Rain in the City. Click on image for full size (95 kB JPEG) Copyright 2002 University Corporation for Atmospheric Research

As an example, consider two different computer simulations of the path of a hurricane. Both of them start from some initial state, estimated from satellite and local measurements of wind speeds, temperature, etc. Let's say that these initial states are exactly the same except the temperature over Miami is one degree Celsius warmer in the second one. Although the results of these two simulations may be similar (hurricanes usually travel toward the north-west), the detailed paths may differ by hundreds of miles.

This is not a problem with the computer program - the real atmosphere is like this, as is any turbulent flow. The presence of rain in Colorado next Monday may depend on whether or not a butterfly in China decides to flap its wings today. The tiny amount of air moved (or not moved) by the butterfly may set off a chain reaction which could effect weather all over the Earth. This is where the butterfly effect gets its name.

The butterfly effect is why we have to use statistics to describe the weather and other turbulent flows. We can estimate the probability that it will rain in Colorado next Monday but we can't predict exactly where and when with 100% certainty. Weather forecasts will continue to improve but they will never be perfect. The atmosphere is just too complicated for that.

The butterfly effect is closely related to the concept of chaos. Chaos is commonly used to mean disorder but to a scientist it has a very precise meaning. Consider two small packets of fluid which initially lie close to one another. A flow is chaotic if the distance between these two fluid packets increases exponentially with time. A chaotic flow is very complicated but it is not entirely random.

Rain in the City. Click on image for full size (95 kB JPEG) Copyright 2002 University Corporation for Atmospheric Research

As an example, consider two different computer simulations of the path of a hurricane. Both of them start from some initial state, estimated from satellite and local measurements of wind speeds, temperature, etc. Let's say that these initial states are exactly the same except the temperature over Miami is one degree Celsius warmer in the second one. Although the results of these two simulations may be similar (hurricanes usually travel toward the north-west), the detailed paths may differ by hundreds of miles.

This is not a problem with the computer program - the real atmosphere is like this, as is any turbulent flow. The presence of rain in Colorado next Monday may depend on whether or not a butterfly in China decides to flap its wings today. The tiny amount of air moved (or not moved) by the butterfly may set off a chain reaction which could effect weather all over the Earth. This is where the butterfly effect gets its name.

The butterfly effect is why the weather is so hard to predict. We can estimate the probability that it will rain in Colorado next Monday but we can't predict exactly where and when with 100% certainty. Weather forecasts will continue to improve but they will never be perfect. The atmosphere is just too complicated for that.

Rain in the City. Click on image for full size (95 kB JPEG) Copyright 2002 University Corporation for Atmospheric Research

As an example, let's pretend a hurricane has just formed in the the Carribean Sea, southeast of Florida. Hurricanes usually travel toward the northwest but their detailed paths can be very different - some hit the U.S.A. and others don't. It would be nice to be able to predict where a hurricane will go after it forms.

It turns out that this is not an easy thing to do because the atmosphere is turbulent . This means that a tiny change, like pushing the hurricane north just a few inches after it forms, can make the hurricane travel in a completely different direction. Changing its path by just a few inches today could change its path by hundreds of miles by the time it hits (or doesn't hit) land.

Let's take another example. Believe it or not, the presence of rain in Colorado next Monday may depend on whether or not a butterfly in China decides to flap its wings today! The tiny amount of air moved (or not moved) by the butterfly may set off a chain reaction which could effect weather all over the Earth. This is where the butterfly effect gets its name.

The butterfly effect is why the weather is so hard to predict. We can make a good guess whether it will rain in Colorado next Monday but we can't predict exactly where and when. Weather forecasts will never be perfect. The atmosphere is just too complicated for that.