Box 1. A primer on monitoring and predicting the Earth’s atmosphere Box 2. Importance of observations beyond national boundaries:

Box 1. A primer on monitoring and predicting the Earth’s atmosphere

Weather and climate prediction is global by necessity, not by choice. Weather systems develop and move across the planet regardless of political boundaries. The atmosphere has no horizontal boundaries and only in its entirety can it be simulated mathematically.

The foundation of all weather and climate monitoring and prediction is global NWP done by the Global Producing Centres that are operated by WMO Member

states and territories. An NWP prediction starts by assimilating vast amounts of meteorological observations from the entire globe into an Earth system model to build a worldwide model estimate of the instantaneous weather. The model then uses the laws of physics to evolve this “initial weather” forward in time. The quality of NWP output can be objectively quantified, and progress in NWP relies on extensive use of broadly agreed measures of lead time, or range - the extent forward in time that can be predicted -, and skill - the quality of the prediction at a given time.

The only practical way to limit the errors – in other words, to improve the range and skill of the prediction – is to ensure that the “initial weather” of the model is as accurate as possible everywhere, and this can only be achieved with frequent observations made everywhere. The ability to accurately predict the weather is limited by what is known as the “butterfly effect”. The impact of the butterfly effect is profound: Any small, local gaps or errors in our knowledge of weather anywhere will propagate and amplify in the models, eventually degrading forecast skill everywhere.

The international exchange of observations is therefore just as important as the observations themselves – observations that are not made cannot be exchanged, and observations that are not exchanged have no value for prediction.