Cumulonimbus clouds are a visible manifestation of weather on Earth. Click on image for full size (234K GIF) Image courtesy Aris Multimedia Entertainment, Inc. (1994).

How is space weather similar to Earth weather? How are they different? Does space weather affect weather on Earth, and if so, how? What sorts of short term "weather" and longer term "climate" changes are there in space weather?

When we describe weather on Earth, we talk about several interrelated factors such as temperature, wind speed and direction, humidity, precipitation, barometric pressure, cloud cover, and so on. Similarly, when we speak of space weather there are several key values that we mention, such as the speed of the solar wind, particle density of electrons and protons, the strength and orientation of the IMF, and so on.

Weather on Earth varies from place to place and over time. The same is true for space weather. Space weather might be "stormy" in the vicinity of Mars while it is calm in the neighborhood of Jupiter. A CME aimed at Earth might disrupt space weather in our neck of the woods for a day or two, but could be followed by a period of less disturbed conditions. Changes in weather, both the Earth and space varieties, take place over a wide range of time scales. Some changes happen in matters of minutes and hours, others take days or weeks, and some span periods of years or longer. Long term trends in weather on Earth are considered aspects of our planet's climate; space weather also has long term trends that can be considered "space climate".

When a large, regional weather system on Earth (such as a cold front) encounters certain types of terrain (such as a mountain range or a coastline) the local weather patterns are a result of the combined influences of the weather system and the local terrain. Similarly, space weather systems interact with solar system objects, such as planets and their magnetic fields, in complex ways that determine the exact nature of the "local space weather" in the vicinity of a planet (or moon or comet or...).

We should be careful, though, not to overdo the analogy between weather on Earth and space weather. Space is a very sparse vacuum; the solar wind has such a low density that although it "blows" at rates of hundreds of kilometers per second it wouldn't begin to ruffle your hair. Space weather exists in a realm of powerful radiation, swirling streams of electrically charged particles, and strong magnetic fields. Weather on Earth is mostly confined to a thin shell of the lower atmosphere a few kilometers thick, while space weather affects an area surrounding our Sun which is hundreds of millions of kilometers in extent.

Because heat from the Sun is the primary driver of weather on Earth, you might suspect that space weather has a strong influence on Earthly weather. Scientists believe that this is likely, but convincing evidence in support of this assumption has been difficult to come by. Although the Sun's brightness varies over the course of the 11-year sunspot cycle, the variation of output in the visible portion of the spectrum from the highest to the lowest point in the cycle is a slight 1/10th of one percent. However, there have been periods of extreme weather, such as a severe cold snap in the early 1700's, that correspond with unusual periods in the solar cycle. Both space weather and Earth's atmosphere are extremely complex systems, so the precise effects of variations in one upon the other are difficult to identify and predict. The links, if they exist, between space weather and Earth's weather are an area of ongoing research.

Cumulonimbus clouds represent one type of weather on Earth. Click on image for full size (234K GIF) Image courtesy Aris Multimedia Entertainment, Inc. (1994).

How is space weather similar to Earth weather? How are they different? Does space weather affect weather on Earth, and if so, how? What sorts of short term "weather" and longer term "climate" changes are there in space weather?

When we describe weather on Earth, we talk about several interrelated factors such as temperature, wind speed and direction, humidity, precipitation, barometric pressure, cloud cover, and so on. Similarly, when we speak of space weather there are several key values that we mention, such as the speed of the solar wind, particle density of electrons and protons, the strength and orientation of the IMF, and so on.

Weather on Earth varies from place to place and over time. The same is true for space weather. Space weather might be "stormy" in the vicinity of Mars while it is calm in the neighborhood of Jupiter. A CME aimed at Earth might disrupt space weather in our neck of the woods for a day or two, but could be followed by a period of less disturbed conditions. Changes in weather, both the Earth and space varieties, take place over a wide range of time scales. Some changes happen in matters of minutes and hours, others take days or weeks, and some span periods of years or longer. Long term trends in weather on Earth are considered aspects of our planet's climate; space weather also has long term trends that can be considered "space climate".

When a large, regional weather system on Earth (such as a cold front) encounters certain types of terrain (such as a mountain range or a coastline) the local weather patterns are a result of the combined influences of the weather system and the local terrain. Similarly, space weather systems interact with solar system objects, such as planets and their magnetic fields, in complex ways that determine the exact nature of the "local space weather" in the vicinity of a planet (or moon or comet or...).

We should be careful, though, not to overdo the analogy between weather on Earth and space weather. Space is a very sparse vacuum; the solar wind has such a low density that although it "blows" at rates of hundreds of kilometers per second it wouldn't begin to ruffle your hair. Space weather exists in a realm of powerful radiation, swirling streams of electrically charged particles, and strong magnetic fields. Weather on Earth is mostly confined to a thin shell of the lower atmosphere a few kilometers thick, while space weather affects an area surrounding our Sun which is hundreds of millions of kilometers in extent.

Because heat from the Sun is the primary driver of weather on Earth, you might suspect that space weather has a strong influence on Earthly weather. Scientists believe that this is likely, but convincing evidence in support of this assumption has been difficult to come by. Although the Sun's brightness varies over the course of the 11-year sunspot cycle, the variation of output in the visible portion of the spectrum from the highest to the lowest point in the cycle is a slight 1/10th of one percent. However, there have been periods of extreme weather, such as a severe cold snap in the early 1700's, that correspond with unusual periods in the solar cycle. Both space weather and Earth's atmosphere are extremely complex systems, so the precise effects of variations in one upon the other are difficult to identify and predict. The links, if they exist, between space weather and Earth's weather are an area of ongoing research.

Clouds are one part of weather on Earth. Click on image for full size (234K GIF) Image courtesy Aris Multimedia Entertainment, Inc. (1994).

How is space weather similar to Earth weather? How are they different?

One way they are similar is that weather on Earth changes from place to place and over time. This is also true for space weather. Space weather might be "stormy" around Mars, but calm around Jupiter. A CME aimed at Earth might disrupt space weather around Earth for a day or two, but could be followed by a calm time. Changes in Earth and space weather take place over a wide range of time scales. Some changes happen in minutes and hours. Others take days, weeks or even years.

Earth weather has different parts then space weather. That is one way they are different. When we think about weather on Earth, we talk about if it's cloudy, or rainy or hot or windy. When we think of space weather, we talk about the speed of the solar wind, how many electrons and protons are there, and if the magnetic field (IMF) out in space is changing.