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April 12, 2017

Infrequently Asked Questions: What's the difference between weather and climate?

It's OK to ask

Climate and weather: two concepts that go hand-in-hand, but are very different in how we observe them. And, more often than not, the difference between the two proves to be a pretty fundamental hurdle to understanding climate change.

Eager for a simple breakdown of what distinguishes one from the other, we reached out to Peter DeCarlo, assistant professor of environmental engineering and atmospheric chemistry for Drexel University's College of Engineering and College of Arts and Sciences.

In a nutshell, what's the difference between weather and climate?

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Weather is the clothes you put on in the morning, while climate is more like your wardrobe. Weather, for the most part, can be summed up by asking, "What is the high (and low) temperature today, and will it rain or snow?" Climate is how hot the summer will be this year -- in an average sense -- and will I wear this goose down coat I bought ages ago again, or should I get rid of it?

What are the components we measure with weather versus climate? 

There is a lot of overlap with what we measure for weather and climate. Temperature, relative humidity, rain amount, snow amount, and intensity are all important measurements for both weather and climate. Weather is focused on the instantaneous value of the measurement, while, for climate, we are more interested in the longer trends in these measurements. We also measure additional climate-specific variables that are not as relevant to weather, such as the atmospheric concentration of greenhouse gasses -- carbon dioxide and methane -- and ocean temperatures, sea levels, etc.

In a study, 30 years is the standard measure of time with climate. Why 30 years as the magic number?

I wouldn’t say 30 years is the “magic” number, but it is a long enough slice of time to make sure the trends in monthly average temperatures are clear. Five-year timescales are too short since they will be influenced by natural cycles of the earth. The El Niño/La Niña cycle is a common one that can have a significant influence on year-to-year temperature changes and rainfall patterns. El Niño years are typically warmer than La Niña years. By using climate data that spans 30 years or so, this shorter term variability from natural fluctuations can be accounted for and the increases in temperature from climate change are clearly discernable in the measurements of temperature and other variables.   

You hear a lot of people talk about major snowstorms "back in the day," with three feet of snow and sub-zero temperatures, etc. And today we obviously seldom see that. What's the basic explainer for that, and what changes we see are climate-change-related and not just a natural change in climate pattern?

Here in the Philadelphia region, heavier snows are less common, but that isn’t strictly true across the U.S.  Areas downwind of the Great Lakes can actually experience higher snowfall due to more available moisture and a longer time before the lakes freeze over in the winter. In this area, however, warmer overall winters have led to fewer days of subzero temperatures and changes in snowfall.  I grew up elsewhere, so I don’t have the nostalgic childhood memories of winters in this to compare to winters of today. 

So when Great Aunt June -- or, as is the case, the President of the United States -- says that climate change isn't real because it snowed in March, what is, conversationally, the best way to explain the reality of the situation? Surely you encounter this scenario often.

This does occasionally happen, but generally, I just try to avoid Great Aunt June at family functions. (Kidding.) Really, one of the most straightforward ways to think about climate that people can intuitively appreciate is to look at the number of days per year when a high or a low-temperature record is set. If the climate wasn’t changing, then the number of record-high temperatures should be roughly the same as the record-low temperatures. This isn’t the case, and across the U.S. and globally, we are setting record-high temperatures much more often than we are setting record-low temperatures. Climate change and general warming of the climate doesn't mean we won’t have days when it snows in March, or record-cold days. It just means those events will be a lot less frequent, whereas, warmer weather and record highs will be a lot more common.  (Some official data on that can be found here.)

What effects are we seeing in Pennsylvania (and/or the southern portion of New Jersey) related to climate change?

Increasing average temperatures is the most straightforward and common effect of climate change that we observe. This increase in temperature leads to longer growing seasons, but also the possibility of temperatures too high for some plants and crops to grow optimally. This can be bad for area farmers and lead to reduced crop yields. Higher summertime temperatures are hard on many area residents and cause many heat-related health problems. Adding to negative health impacts, summertime high temperatures also increase air pollutants like ozone, leading to respiratory problems for many Philadelphians. Other climate-change-related impacts we can expect are increasing storm intensities and rising water levels on the tidal rivers in the area.

Anything to add? What might people like to know -- or need to know -- about climate v. weather? 

Weather forecasts are reasonably accurate for five or so days out and getting better, but beyond that, it can be difficult to predict the weather. Climate forecasts have pretty good accuracy over longer timescales. We can expect that the next few decades will continue to get warmer based solely on how the climate system works, and how long greenhouse gasses stick around in the atmosphere. How much warmer and what direction the climate system goes after that largely depends on us. How we choose to produce and use energy domestically and globally will determine the climate for future generations. This is an issue that must transcend partisan politics and start with open and honest discussions leading to solutions. Continuing to push those discussions down the road will only make the problems worse and the solutions more difficult.