Based on the information in Box 5-2, do you think that astronauts in orbit hundreds of kilometers above the Earth see the stars twinkle? Why or why not?
(Reference Box 5-2)
Box 5-2 GPS and Water Vapor
The same technology used in cars to tell you where you are as you drive around also is used to tell how much water vapor is in the atmosphere and to improve weather forecasts.
Measuring atmospheric water vapor is important in weather forecasting and climate monitoring. Short-term cloud and precipitation forecasts require timely and accurate moisture data. Water vapor releases huge amounts of energy as it condenses to form clouds and precipitation, providing an energy source for storms. Water vapor is a greenhouse gas that plays a critical role in the global climate system.
Weather balloons measure the vertical profile of water vapor, which can be used to determine the total amount of water vapor in a column of air. These balloon-borne water vapor sensors, however, are expensive and do not provide the frequent measurements needed for accurate weather forecasting. Remote sensing is an approach to reduce costs and decrease the time between measurements.
The GPS tracking network was established to provide high-precision navigation. The system consists of satellites and a ground-based network of support stations that receive data. Because the GPS uses radio waves, it works in every type of weather—clear, cloudy, or rainy conditions—which is important in weather forecasting. A constellation of GPS satellites transmits radio signals to receivers on Earth. These GPS satellite radio signals are slowed by the atmosphere, which results in a delay in the arrival of a signal as compared with proposition in space. Data recorded by GPS receivers at fixed locations will show delays caused by a variety of effects, one of them being the amount of water vapor in the atmosphere. The measured delay is used to determine the amount of water vapor in the atmosphere.
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