Often times when people discuss climate change, the conversation drifts towards something completely different: weather. So what is the real difference between weather and climate, and why is it important to differentiate the two from each other?
To start with the basics, weather is the day to day change in atmospheric conditions, such as precipitation, temperature, pressure and humidity. Simply put, when you look outside your window, the current conditions, are that day’s weather. This means it could be pouring down rain, snowing, overcast, sunny, or any combination of weather phenomena that are known to exist.
Climate, on the other hand, is the prevailing long term changes in atmospheric conditions. We can break climate up into two broad categories: regional climate and global climate. The topography and location of an area on the Earth’s surface directly affects the climate of a perticular region. This is the reason why a cold, snowy, winter day in New York state differs so much from that same day in southern California. Why is the climate so different between these two locations? Well, let’s think about their location on the Earth’s surface: Southern California is located approximately 35 degrees North latitude while New York is located approximately 41 degrees North of the Equator. This seemingly small six degree difference in latitude, is large enough to create two completely different climate zones. We can then fairly conclude that regional climate is a climate which dominates only a small area, and is dependent on local topography and geography.
Take a moment to consider the shape of the Earth, spherical, with a slight tilt about it’s axis (~23.5 degrees). Due to the curvature of the Earth, incoming solar radiation decreases as you move away from the equator (Figure 1.). Instead of radiation (both light and heat) directly striking the earth along the equator, it spreads across the Earth’s surface at a more oblique angle as latitude increases (towards the poles).
Regional climate is also affected by the local geology, and topography. For example, while southern California is considered a Mediterranean climate, and receives little precipitation, it still gets more precipitation than Nevada, which is mostly classified as desert or semi-arid desert. This is largely in part due to the Rocky mountains, which casts a rain shadow in a process known as orographic uplift (Figure 2.). As cool moist air moves towards a coastal mountain range, the airmass is forced upward along the slope of the mountain (windward side), condensing it and producing precipitation. As the air mass continues over the mountain, to the leeward side, is has now lost most of it’s moisture thus reducing the likely-hood of cloud formation or precipitation.
Formally climate zones are given a name based on the Koppen climate classification system (Figure 3.). This system was first developed by climatologist, Wladmir Koppen in 1884, and has since been ratified as more has been learned about the Earth, and as climate has shifted moving into the 21st century. The Koppen classification puts regional climates into 5 different groups, and smaller sub-groups to create a climate ‘code.’ For example: Dsb would be a continental climate with dry, warm summers.
Inside of each of these regional climates, different weather phenomena are experienced, from blistering hot days to non-stop rain. Understanding that weather is related to climate, but not an example of climate is extremely important. It is possible to have a warming climate, based on long term averages, yet still experience extreme weather phenomena that seem to contradict the measured trend. Think of it like this, each day is a single point of data (for a given location). So day 1 could be 75 degrees and mostly cloudy, day 2 could be 68 degrees, overcast with a chance of rain, and so on…each day will likely change, with some similarities and differences because the regional climate dictates the weather experienced. However, you may notice that over the course of a year, if you recorded each day’s weather, there would be a few days that appear to be outliers, being unusually hot, cold, or some other strange characteristic that doesn’t fit the observed mean. Now we could take each point of data and graph it over time to understand the long term trend. Is it cooling, warming, getting more precipitation, becoming more dry, or staying the same? Ultimately the data will show a trend that either relates to your local climate, or shows a shift from the norm. Imagine doing this around the world, from the equator to the poles. After some time you would be able graph out a global trend, and that is exactly what climatologists do to understand the global climate system and how it is changing in the modern era. Global climate must then be defined as the climate which extends over the entire Earth, based on the average of all regional climates.
Formally, humanity started taking an interest in quantifying the physical world beginning in the 1800’s, in what’s known as the instrumental era. This era has obviously continued until today, only we have gotten much more accurate and precise with our measurements. Although physically measured data only goes back to the 1800’s, we have data that goes back hundreds of thousands of years in both sediment and ice cores collected from around the world. Advanced coring techniques allow climatologists to understand global climate shifts on a long term scale, enabling us to compare what we observe today to what has happened in the past. From these comparisons we can understand what is causing a changing climate in the modern era, and how far it is deviating from the historical norm.
Ahrens, C.D., 2003, Meteorology today: an introduction to weather, climate, and the environment, seventh edition: Brooks/Cole, p. 488-515