Why is the salad tray at the *bottom* of the fridge? In a badly-adjusted (too cold) fridge, I notice that things like celery & lettuce are more likely to be spoiled by becoming frozen if they're put on a middle shelf, rather than the salad drawer at the bottom.
I realise the salad drawer is "enclosed". But it's far from air-tight, so over time I would expect denser cold air to sink to the bottom and end up filling the salad drawer.
I don't know if it's relevant that I've only seen this phenomenon in fridges with an ice-making compartment at the top of the fridge (as opposed to a "larder fridge" that doesn't have an ice-making compartment). 
I know water expands as it cools below 4°C (the specific part of the temperature scale relevant here), but I'm pretty sure air doesn't do this. So why should the bottom of a fridge end up being the warmest, rather than the coldest part?
 A: The heat transfer inside the fridge are almost completely based on convection. So the air moves around, changing it's temperature - there is not much separation of cold and warm air normally.   
That is intended, of course, and to distribute cold air through the fridge, it's best to put the cooling element at top end - I assume that's the case in your fridge. Now, warm air can go up to the cooling element, and cold air comes down from there.  
In normal operation, the only other relevant heat flow is the leaks through the walls and door. 
In comparison, outside of the salad tray, there is some heat leaking from the walls, and some convection up and down to the cooling element.
Inside the salad tray, there is less leak from the walls because of the additional isolation from the tray itself, but very few convection transporting away the heat that leaked in.   
Because of this, it gets somewhat warmer in the tray. The heat conduction through the tray wall and through the fridge wall (plus minor convection leaking out of the tray into the fridge, added to tray wall conduction) will find a balance, with the tray at a slightly higher temperature.
(The effect is much stronger if the bottom of the fridge is also an outside wall, leaking heat in. But it may also be the top of a colder compartment of the fridge, so I ignore it for simplicity.)
A: Older fridges (and inexpensive modern ones) would only cool the freezer.  To cool the fridge there would be some airflow between the freezer and the fridge which obviously came in from the top of the fridge part.  This is why it was common in older fridges to find liquid things partially frozen on the top shelf.  So, even though the cold air falls the active cooling at the top still establishes a temperature gradient which is warmest at the bottom.
A: As already mentioned by others, inexpensive fridges only cool the freezer on the top. Additionally they often are not isolated very well. And finally, the temperature regulation usually isn't the best too.
Cold air is going downwards through the fridge. But as your fridge might contain a lot of stuff on many shelf, the air flow is reduced. At the same time heat is coming in from all four sides and from the bottom. If you don't open your freezer for a while, all the freezing he does is just to compensate the heat which came in through the surroundings.
Therefore usually the top is the coolest part, cool air is slowly streaming downwards but is warmed by the fridge walls and eventually warm products.
For the low cost temperature regulation. If you put warm stuff in your freezer, of left the door open for a while, your fridge might work harder afterward to get back to the expected temperatures. Cause the whole system does not react that fast, the regulation might cool the fridge too low or not enough. (He might get over the target after cooling hard.) On very cheap fridges, the two temperature ranges (stop cooling temp and start cooling temp) might be wider appart. Near the freezer, one of them usually is below zero degrees. The further away from the freezer your products are, the less they are affected by temperature peaks.
As you can imagine, if you have two compartments, a freezer and a normal fridge, cooled by the same thing, it can be very hard to have both exact on the same temperature. Either one is too warm or the other one too cold.
The same might happen if you put something very warm inside. It might be too warm there, but too cold on the other side. The special compartment helps there, to average the temperature a bit.
I have seen friends putting the salad on top, just below the freezer. This gives nice Salad pieces frozen to the walls which can stay there for many month, until you defrost your fridge the next time.
