Tuesday, July 22, 2014

Weather Observations. Climate Observations. What is the difference?

Credit: www.easterbook.ca

This is a question that pops up regularly, often times because 30 years ago most experts might have stated that, except for a few important exceptions such as the NOA NWS Cooperative Observer Program, there was essentially little difference. However, today the answer is somewhat more complicated.

As most of us know, weather describes the evolving state of the atmosphere over short periods of time (from a few minutes to two weeks at most) while climate describes the behaviour of the Earth's climate system over long periods of time (from the order of seasons out to years to decades or even longer). In fact the "gap" from two weeks and three months (length of a season) has become a hot topic of current research (but more on that in a future post).

Whereas weather forecasts have only recently come to be expressed in probabilistic terms, climate descriptions have always been in terms of means values (normal's) and probabilities of departure from those normal's (natural variability). Until the last few decades, climatology was largely about analysis to reveal the different types of climate to be found on Earth and research to resolve and understand the often subtle changes in climatic patters on various time scales in the past and at present.

A major change came with the arrival of computing capabilities sufficient to support numeric global-scale climate models, the appearance of both satellite and in-situ ocean observation as well as the finding that the greenhouse gas content of the atmosphere was increasing. A new generation of climatologists became interested in puzzling out the contributions of anthropogenic (man made) activities to climate change and to using numerical models to simulate past climates and predict likely future ones.   

Today, observations from over 11,000 observing stations support operational weather forecasting, air pollution modeling and emergency and military operations, as well as a whole host of industrial and financial applications. Additional data flow in from observations made from balloons and aircraft, buoys and ships at sea, special monitoring systems and a host of satellites.

Many of the stations making primary 3- or 6- hour weather observations (temperature, pressure, humidity, wind speed and direction, precipitation, etc.) are located at airports or in meteorological observatories operated by National HydroMeteorological Services. Such measurements- particularly where supplemented by hourly observations from airports and other locations- are highly useful for resolving synoptic (continental) and sub-synoptic scale events in the atmosphere. In the past, climate information has been extracted from the data produced by these weather observation networks. However, that has now changed.

While modern-day climatologists still use the above mentioned weather observations for many traditional applications, they now need climate-specific observations that provide a much broader range of data (including chemical constituents) to more completely describe Earth's climate. Further, much more extensive observations of the Earth's oceans are required. Such climate observations are in demand not only to monitor the evolution of Earth's climate in a more precise way, but also to validate and verify the numerical models used to predict how Earth's climate system is likely to evolve.

In addition, not all climate observations are the same. The signals that climatologists are trying to tease out of the observational data are quite small and often buried in environmental and sensor noise. Thus, climatologists have developed requirements for climate measurements that are more stringent than those required for weather observations.

Other important factors in climate observations include assuring that observation sites remain at a fixed location for a long period of time and that its observations  be made in a consistent manner using established standards for data from that site in order to have a significant value to a climatologist. This allows computation of a normal for that site that has a high statistical confidence (typically requiring at least 30 years of continuous observations).

A climate observations must also be associated with metadata that provides data users a detailed history of how the observations were collected. Such "metadata" guides how the observation should be interpreted and used.

The stringent requirements for climate observing networks has led to the development of special networks at a national (Reference Climate Stations), regional (Regional Basic Climatological Network) and global scales (the Global Climate Observing System- GCOS). Coordinated by the World Meteorological Organization (WMO) to suppor global climate applications, two networks of observing stations have been established as GCOS Baseline Networks. These are the GCOS Surface Network (GSN)- with 1028 stations-, and the GCOS Upper-Air Network (GUAN)- with 169 stations.

As climate becomes an ever more important issue for individuals, governents, industry and commerce around the worls, we can expect that the number of climate observing networks to grow at all levels. Not only will these be used to monitor the evolution of Earth's climate, but they will become the foundation for long range policy and business decisions in much the same way that weather observations support short term decisions.

The CIRDA Programme is the UNDP Multi-Country Support Programme to Strengthen Climate Information Systems in Africa. Click here to learn more about it.
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