This is because the surface of the water
absorbs heat from the air and the sun! Heat
from the sun is transferred to the pool via
radiation, and heat from the air is
transferred to the pool via convection.
However, the heat transfer can only occur at
the surface of the pool, at the top. This is
because heat transfer can only occur across
boundaries. In this case, the boundary of the
pool is the top layer of water. This surface layer
is in contact with the air, which allows it to
exchange heat energy with the atmosphere.
Also, the sunlight shines on the surface of the
pool, which means that it is also the boundary
for radiation. This means that the surface of
the pool heats up first.
The bottom of the pool isn’t in contact with
the air, and it doesn’t receive direct sunlight,
so it stays cold. If you let the pool sit
still for a long time, some of the heat from the
top of the pool will go to the bottom. This is due
to diffusive heat transfer. Basically, even
if the water does not look like it is moving
overall, the water molecules are still bumping
into each other and moving around in the pool.
The water molecules at the surface which are
moving faster due to all the heat energy they have
absorbed from the air and sun begin to bump into
water molecules deeper in the pool. They
transfer energy through these collisions, and
eventually the heat begins to reach the bottom.
However, the bottom of the pool will never be
the same temperature as the top of the pool.
Why is this? The water at the bottom of the
pool is in contact with the cement and ground! So
just like the top of the pool exchanges heat with
the air, the bottom of the pool exchanges heat
with the ground. So the ground will absorb
heat from the bottom of the pool, and the bottom
of the pool will never be able to get enough heat
to be the same temperature as the top of the pool!
This phenomenon, where one end of a substance
is hotter than the other, is what scientists call
a temperature gradient.
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