How would you determine whether the 1991 Mount Pinatubo eruption affected the average monthly temperatures in?

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Answer 1

You can't tell how the eruption affected the temperature in any ONE location.  There are other effects that can have a bigger impact.  The best way to tell is by looking at the global effect.  It's reasonable to say though that chances are high that temps at that spot followed what happened globally.

The global effect of Pinatubo was a decrease in average global temps of about 1 degree Fahrenheit (see link).  But you can't positively say what the effect was in any one specific place.  As an example consider the following:

Take the coast of Peru.  The temperatures along the coast are strongly influenced by El Nino/La Nina.  Coastal temperatures closely follow the adjacent ocean temperatures.  Volcanic eruption doesn't have a DIRECT effect on whether an El Nino or La Nina is happening.  Let's say for sake of argument that an El Nino just happened to occur during that year.  When that happens, water temps along the coast of Peru are between 75-80F, while during La Nina they are around 60F.  Temps there are normally only in the 60s.  If an El Nino were happening, coastal locales in Peru would have much warmer than normal temperatures, even though Pinatubo caused a 1 degree decrease in global temps.

So there's no clear answer to your question.  It depends on other things.  But on a global basis, the decreased amount of sun (insolation) caused average temps to cool, and it's more likely than not that temps at a specific place were less than normal.  But you can't bet money that average temps at that location definitely were lower than normal.

I hope that makes sense.