How can there be global warming if it is snowing outside in April when it should be 50 to 60 degrees Fahrenheit? This is a very common question, and the answer lies in the difference between weather and climate.

Weather, which is highly variable, is made up of specific atmospheric conditions including temperature, rainfall, wind, and humidity, that occur at any given place and time. Weather occurs over a short term (today, tomorrow, last week, etc.).

Climate (according to the National Weather Service’s definition) is the average of weather over at least a 30-year period. Climate is much less variable; it is the typical weather for any given area, averaged out over many years. General weather conditions such as temperature, precipitation, humidity, air pressure, sunshine, cloudiness, and wind are averaged out over many decades to characterize climate.

Climate (according to the American Meteorological Society’s definition) is defined as “the slowly varying aspects of the atmosphere–hydrosphere– land surface system. More specifically, climate is frequently defined to be the average of weather over at least a 30-year period. Climate is much less variable; it is the typical weather for any given area, averaged out over many years. General weather conditions such as temperature, precipitation, humidity, air pressure, sunshine, cloudiness, and wind are averaged out over many decades to characterize climate.

A climagraph is a way to represent the three most important elements of climate: average temperature, average precipitation, and seasonality. Typically when climatologists talk about the mean temperature, they are referring to the average of the maximum and minimum temperatures. Each day in a month has a maximum and minimum temperature so the monthly mean is the average daily temperature for the month in any one year.

The monthly temperature and precipitation for each year over a period of time, usually 30 years, is averaged and represented in the climagraph.

There are many different terms associated with climate, including global warming, climate change, and global change, but these terms cannot be used interchangeably. Climate is commonly defined as the average weather for a specific location, region or the entire globe over an extended period of time (decades).

Atmospheric scientists investigating the possibility that human influences are changing the earth’s climate confront a significant problem: how do we actually detect climate change? We know that weather can be highly variable, but climate, which is based on longer time scales, can be variable as well. If the last 30 years were generally warmer worldwide than the previous 30 years, would this be solid evidence that the climate is changing in a particular direction? Or could this only be a longterm, normal statistical fluctuation in climate? This is a critical and surprisingly difficult question for atmospheric scientists to answer. While computer models may predict climate change, citizens are unlikely to support significant social, economic, and/or technological changes to slow the rate of change unless they are sure that the climate is truly changing, not just experiencing random variability.

It is important to understand what constitutes normal climate variability versus actual climate change. Climate variability is the way climatic variables (such as temperature and precipitation) depart from some average state, either above or below the average value. Climate change can be defined as a trend in one or more climatic variables characterized by a fairly smooth continuous increase or decrease of the average value during the period of record. As we look at 30-year average values, however, we also detect variability.

The term global warming refers to a sustained increase in global average surface temperature and the lowest layer of the atmosphere and is just one aspect of climate change. Global warming does not imply that the world will warm uniformly. In fact, as with any average, there will be places that warm more or less than the average. Some areas may even cool.

Climate change refers to a long-term shift in climate measured as a change in some or all of the features associated with weather, such as temperature, wind, precipitation. It is a long-term continuous change to the average weather (e.g., warming or cooling as indicated by the average temperature) as well as changes to the range of various weather conditions (e.g., high and low temperatures) and extreme events (e.g., frequency of tornadoes). Climate change can result from either natural or anthropogenic (human-influenced) causes. For example, natural factors affecting climate include changes in the Sun’s energy or slow changes in the Earth’s orbit around the Sun. Human activities that change the atmosphere’s make-up (e.g., burning fossil fuels) and the land surface (e.g., cutting down forests, planting trees, etc.) also affect the climate.

Global change is the broadest term and it encompasses more than just climate change. According to the U.S. Global Change Research Act of 1990, global change is defined as: “changes in the global environment (including alterations in climate, land productivity, oceans or other water resources, atmospheric chemistry, and ecological systems) that may alter the capacity of the Earth to sustain life.”