is everywhere on Earth has a day of 12 hours of daylight, 12 hours of darkness, more or less. After that, at least here at our middling  northern hemisphere latitude, the darkness increases dramatically, making the light sensitive among us go into mourning, and the snow lovers among us optimistic.

I recently received a weather alert on my phone, telling me that frost and freeze advisories had been suspended for northern Maine, as they had officially reached the end of the growing season there. The alert added that frost and freeze warnings would continue in my neck of the woods, coastal Maine, until the official end of the growing season here, noted as October 10, or earlier if we received a hard freeze before then. It made me wonder, what exactly is the growing season? How is it calculated and what does it really mean? How do we know the exact date when it ends?

Here in Maine, midway between the equator and the north pole, there are three components to growing plants, which when we talk about the growing season, is what we mean. We need light, water and we need warm enough temperatures. The light aspect is very predictable, at least in terms of day length. We know to within seconds how much time the sun will be above the horizon on any given day of the year. Temperature, however, is harder to predict. We can look at averages over time; the date of the first frost for a region, averaged over a period of years for example. I suspect that is where the October 10 deadline came from. Water is, for the most part, not a significant issue here, as we usually have plenty of it, with one major exception. In winter, there may be lots of water around, but it is frozen, and thus not available for use by living plants.

Information for farmers about the growing season is all about mitigating the effects of cold temperatures, really, unless you want very powerful grow lights, temperature is the only aspect of the growing season we can control, and only on a small scale at that. When it comes to light, 10 hours seems to be the magic number. Below that amount of daylight, plants can simply not fix enough of the sun’s energy to meet their metabolic needs and grow. They may be able to maintain, but not get any bigger, or reproduce. For that reason winter is a time of dormancy, a holding period that plants simply wait through. Here in Maine, our true non growing season runs from the beginning of November until the beginning of February, based on that magic 10 hours of light. Wild plants obey the same rules as their domesticated cousins, if my observations are correct. Trees lose their leaves, annual grasses and forbs die back to their under ground roots, or over winter as seeds. Even the hardy ever green trees are limited in their abilities to photosynthesize over the dark, cold winter months by low light, cold slowed metabolism, and lack of available liquid water.

We are tropical animals, living in a temperate climate. Even in the tropics there are patterns of growth and rest, wet and dry, exuberance and senescence. As the calendar rolls towards October 10th, the last day they will bother to warn us about a possible frost, or the beginning of November, when the daylight dips below the 10 hour mark and stays there for three whole months, take heart that this time of year plays a valuable role in the yearly cycle of plants, and people. Take this time to show your other, hidden colors; draw in and hunker down. The dark provides our excuse, our opportunity, finally, after the excitement of summer, to simply stay home and regroup. So don’t fight it. Look around you, are the trees in your yard, the weeds in your garden, or the ferns in the woods arguing with the solstice? Neither should you. Happy fall everyone.

References:

Interesting info from Maine’s own Johnny’s Seed company http://www.johnnyseeds.com/t-catalog_extras_vegetables.aspx

A frost free dates for my neck of the woods, from the National Climatic Data Center, featured on a gardening website: http://davesgarden.com/guides/freeze-frost-dates/index.php?q=04676&submit=Go