Desert Air, Ocean Air and a City’s Power Use

A Hot Week at the Planning Office

1Puerto Arena is a coastal city in southern California, with the Pacific Ocean to its west and mountains and high desert to its northeast. The city and its numbers are made up, but they are based on real fall weather in the region. Ms. Yolanda Petrosyan runs the city office that plans the supply of electricity and water. In October, sixth grader Ravi Duchamp spent a week shadowing her for a school career program.

2Ms. Petrosyan explained that an air mass is a huge body of air with about the same temperature and humidity throughout. On most days, the wind carries air into the city from the west, off the cool Pacific. That air is mild and humid, and it often brings fog or low clouds in the morning. A few times each fall, winds push air from the high desert toward the coast. That air holds very little water vapor, and as it flows down the mountain slopes it is squeezed and warms up, so it often reaches the coast hot and very dry. To compare air masses, the office uses the dew point: the higher the dew point, the more water vapor the air holds. Fog can form when the air cools to its dew point.

3Her office tracks two kinds of use. Peak power is the most electricity the whole city uses at any one moment of the day, in megawatts (MW). On hot days, much of it runs air conditioners and fans. Water use is the total water the city supplies in a day, in millions of liters (ML). On hot, dry days, many people water lawns and gardens more often.

4Ravi copied the noon weather and the daily use into Table 1. On Tuesday at noon, Ms. Petrosyan also checked three other weather stations on a line from the high desert to the ocean (Table 2). She said that the office uses readings like these to decide when to ask power plants to get ready for extra demand.