Making Sense of Our Changing Climate: 2. Weather and Climate

Index of Blogs and Courses

I often find people using the terms weather and climate interchangeably - this creates much confusion, and makes any serious discussion of the important subject of climate change (CC) very difficult.  The media, political leaders, industries with vested interests have conveniently exploited this confusion. 

Both weather and climate describe certain properties of the atmosphere - like temperature, wind speed and direction, humidity, rainfall etc.  The difference between weather and climate is that 

weather describes the atmospheric conditions at a particular time and location, while 

climate is the averaged atmospheric conditions of a region over a period of time (normally 30 years or more).
  
Weather in London at say 3 pm tells us what the temperature is, whether it is sunny, cloudy, raining or snowing, what the wind speed is and what is the direction of air flow etc.  The weather in London after a little while may be vastly different - weather can change rapidly. Likewise, weather in Manchester at 3 pm might be completely different from that in London at that time.
Climate pertains to an average of atmospheric properties over a 30 year period and refers to an extended region.  The region may be a county or a country or even larger areas.  
Atmospheric gases are always moving causing weather to change continuously.   Their mixing over an extended period of time enables us to describe an average atmospheric condition for the region.  The set of such mean atmospheric conditions (temperature, rainfall, snowfall etc.) is the climate for that region.

Weather is the atmosphere that you feel while climate is what you expect the atmospheric conditions to be like - and the two can be very different.  For example, for UK climate, I expect that the average temperature in July will be about 17C.  In London the temperature in July may routinely change over a range from 10C to over 30C.  Even within the UK, we can have several different climatic zones - Scottish climate is colder and wetter than the English climate.

Following slides demonstrate the difference between weather and climate

SLIDE 1:  Data from

The data in slide 1 demonstrates the variability inherent in the weather at a location.  Red and blue bars represent larger variations from the more normal weather (green bars) that one encounters.  Even the green bars have significant range.  The grey curve shows the observed maximum temperatures since records were kept.  Interestingly, in many of the months in 2018, maximum monthly temperatures almost reached recorded maximum - an indication of global warming trends (more of this in later blogs).
Similar variability is observed in other atmospheric parameters like precipitation, wind speed etc.
The main take away from the figure is that weather does change rapidly over a large range - and it is highly unpredictable.  

Climate, on the other hand averages out short-term variations and gives us a very good idea about what atmospheric conditions to expect (on average) in the vicinity of a particular location.

Slide 2 shows monthly averaged temperatures in the UK over 5 years.  For a given month, there is still a spread but is typically less than 3C. Looking at slide 2, we could say that February in 2018 was 2C colder than January (but 3C warmer in 2019) - against an expectation of similar temperatures in both months. 

SLIDE 2:

Climate of a region is defined as average atmospheric condition over 30 years.  Atmospheric data are obtained from a large number of weather stations distributed in the region and a mean value calculated for each parameter.
One can then start to talk about annual mean temperatures (AMT) for a region or annual mean rainfall etc.  Staying with the UK,  Slide 3 shows how AMT has varied from 1910 to 2017.  The fluctuations in AMT are typically half a degree centigrade from one year to the next.
  
SLIDE 3:

We make some interesting observations from this figure. Between 1961 and 2010, the 30-year mean temperature has increased by 0.4 to 0.5C for the UK and England. We can state that the UK climate has warmed by half a degree during the second half of the 20th century.  The warming trend is particularly pronounced since 1990, although one can have (as in 2010 and 2011) a sharp drop in AMT for a brief period.  From the data in the slide, I also notice that the English climate is 2C warmer than the Scottish climate.

I have discussed the case of the UK climate to demonstrate the difference between weather and climate.  The warming trend over the last century or more is not particular to the UK only but the Earth's climate is showing a consistent warming trend - generally called Global Warming (GW) or Climate Change (CC). 

Of all the atmospheric parameters, the temperature is the most important and is universally used as a primary indicator of the climate.  Most input of energy to the Earth is from the Sun and majority of this energy is absorbed by the land and seas in the low to mid latitude regions lying on either side of the equator.  The resulting increase in temperature of the tropics warms the atmosphere causing circulation of atmospheric air that then distributes the energy to northern latitudes.  This drives wind currents which also carry the evaporated water from the oceans to create rain and snow.  Distribution of air currents and precipitation determines the climate in different parts of the globe. See my blog on Air Circulations etc.

I shall discuss the global situation in my next blog.