Bill Post has shared a detailed historical account regarding the use and purpose of the Fog Pressure Units deployed by the Chicago Fire Department in the early 1960s. The primary goal of the Fog Pressure Units was to provide a rapid water delivery system to control fires before traditional engine companies could connect to a hydrant and lay out a canvas line. These units were particularly useful in areas with limited access, such as expressways, alleys, near railroad tracks, or locations where hydrants were sparse or had unreliable water supply. Between 1961 and 1966, when these units were purchased and put into service, the Chicago Fire Department had only around 11 or 12 engine companies equipped with booster tanks. For nearly a decade, from 1955 to 1966, the CFD did not acquire any new engines. The last batch of engines purchased—ten 1956 B (cab model) pumpers—were the only ones initially fitted with 200-gallon booster tanks. A few older models were later retrofitted with similar tanks, but this was rare. Each Fog Pressure Unit came with a 300-gallon water tank and two reels of narrow-diameter hose. They were equipped with specialized fog nozzles that created a fine mist at high pressure, which is why they were called "Fog." The first unit was built on a Willys Jeep chassis with a John Bean pump, while the rest used International Harvester chassis with Darley pumps and bodies. Although some units were stationed near expressways, most were located in high-fire-volume areas on the city's west and south sides. Fog Pressure 2, for example, was based at O’Hare Field and originally used the original Willys Jeep apparatus. In addition to their primary role, Fog Pressure Units were assigned as secondary units to all three Snorkel Squad companies. They provided tactical flexibility, allowing firefighters to carry extra personnel and equipment. In 1964–1965, the CFD acquired three longer-bodied Fog Pressure Units specifically for the Snorkel Squads. As newer engines began to roll out in 1967, many of them featuring 500-gallon booster tanks, the department started moving some Fog Pressure Units away from stations receiving the new engines. By 1970, most had been retired, with only two remaining: Fog Pressure 2 at O’Hare Field and Fog Pressure (SS1A) assigned to Snorkel Squad 1. Most units were taken out of service between 1968 and 1969, including those supporting Snorkel Squads 2 and 3, which were both disbanded that year. One notable example was Fog Pressure 31A, which was temporarily stationed at Truck 31’s new location for over a year until Engine 64 was relocated there. At its peak in 1966 and 1967, the CFD operated twelve Fog Pressure Units, not counting the three assigned to the Snorkel Squads. However, within just two years, nearly all of them were retired, leaving only the exceptions mentioned above. Bill Post KingKong, Intelligent Equipment (Xuzhou) Co., Ltd , https://www.impimaq.com