Wei Liu: A Measure to Reduce W
Wei Liu: A Measure to Reduce Wildfire in America
Wei Liu, U. S. citizen, U. S veteran, California, September, 2025
From 2001 to 2025, wildfires have burned an average of about 4,000 square kilometers of land each year in California. One measure to reduce or slow down the potential for large fires is for residents to spray the outside of their houses or apartment buildings using the faucet on their outside walls. Wet material does not burn. This essay uses thermodynamics to analyze the method of spraying the outside of a house to reduce wildfire risk. Thermodynamics is the study of heat energy, including the four laws of thermodynamics. For example, the First Law of Thermodynamics states: Change of Internal Energy = Heat + Work. Change of internal energy is measured in joules; heat is in joules; work is in joules (Cengel, 2024).
A typical house of 150 m² contains about 2,000 kg of wood. The specific heat of wood is 1,700 J/kg·°C. The ignition temperature is 300 °C (572 °F). Suppose the outside temperature is 25 °C (77 °F). Find how much heat is needed for the wood of the house to burn.
Solution: Heat required to burn = specific heat of wood × mass of wood × change in
temperature (Cengel, 2024)
= 1,700 × 2,000 × (300 − 25) = 935 million joules
The showering should cover all four sides of the house as well as the roof, with windows and doors shut. Residents can buy a 50-foot or16-meter water hose for $29 and a nozzle at Home Depot for $12. The total cost, including the water bill, should be around $100 per home. The faucet on the outside wall is shown in Figure 1 below (Photo by Author, 2025).
If it is an apartment building, the procedure should be carried out by the apartment office, with the water reaching at least the second floor. The effect should be similar to the house experiencing heavy rain. Residents should also spray the ground, the sidewalk, the road, and everything outside, as far as the water from the hose can reach. Spraying the outside wall is shown in Figure 2 below (Photo by Author, 2025).
When the same house with 2,000 kg of wood is showered completely wet, its temperature and the temperature of the water on it are around 18 °C, or 64 °F. The specific heat of wet wood is 3,000 J/kg·°C. The specific heat of water is 4,180 J/kg·°C, and the latent heat of water is 2,260 kJ/kg for evaporation. The ignition temperature of wood is 300 °C, or 572 °F. After draining into the soil, about 80 kg of water remains on the walls and ceiling of the house. Find how much heat is needed for the wood of the house to burn.
Solution: Heat on wood = Specific Heat wood*Mass wood*Change of Temperature
= 3000*2000*(300—18) = 1692 million Joule
Heat evaporating water = Specific Heat water*Mass water*Change of Temperature
+ Latent Heat water*Mass water (Cengel, 2024).
= 4180*80*(100—18) + 2260000*80 = 208 million Joule
Total Heat needed for house to burn = Heat on wood + Heat evaporating water
= 1692 + 208 = 1900 million Joule
Percent of Heat needed more than Dry House = (1900 – 935)/935 = 103%.
So, showering the outside of houses and buildings doubles the heat required for them to burn. Instead of an entire city such as Pacific Palisades in Los Angeles burning down in 2025, following the above method could save half a city. Below, Figure 3 shows a 50-ft hose costing about $29 and a nozzle costing about $12 (Photo by Author, 2025).
Some people may wonder whether everyone doing this on the same day could cause a shortage of running water.
Los Angeles has about 4 million people. Each person takes a 10-minute shower every day. A 10-minute shower uses about 25 gallons of water. The time range for people taking showers is from 6:00 p.m. to 12:00 a.m., a total of 6 hours each day. Find the average running water usage per hour for showers.
Solution: City Average Shower Water Use Per Hour
= Population*Show Water Amount/Hours
= 4 million*25/6 = 17 million gallon/hour
This means Los Angeles uses 17 million gallons of water per hour for showers from 6:00 p.m. to 12:00 a.m. every day.
Counting an average of two people per house or apartment unit, Los Angeles has about 2 million houses or apartment units. Showering the outside of an apartment unit should take only about 10 minutes. On average, we estimate that it takes 15 minutes to shower the outside, including the roof, of a residential unit. The time range for doing this should be during daylight, from 6:00 a.m. to 6:00 p.m., a total of 12 hours. Find the average running water usage per hour for showering the outside of a house.
Solution: 10 minutes of running water is 25 gallons. 15 minutes is 38 gallons.
City Average Shower Outside Wall Water Use Per Hour
= Number of Units*Water Per Unit/Hours
= 2 million*38/12 = 6 million gallon/hour
This means that if we shower the outside walls, Los Angeles would use about 6 million gallons of water per hour from 6:00 a.m. to 6:00 p.m. This is much lower than the 17 million gallons per hour used for showers at night and should not cause a significant shortage. In the 2025 wildfire in Los Angeles, two cities were wiped out by fire. Our houses should not simply fuel the wildfire; they should become a burden to it, making the fire lose energy along its path. Our analysis based on thermodynamics shows that showering the outside of a house before a fire is an effective way to reduce wildfire damage.
When there is a wildfire warning, it is worthwhile for all 88 cities in Los Angeles City to shower the outside of houses. Even if it saves only half a city, the benefit outweighs the cost and labor by more than 100 times. Spending about $100—including equipment, water, and 30 minutes of time—could potentially save one’s million-dollar home or a neighbor’s million-dollar home in California. This practice can also increase moisture in the air and may even bring some rainfall in the summer. One hundred dollars versus one million dollars is a very good deal.
References
Cengel, Y. A., Boles, M. A. & Kanoglu, M. (2024). Thermodynamics: An Engineering Approach.
McGraw Hill LLC.
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