| David Wood, , Study, Retrieved 2020-11-17 | , American Meteorological society, 2012-01-25, Retrieved 2020-11-17 |
|---|---|
| Berg 2020-11-04 , , Britannica, Retrieved 2020-11-17 | Berg 2020-11-04 , , Britannica, Retrieved 2020-11-17 |
David Wood, , Study, Retrieved 2020-11-17.
4.
21.
17