Ground fog that stays on the ground

Why it rises

A fog machine heats fluid to several hundred degrees and pushes out vapor that is warmer than the night air. Warm air rises, so the fog does too. To make it crawl across the lawn you have to cool it below the surrounding air on the way out. Everything else, the fluid, the wattage, the timer, comes second.

Three ways to chill it

• A fog chiller: a cooler with an inlet and an outlet, packed with ice, so the fog snakes through the ice before it exits. A 48-quart cooler with two bags of ice runs most of an evening and drops fog to ankle height.
• Dry ice in a chiller does the same with more effect and less melt, but treat it with gloves and keep it out of a sealed car on the drive home.
• A dedicated low-lying fog machine, which chills internally. It costs more and does one thing very well; the standard 1200 to 1500 watt machine plus a chiller does the same for less if you do not mind the cooler on the porch.

Fluid and timing

Use a dense or ground fog fluid, not the thin haze made for concerts. Dense fluid holds together longer once it is cold. Then run the machine in bursts, not continuously: five to ten seconds every minute or two, on the machine's timer. A continuous run makes a wall of fog that hides the yard and the props you paid for. Bursts make drifts that reveal and cover the scene as they move.

Placement

Put the outlet low and upwind of the scene, behind a tombstone or a fence section so the machine itself is hidden. Fog follows the ground downhill and with the wind, so on a still night it pools in the low spot of the lawn, and on a breezy night it goes wherever the breeze goes. Humid, cool, still nights are the best fog nights of the year. A warm, dry, windy night will not give you fog no matter what you bought.

The two rules people forget

Never fog the walkway. A kid who cannot see the step is a kid who falls, and a parent who cannot see the kid is a parent who does not come back. Fog the lawn and leave the path clear. And never run the machine dry: when the fluid is gone the pump burns, and it goes quietly. Check the tank at the start of every night.

Will fog stain the siding or kill the grass?

Water-based fog fluid leaves a faint glycerin film on hard surfaces that hoses off, and does nothing to grass. Keep the outlet a few feet from the house so the film does not build up on paint.

How much fluid for a night?

A 1500-watt machine in bursts uses roughly a quart over three hours. Buy a gallon for the season and you will finish October with some left.