-Our weather changes because we live on a round, rotating planet with a bumpy surface. The roundness of the planet causes its equator to be heated more than the poles by the sun. The relatively warm air over the equator vertically expands, causing there to be more air over horizontal surfaces aloft over the equator than over the poles. This in turn causes air aloft over the equator to spread toward the poles. This poleward spreading air is deflected to the east by the turning of the earth. This is why winds aloft generally blow from west to east.
-This eastward moving air strikes mountains on the earth's surface, creating atmospheric waves which move eastward, too. Air rises on the east side of each wave, and sinks on the west side. Clouds and rain or snow form in the rising air, while the weather is usually fair under the sinking air. As the waves move eastward, so do the alternating regions of fair and stormy weather. New waves continue to form by winds striking the mountains, resulting in new patterns of fair and stormy weather which move eastward with the waves. On the earth's surface, we observe these alternating patterns by changes in the weather: Fair one day and stormy the next.
-Since there are many mountains of different sizes on the earth, and since winds don't always strike them with the same speed, then many different kinds of waves can form. These waves can interfere with each other, producing very complicated weather patterns. This is why the weather sometimes doesn't change, with prolonged periods of fair or stormy conditions. This is also why weather changes are sometimes difficult to predict, especially more than a few days in advance.
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