Why is the departures level always above the arrivals level?

There actually is a right answer to this. Surprised it’s not here yet.

Look at the airport this way:

                                                        |
                                                       (*)
          Departures                    Jet Bridge     _|_
 ________________________________________________  (*)/===\(*)
|_____________________________________|__________|___(  .  )___________   
|_________//__________________________|     (o)       \___/
 Arrivals         Airport Guts

How high are planes? The jet bridge needs to be one story above ground for passengers to walk more-or-less level when embarking. If the jet bridge was on the ground floor, one would need stairs. Stairs are hard to navigate for many passengers, wheelchairs etc.

The fact that having the airport guts at ground level lets them connect to the airport basement is bonus. There’s also plenty of room for arrivals at ground level.

Edit: sorry if this wasn’t clear. There has to be a second floor to the airport, but why pick departures over arrivals? To the other posters’ points: because arrivals requires much less space than departures. Though, in a modern airport, you can simply use departures as shorthand for “inside security” which may help explain.

In addition to the point mentioned about baggage generally flowing downhill (which definitely makes baggage collection on the ground floor / basement a good idea), and the tendency for for drivers waiting to pick up spending longer than drivers waiting to drop off, there is one other point that has not been mentioned.

Passengers generally spend very little time in the airport once they have arrived. It is in the interest of both the airport and the passengers for them to be gone as quickly as possible.

That often means they walk straight off the plane, through immigration, baggage claim and customs, and out to the arrivals pickup area, without a single change in level.

Departures passengers, on the other hand, have to queue for check in, wait for security, then hang around for ages waiting to be called for boarding. While none of this is particularly agreeable, it is unavoidable, and level changes are a trivial additional inconvenience.

Two fairly new terminals I visit frequently (London Heathrow Terminal 5 and Madrid Terminal 4) are prime examples of this.

A departing passenger has to go to the top floor to check in and go through security, then goes down a level, to one or two levels of departure lounge / shopping. If they are not flying from the main building, they may then have to go down to the bottom level to catch the underground monorail to the satellite building(s) where shopping is minimal.

On arrival the passenger either arrives on the ground floor of the main building, or is brought there from the satellite by the monorail. There may be some twists and turns for immigration, but he/she then walks pretty much straight out through baggage claim and customs to the onward transport facilities.

Rain.

When someone drops you off at an airport, you can go inside immediately. When you are being picked up, you usually have to stand around for a while.

If departures were on the upper level, either they would have to build awnings or everyone would get wet and miserable. As it is, the people waiting for a ride are sheltered by the departure lanes overhead.

In addition to the reasons others have listed, an additional advantage to having the departures on top of the arrivals is that you can then add another floor for departures, if desired. As others have mentioned, departures is where most of the airport amenities will be on the airside. Thus, in order to add more space for amenities, many airports actually have another departures level above the main departures level. Just off the top of my head, I can think of many major airports that do this, such as ATL (at least in concourses A and F,) ICN (in both buildings,) DTW (A concourse,) LAX, and SFO (international terminal.) Even some medium-traffic airports do this, too, though, such as concourse C at BNA.

I think the basic reason comes down to baggage and gravity. If you want gravity to help move the baggage forward, it makes sense to have the baggage check in at the top, the baggage claim at the bottom, and the loading dock at tarmac level somewhere in between.

This not only saves energy in moving baggage along the conveyer belts, but also cuts down on wear and tear.

I think some of the earliest baggage conveyer systems weren’t even motorized. They just had lots of rollers mounted on ball bearings, and let gravity do the work. That would have been enough to set the precedent.

I’ve been at a few airports that only have one level, but they are the exception.

Note that these are just assumption based on observation.

To add to the other answers:

Good design can be born in many ways. Either by creation, but also by trial and error (or a mix of both). With airports people just realized that this setting works.

Note that this setting (2 floors) is typical in large airports. A good reason for having 2 floors is because it saves horizontal space and that is precious for airplane parking. Many small airports only have one floor because they have little traffic and enough space to keep everything at the same level (stairs, elevators, etc all occupy space and cost money).

Once you have the need for 2 floors you start thinking what to do with them. Since we have 2 types of airport users (arriving and departing) it’s natural to keep each group together, at the same level. You could split the building differently, but using one floor for each group is, in principle, more rational.
Also we can easily assume, as others suggested, that airports worry more with departing passengers than arriving ones. Not that arrivals are not important but they stay less time and require less space. The comfort on departures is probably an important factor in an airport and it’s where the airport can make extra money.

With this setting you can occupy part of the space in ground floor with technical areas, car parking, luggage handling and also arrivals. It;s also easier for employes working on the field to enter leave the building. And you might want to have those technical areas not only at ground level but also underground. This way you keep them adjacent.Having departures in the first floor allows not only for larger areas but also for natural light, either by using large windows or sky lights. Natural light and the sense of depth through the windows is a factor of comfort.

Probably as a consequence airport designers also realized that having the departures above puts passengers approximately at the airplane level and therefore jet bridges were born. I think they were born as a consequence but it’s now a reason to support keeping the same design. It’s also more comfortable to the departing passengers to just walk in to the airplane.

Currently in most airports I know departing passengers cross with arriving ones in the commercial area. But I’ve seen a few older airports where this didn’t happen. Arriving passengers would actually be dropped on the ground floor. This kind of setting makes it easier to route persons (to add signs, etc). Also, you require less space since people always follow the same route and arrivals don’t cross with departures. I now see many airports that drop passengers exactly in the same floor as departures. Although airports still try to separate them early, by routing arrival passengers down this is probably a commercial decision since it might induce consumption in the shops. After all many people still have to wait a couple of minutes for their luggage. Better have them around the shops than looking at a luggage belt.

In addition to SpaceDog’s answer, the main issue which forces airport designs to make arrivals at the lower level and departures at the higher level is land space due to vehicles.

Cars, taxis, buses, etc. when dropping people for departures they take much less time to do so, just drop and leave. Which means less space is required for that and upper levels fulfill that.

Unlike arrivals, taxi queues, buses and private cars picking up people need more waiting time because people do not come out from flights at once (Immigration, Customs, etc.). In addition to that, there are rentals as well. All of these require more land space which is available at the lower level.

Warning, guesswork ahead — I couldn’t find anything definitive.

Checked baggage is handled at ground level (i.e. that’s where it comes out of the plane). Therefore it makes sense to have baggage claim on the same level to save the not inconsiderable energy it would require to move baggage up a floor (and then inconvenience people having to take it down again). Once baggage claim is on the ground floor it makes sense to have arrivals there to.

The other concern is that you need more space for departures — people spend more time there and that’s where an airport can make money with shops and things. In arrivals people are generally moving through the airport and leaving; as such not as much space is needed. That means you can use the extra space on that ground floor for all the behind the scenes stuff (such as baggage handling).

Also, many airport terminals are big ‘hanger’ designs and having departures (where people spend more time) upstairs means they get the space and the high ceilings, etc. People coming off a plane are not going to care that much about how high the ceilings are. I borrowed this last point from this thread discussing the same thing. It also notes there are more than a few exceptions to the convention.