To limit the amount of light coming in from the side so the sensors can more readily read the projectiles interuption of the overhead light the path of the projectile over the sensors and between the light screen momentarily casts a shadow over the sensor thats how those type of chronographs function.

As you would get that much glare from the snow all about it could lead to erronous readings or the chronie not working at all due to the relected sunlight or ambient light.