where L is the length of the pendulum. Since H depends on the temperature, the period of (atmospheric) oscillation also depends on the temperature and on the rate at which it changes with height. The Brunt-Vaisalla period for the Earth's atmosphere varies as a function of height and solar cycle conditions, ranging from a few minutes to about 15 minutes (see Figure 8 ).
Such a purely vertical oscillation does not propagate and corresponds to what would happen directly above a volcanic eruption as hot gases push upward on the ambient atmosphere. Far away, however, like the effect of a pebble thrown into a pond, a set of waves is observed to propagate outward from the disturbance with frequencies less than the buoyancy frequency and longer periods. For example, the disturbances from an earthquake shown earlier had periods of the order of about an hour. In general, buoyancy waves have periods longer than the Brunt-Vaisalla period and sound waves have periods shorter than those of buoyancy waves.