The most critical single factor regarding the physical siting of your home is the slope of the ground. Heat rises and fire follows it. Fire usually will move much faster uphill than down.

The next factor is the interaction between wind and the ground. High wind will blow embers far ahead of the flame-front itself. This can cause the apparent speed and direction of the fire's spread to change erratically-another reason to be sure someone is watching for fire in a full circle all around you.

Often the fire will jump from one side of a valley to another without actually burning right down to the bottom and up the other side. This may be due to the effects of micro-climates. The valley bottom may be shaded much of the day and thus remain cooler. The fire is very sensitive to temperature differences and to relative humidity.

If, however the wind is blowing along the length of the valley or canyon or arroyo, it may blow in enough hot air to evaporate any moisture. Moreover, if the canyon is narrow and steep it may begin to act as a chimney, actually drawing air in and accelerating it uphill. This can preheat the vegetation in the canyon enough to drive combustible oils and gasses out of the vegetation. Under these conditions the fire might flash up the canyon at very great speed-almost explosively.

These rather technical considerations can have vital implications for you. First, if your home is at the top of a slope or in the middle of a slope you need to pay close attention to the downslope side of your place. Second, you need to do what you can to remove or cover up features of your home that may trap heat flowing uphill toward your house.

To take full advantage of the view, houses are often sited right on the edge of the crest or projecting out over the ground. The deck may be constructed to give unobstructed views. The undersides of such houses and decks can trap air that may become hot enough to ignite before actual flames reach the house. Boxing in these areas with non-combustible materials would be wise. Similarly, closing the openings to carports or sheds will prevent hot air being trapped inside structures which could be ignited thereby.

Outside of the house you also want to look for places where wind currents cause leaves and other litter to pile up. These are often inside corners formed by the main house and an L-shaped projection. These are areas to which the fire winds will tend to carry embers. Pergolas, arbors and sun shades can also cause the same problems. Hedges and trellis-borne flowers are also vulnerable to the drying and combustion of flowing hot air. Stand away, downhill from your place and look up at it to get a perspective on these potential hot air traps.

If you cannot or will not eliminate these traps by boxing or cutting away the vegetation, then your fire plan must include action to be taken in the event of a fire. Otherwise you are inviting direct impingement of flame on your house-a very difficult situation with which to deal, and not an attractive feature for firefighters assessing their options for defending the neighborhood.