Background Material
Sea and Land Breezes
If you have spent much time at the beach during the summer at
the beach, absorbing UV radiation to
darken your skin or just beachcombing, you've probably noticed
that at around 3:00 p.m. there often is a strong steady wind blowing
in from the water. This steady wind, the sea breeze, is a result
of the uneven heating during the daytime between the land and
the adjacent water. At night the wind often reverses direction
and blows from the land to the water (a land breeze). Land and
sea breezes are referred to as direct thermal circulations. Let's
examine the sea breeze.
An object will heat up or cool down depending on its total energy
budget. If the object gains more energy than it looses, the object
warms. How much the object's temperature increases depends on
its thermal properties -- heat capacity
and heat (or thermal) conductivity.
During the day the land, which has a low specific heat and is
a poor conductor, heats much more quickly than water. As the land
warms up, the air next to it heats by conduction and
rises, warming the air above the land by convection. As the air
rises it generates a pressure gradient,
and thus a pressure gradient force,
generating the thermal circulation. To understand this, let's
first imagine that the land, sea and the air above them are at
the same temperature and that the isobars are parallel. As pressure
is defined as the weight of the air molecules above us, the isobars
must decrease in magnitude with increasing altitude (Figure 1).
In this simplified model, the temperature of the ocean and the
air above the ocean are not changing and thus initially no air
molecules are moving. As air over the land warms (due to absorption
of solar energy, and conduction to a thin air layer above) it
rises and there is a vertical displacement of air molecules to
a higher altitude (Figure 2). These
rising thermals of air change the orientation of the initial isobars.
For example, the height of the 980 mb isobar
in Figure 1 must increase if we are putting air molecules above
this height. The result is that above the surface, the isobars
begin to slope upward. Now, at a given altitude (say 100 meters),
the pressure is no longer the same over the ocean as it is over
land, it is higher over land. Note, at this time in our simple
model, the surface pressures over the land and ocean are the same
as we have not transported molecules horizontally. But now we
have a horizontal pressure gradient above the surface (Figure 3),
and thus a pressure gradient force generating a wind from over
the land out towards the ocean (Figure 4).
This wind removes air molecules from over the land (less molecules;
thus lower pressure at the surface) out over the ocean increasing
the surface pressure over the water. Thus at the surface over
land low pressure is developing while over the ocean high pressure
is developing, generating a horizontal pressure gradient force
at the surface acting from over the ocean towards the land. To
replace this surface air moving from over the water towards land,
air sinks from above, completing the circulation (Figure 5).
This sinking air moves air molecules towards the surface, causing
the pressure above the surface to lower as the molecules from
above descend. Notice the slope of the pressure surfaces in Figure
5. As the temperature difference between the land and water increases
throughout the afternoon, the circulation increases in strength
and winds pick up, reaching a maximum in the middle to late afternoon.
Over land the distance between two isobars (i.e., 980 and 960
mb) is greater than over the ocean. This difference is what keeps
the circulation moving and is due to the air over land being warmer
than the air over the ocean.
The important concept is that heating (or cooling) of a column
of air leads to horizontal differences in pressure, generating
a pressure gradient force which causes the air to move and a circulation
to develop. During the evening, the land cools faster than the
water and the process is reversed (Figure 6).
The net result is a land breeze, surface winds blow from the land
out to sea.
If you are not at the beach to feel the sea breeze, you can monitor
its existence and intensity by analyzing satellite imagery. A
rising parcel of air expands, and cools and the relative humidity
increases -- conditions favorable for the formation of clouds.
For this reason, the upward branch of the sea breeze is often
visible from satellite pictures in the form of cumulus clouds.
During the day, the upward branch moves inland and is an indication
of the strength of the sea breeze. If the atmospheric conditions
are favorable for the formation of thunderstorms, the sea breeze
may provide just enough lifting to cause thunderstorms to develop.
An example of such a case is given in a sequences of satellite
images accessed below. Before going to the sequence of satellite
images, or satellite loop, let's inspect some of the individual
scenes. These images were made using the visible channel of the
GOES-8 imager which has a spatial resolution of 1 km. In all the
images, which have been enlarged to twice their original size,
the imagery is showing the coast of North Carolina. The first
image is at 1545 UTC,
which is approximately 8:45 am local sun time (0845 LST). Notice
that along the North Carolina coast, there are few clouds over
the Atlantic Ocean, and many scattered clouds over land. There
is a distinct cloud-free boundary at the coast line. By 1815 UTC
(1115 LST) the clouds have moved inland, marking the boundary
of the upward branch of the sea breeze, or the sea breeze front.
As the day progresses and the temperature difference between the
land and ocean increases, the circulation gets stronger and the
sea breeze front penetrates farther inland (2015 UTC).
On this day the atmosphere is susceptible convective activity
and the lifting associated with the sea breeze is enough to 'kick
off' convection by 2145 UTC
(1545 LST).
The evolution of the sea breeze phenomena is demonstrated in a
satellite loop of a sea breeze
from approximately 1545 UTC (8:45 am) through 2215 (3:15 pm) at
half-hour intervals.
Whenever large land and water bodies are adjacent to one another,
sea breezes may develop and may cause thunderstorms. Florida's
abundant summertime rainfall is a result of sea breezes. One sea
breeze front advances from the east and one from the Gulf of Mexico
side.
If the synoptic scale winds are weak, lake breezes may develop
due to the differential heating between the lake and the surrounding
land. The lake breezes are often observed around the Great Lakes.
Questions for thought.
- Why would a sea breeze develop on one summer day and not another?
- How big does a lake have to be for a lake breeze to develop?
- This is satellite view of Florida one day in April.
Can you identify the sea
and lake breezes in this image? Why do the cumulus clouds develop
over the land but not over the adjacent water?
- Give examples of other circulations that are driven by differential
heating.
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