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A
frozen Bay
Several
times in history, in 1780, 1922, and 1977, the Chesapeake
Bay was frozen, and people could walk across the ice to
the other shore. But what exactly happens when "the
Bay freezes_" Does the Bay freeze solid or is there
still some water underneath the ice_ Fortunately for the
animals and plants in the Bay, the Bay does not freeze
solid. "Why_" you wonder_ To understand why
the Bay does not freeze solid, let's take a closer look
at water, how its density depends on temperature and salinity,
and what that means for the chances of getting a Bay that
is frozen solid.
Why
ice floats on water
As
you may know, pure water boils at 100°C (or 212 F)
and freezes at 0°C (or 32 F). The density of water
(the weight of the water per unit of volume) changes with
temperature, and for most of the range between 100 and
0 degrees Celsius, the density of water, like that of
many other substances, increases as the temperature goes
down. The colder the water, the denser it becomes, or
the more one gallon of water will weigh.
When
the water temperature, however, drops to 4°C (or around
39 F), something extraordinary happens: the water becomes
less dense as its temperature decreases from 4°C until
it freezes at 0°C. So, below 4°C, the colder the
water, the less dense it is. Wow, that is quite something!
Why
exactly the density of water decreases below 4°C can
only be explained at the level of the water molecules,
the tiny building blocks that form water. When the temperature
of water goes below 4°C, the molecules start to rearrange
themselves in a way that creates more space between the
molecules, and, because of these spaces, the same volume
of water will now weigh less: its density has decreased.
This
also explains why water expands when ice is formed. The
same molecules now simply take up more space.
Image:
The water molecules, on the left of the image, are nicely
packed together. When water turns to ice, however, the
molecules start to rearrange themselves, creating spaces
between the molecules. Images courtesy of the University
of New York. Originals and more information is available
at the Ice
and Water
website.
Because
of this phenomenon, ice—which is water in its solid
form—is less dense than water. So, as water cools
to temperatures less than 4°C, its density decreases
and it rises to the level of the surface. If ice would
be more dense than water, it would sink, upon which the
next layer of water would freeze and sink, and so on,
till the entire Bay would be frozen. Because
ice is less dense than water, it floats to the top and
shields the rest of the water from being frozen.
Salt
water needs lower temperatures to freeze
There
is another reason that the Bay probably never gets frozen
solid. Salt water, which is found in the Chesapeake Bay,
takes longer to freeze than freshwater. If you sit in
a boat in the middle of the channel of the Bay where the
water depth is 90 to 100 feet, and you drop a penny overboard,
that penny will pass through layers of water that have
increasing amounts of salt, or an increasing "salinity."
Oceanographers define the "salinity" of water
as the number of grams of salt per 1000 grams of water.
Salt
water is denser than freshwater, so the surface water
will be fresher than the bottom of the Bay, which can
be as salty as the ocean itself, around 35 parts per thousand.
What is important for our story is that the temperature
at which water freezes depends on the water's salinity.
Here's
a table that gives the freezing point of water at a number
of different salinities.
|
Salinity
Parts per thousand
|
0
|
10
|
20
|
24.7
|
30
|
35
|
|
Freezing
Temperature (°C)
|
0
|
-0.5
|
-1.08
|
-1.33
|
-1.63
|
-1.91
|
The
table shows that as the water gets more and more salty
(up to 35 parts per thousand), the temperature at which
the water freezes goes down. The salty water at the bottom
of the Bay would only freeze when the water temperature
would go down to -1.91 °C.
With
what we now know about the density of water and ice and
the freezing point of salt water, we can see that there
isn't a chance for the entire Bay to freeze solid. Thanks
to the amazing properties of water, ice will float on
the surface and protect the denser, salty water below
from freezing. Although the surface waters of the Bay
are exposed to weather conditions that will fall below
0°C for extended periods of time, the deeper waters
are protected from the freezing atmospheric temperatures
and therefore do not turn to ice.
References
and further reading
Related
Watershed Radio programs
- Search
for other Watershed Radio programs in the Archive.
The archive includes a program about the Chesapeake
Bay Bridge and several articles related to the history
of the Chesapeake Bay watershed.
Other
resources
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