When a HE shell explodes, most of the fragments shoot out in a
circular pattern from the sides of the shell, with some amount of
fragments from the nose and tail. Unless the angle-of-impact is
very high, the fragments that shoot upwards are ineffective:
The burst area in yards for a single HE shell with quick fuze,
defined as the area in which there is at least a 50% chance that
a standing man will become a casualty, is given below for various
guns. Depth is in the direction that the shell is travelling --
due to side-spray, the area is longer along the perpendicular.
Large fragments can be thrown over a larger area.
The following table shows burst areas for various weapons.
Remember that this is the burst area for a
single shell, not a volley or sheaf:
|155mm Gun or Howitzer||18||60||550|
|8" Gun or Howitzer||20||80|
If a smoke shell penetrates the ground or hits it too hard, the smoke effect will be minimal. Base-ejection shells throw the smoke generating part of the shell backwards just before impact so it is moving at a slower velocity when it hits the ground and is unlikely to penetrate.
Most smoke is produced by hexachlorethane (HC), which burns cold, so the smoke will tend to drift along the ground as blown by the wind. Rain makes it more effective. Smoke shells take 30-60 seconds to build a cloud; 1-2 minutes to reach maximum effectiveness; and they burn 3-4 minutes.
As the diagram on the right shows, weather can have a dramatic effect on the behavior and effectiveness of a smoke screen. If the wind is parallel to the screened front, the shells can be spaced by as much as 400 yards. If perpendicular, they might need to be as close as 30 yards. Ammo expenditure per minute per point in the screen is 1-2 shells for 105mm, depending on the wind, and 0.5-1 shell for 155mm.
WP produces smoke, incendiary effect, and causualties. Quick fuze is preferred in all cases -- unlike HE, with WP, those fragments that shoot upward are actually useful. Because the burning is very hot, smoke tends to pillar upwards rather than spreading across the battlefield. To build a smoke screen, more ammo must be used than conventional smoke shells.
In terms of casualties, WP should be treated as about half as effective as HE, though all shots should be treated as airbursts. 
Illuminating shells burn for about a minute. In all cases, it's best to have coverage from at least two shells to minimize shadows. They should burst at about 700 yards of height; be spaced about 700 yards apart; and be fired at a rate of about 2 rounds per minute. A single 155mm shell will illuminate an area about 1000 yards in diameter.
[Wesely] 75, 105, and 155 howitzers, as well as 60mm, 80mm, and 4.2" mortars, are capable of firing flares.