In the FAA AFD for Santa Rosa airport (KSTS), the pattern altitude is given as 1,000’ AGL for small aircraft, and 1,500’ AGL for large aircraft.
Why do these altitudes differ? Doesn’t it compromise having a uniform pattern altitude so aircraft can see each other?
Several possible reasons.
Speed differences between aircraft classes. Larger jets cruise and land at much faster speeds than single-engine props. Keeping their pattern separate from slower traffic minimizes problems posed by a faster plane entering the pattern behind a slower one.
Noise abatement. A jet, even a smaller private one at near-idle in approach, is loud. Keeping them higher increases the distance and number of thermal strata to shield nearby residents from traffic noise.
Prevailing instrument conditions and relative pilot experience levels. A ceiling below 1000 feet, the small craft pattern altitude, would ground VFR flights and thus drastically reduce the number of small craft in the first place. Jets, turboprops and other larger planes are much more likely to be IFR-capable (plane and pilot) and so they're placed higher because they can deal with clouds in the pattern (the airport, if towered, would likely alter pattern height to clear cloud layers but traffic holding above the clouds would still require an IFR approach).
Larger aircraft generally are faster and fly wider patterns than small planes. It would be much more difficult to sequence different types of aircraft at the same altitude while maintaining separation.
Additionally to KeithS' answer, you also have some airfields in e.g. Germany, where a different pattern exists for glider aircraft, ultra-lights and regular single or multi engine pistons. The consideration here is not only speed, but also the ability to maintain level flight. Glider patterns in almost all cases will be very close to the airfields with little impact on surrounding housing areas, as gliders generally create less noise than powered aircraft. They also need to remain closer to the airfield in the pattern, as they can only trade speed vs altitude, but not gain altitude or maintain it as power aircraft. Powered aircraft can follow a pattern that will be suitable for noise abatement and other requirements, such as obstacle clearance or avoiding certain areas for other reasons.
The same consideration that apply above between gliders and powered aircraft do apply between slow powered aircraft and fast powered aircraft, as already explained in the other answer.
gliders generally create less noise than powered aircraft -- under what circumstances would a glider create more noise than a powered aircraft??? :)
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At many of the airports I've flown in (as a helicopter pilot), they will also have rotorcraft fly in the opposite direction from fixed wing. Our descent profile and cruising speeds are different than aircraft, and given that we may do confusing things like slow down into a hover, its for the best. Generally they'll place us at a different altitude as well.
As other people mentioned, the fact that larger planes travel at different speeds is one reason they may place them in a different pattern, and I suspect it helps with wake turbulence as well. The planes can be spaced a bit closer together if large planes aren't comingled with the smaller aircraft.
Similar setups will also happen in commonly used airways. For example, where I live in Chicago, and when flying westward through Midway/O-Hare airspace (they overlap), they'll generally have us fly along the highway. They tend to put the rotorcraft at a much lower altitude than the fixed wing, which I suspect is not only for the same reasons as above, but also because the altitude minimums for rotorcraft are lower than for fixed wing, and it gets more traffic out of the way.
