How do officials warn people about volcanic eruptions

Volcano observatory scientists determine alert levels for a volcano by using monitoring data and their knowledge of the expected or ongoing hazardous activity. ... During volcanic unrest and most eruptions, the Volcano Alert Level and Aviation Color Code will change upwards or downwards together.

Explanation:

Volcano observatory scientists determine alert levels for a volcano by using monitoring data and their knowledge of the expected or ongoing hazardous activity.  During volcanic unrest and most eruptions, the Volcano Alert Level and Aviation Color Code will change upwards or downwards together.

Explanation:

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the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.