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What is Weather?
Is 'average weather' climate?

C-305

What is the difference between partly cloudy and partly sunny, or warm and cold weather? In Australia, in China or in North America? You know, we know, the weather forecaster know! Often we needs much more specification, concerning season, location, temperature and humidity conditions. But we know as we have a lot of knowledge about weather conditions and a lot of experience. Every day we are confronted with weather, we talk a lot about  the weather, and are usually keen to know what is happening next with the weather. For this reason we are grateful for being advised about the weather today, tomorrow, and beyond. As it works fairly well for a couple of days, it is less convincing with regard to months, years and millenniums. This touches the question how science handles the terms: weather, average weather and climate. That is a complex matter and the material and comments provided can only hope for initiating a border discussion. Lets have a look:

What do such notions explain; hardly much more than those that can be found in articles and comments abundantly, such as:

• Weather is a short-term phenomenon, describing atmosphere, ocean and land conditions hourly or daily.
• Weather is the atmospheric condition over a small area and a short period of time. For example rain is a type of weather.
• Weather is not constant. It is dynamic and always changing.

None of the definitions is more explicit as any laymen has experience with it.  However, one can find a more complex description of meaning of weather as the following box shows:

There are presumably hundreds of general descriptions for weather conditions, for example:

• Cyclones (Depression)/Anticyclones, various types of Clouds , Warm/Cold/Occluded Fronts, Condensation, Convection , Dew/Dew Point, Evaporation, Fog, Moisture, Frost, Humidity, Strom, Hurricanes, Monsoons, Precipitation, Rain, Pressure, Sunshine, Temperature, various types of Storms (e.g. thunderstorm, tornadoes, whirlwind), various types of Wind (e.g. sea/land breeze).

There are presumably hundreds of technical descriptions with weather relevance, for example:

• Aerosols, Air Gases, Air Layers, Aurora, Blue Sky, Coriolis Forces, Sun Ray, Energy, Cosmic Ray, Exosphere, Magnetosphere, Mesosphere, Jet Stream, Nitrogen, Carbon Dioxide, Oxygen, Ozone Hole/Layer.

And what do the most prominent international institutions say about the term “weather”? Nothing! The World Meteorological Organization (WMO) and the Intergovernmental Panel on Climate Change (IPCC) offer nothing in this respect, although they refer to the term  ‘weather’ in this way:

WMO at section: Frequently Asked Questions (FAQs)?:
Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
Source: http://www.wmo.ch/pages/prog/wcp/ccl/faqs.html
Ditto: IPCC 2001, Glossary, WG1;
Ditto: IPCC 2007, Glossary, WG1.

COMMENT:
Do we now know what “weather” is? Definitely as long as we talk about weather as the general public does. It works as long as it is explained in general terms, and meet our experience. But our understanding is quite different from that what science make out of it. They fragment it in hundred of pieces. They gave them hundreds of descriptions and terms. The Glossary of AMS lists app. 12’000 items. Their basic tool are instrumental data. But once a collection of data is compiled it is a statistic and  it remains a selection of data, namely a statistic. In this moment it has lost the ability to be ‘weather’.  There is nothing to say about ‘average weather’, but presumably a lot if it is called “Climate”, as a statistic remains a statistic, and only insofar as corresponding data have been compiled and assessed. The trick with the term Climate is well illustrated by AMS when it says:  The “present weather” table consists of 100 possible conditions, with 10 possibilities for “past weather” (see previous box). If the weather consist of 100 possible conditions, how can ‘past weather’ consist only of 10 conditions? Who is making the selection? Who decides over the period of time, whether data are used over a period of time ranging from months to thousands or millions of years? Which mix of data represent the past weather or the future weather? Questions over Questions and only because science uses a laymen term, namely:  Climate, without being able, or willing, to define the term in a scientific manner. The term ‘Climate’ as it is used in politics and science nowadays is unfair towards the general public. It is their term for average weather since ancient times. 
30July2009

Further reading:

• The Term Weather in: „Weather Glossaries” C-310

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