The angle of the glass on hatch backs varies from straight up and down to almost horizontal. They almost all have a rear wiper.
sedans and coupes can have similar glass angles, yet don’t have rear wipers.
why?
In: Engineering
Aerodynamics.
As your car moves around, it has to push a lot of air out of the way. It creates a pocket of low pressure air behind it and an area of high pressure air in front of/above it. The air inside the low pressure pocket circulates quite a bit and kind of gets trapped. This pocket is like an invisible bubble that extends back off the rear most part of the vehicle. True for any car, truck, or hatchback.
Most of the air inside this pocket comes from underneath your car, so this air is full of dirt, dust, oily greasy stuff, mist from rain, etc that gets kicked up from the road.
As the air circulates inside the pocket, it moves up along the rear part of the car, leaving behind much of the mist/debris along the rear face of the vehicle.
The rear face of a sedan or truck includes most of the rear bumper, the vertical part of the trunk, and the tail lights. On a hatchback, station wagon, or SUV, this also includes the entire rear window.
Cars/trucks have the rear window in front of the rear edge of the car, so the rear window of a car is exposed to the air moving over the car, which is pretty clean. However, the parts of a car that touch the air pocket also get dirty much faster, at the same rate as a hatchback. The part that gets dirty just doesn’t include the window.
The rear window on hatchbacks is inside the pocket of dirty air, so the window gets dirty much faster. In the rain, a bunch of mist gets kicked up onto the window. It needs a wiper and a spray nozzle to stay clean and clear.
Truck rear windows actually do get dirtier/wetter than cars, but aren’t as bad as hatchbacks. Usually it’s whatever dirt/dust that’s in the bed that gets onto the back window.
Some SUVs have a [wind deflector](https://www.visualautowerks.com/cdn/shop/files/image_cacfee7e-cc37-4918-a6f1-cca37a56cd51_900x.heic?v=1685126944) which takes clean air from above the car and shoots it down the backside of the vehicle. This makes a downdraft over the rear window, making a barrier of clean air. The 4Runner in the picture has a rear window that can be rolled down, so it has a wind deflector instead of a rear wiper.
Also if you notice how most hatchbacks and SUVs now have a little spoiler lip. As the air moves over the car, it also wants to move on top of the air bubble behind the car. This lip sort of cuts off the bubble and creates a smooth transition for the air moving over the car. This reduces drag and increases fuel economy, which is why every hatchback/SUV/station wagon is getting a tiny spoiler thingy.
You can see all of these effects in action in a wind tunnel.
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