Demonstration
How
a Thermal Holds Up a Feather
(Lesson | Demonstration | Discussion)
Note: This activity must be done under the supervision of an adult
so you don't get burned.
Materials Needed:
- some feathers; colorful chicken and goose feathers are often sold
at craft stores
- thread or string
- an electric stove or hot plate burner
Directions:
- Tie
the feathers with thread or light string to a pencil, ruler,
or something else (you can make them into a "dreamcatcher" if
you like).
- Turn on a front stove burner and wait until it gets hot.
- Hold the pencil or dreamcatcher so the feathers dangle at least
8 inches from the burner. Be careful!
- Think about these questions: Does the rising heat make the feathers
move? Does it hold them up? Would other kinds of feathers work better
or worse?
- Try the experiment again with feathers of other sizes.
|
|
These feathers are hanging
above a cold stove |
These feathers are hanging
above a hot stove |
If birds and monarchs were weightless, they could rise straight up
on thermals (at least if the wind was absolutely calm). But these creatures
are not weightless. In order to stay in the air at all when they're
not flapping, they must be moving forward. (See
our lesson about flight.) And in order to move forward AND stay
over a thermal or updraft, they move in a circle. The rising air carries
them upwards at the same time. This makes their overall movement a
spiral.
In a thermal, eventually the rising warm air cools down enough that it is the
same temperature as the air around it. And air rising in an updraft slows down
little by little until it eventually dies out. At that point, the animal starts
heading in the direction it wants to move. If it is soaring or just not flapping
much, it will lose altitude, getting lower and lower, until it discovers another
thermal. Then up it goes again!