By Dr. Stoos

Severe weather once again struck last week in the Southeast, with three tornadoes in New Orleans and up to 7 inches of rain in Mississippi. Seems like every other week lately we’ve had extreme weather somewhere in the world, whether it be snowstorms, flooding, tornadoes, cold — and don’t forget heat (but that was last summer). Scientists say some of this weather may be due to global climate change, but how can that be and what is the difference between weather and climate anyway? Some facts:

What is weather?

Weather is the day-to-day conditions (such as temperature, rain, humidity, etc.) of a particular place. For example: It rained 2 inches today at my house. Yesterday, it was sunny at your school. Thus it refers to what is happening at a specific time and place.

What is climate?

A dictionary definition of climate is “the composite or generally prevailing weather conditions of a region … throughout the year, averaged over a series of years.” Charts showing the weather over long periods of time can show trends, or changes in the long-term averages. These can be evidence of climate change.

So what is the difference between weather and climate?

Climate is all about statistics and probability. The individual numbers used to computed the statistics are the weather; the statistics (such as means and deviations) are the climate. If it rained one inch in Chicago last Tuesday, that is a weather fact. The average monthly rainfall in Chicago in March is a climate fact.

What is a climate model?

Climate models are big computer programs that simulate the interactions of the atmosphere, oceans, land surface, and ice to predict trends or changes in the climate. In particular, they are not the same as weather models, which only predict what the weather will be in a specific time and place in the near future.

How can they predict what the weather will be like in 50 years when they can’t even get the forecast right for next weekend?

Well, the “theys” in this question refer to two very different groups of people and models. Climate models do not attempt to predict what the weather will be in any given spot on any given day; they only forecast trends and averages. For example, they predict what the average temperatures will be over a season in a broad region, and what the average rainfall will be in a given year and region. They can forecast how many severe storms will occur in a year on average, but not the time or place of any particular storm.

OK, so did climate change cause the tornado in New Orleans last week?

No one can say that. Even if scientists are correct that global warming causes an increase in severe weather, they can never say which storms would or would not have occurred without it.