As the natural gas is stored in a fixed volume tank, the tank's pressure will vary in direct proportion to the temperature of the gas in the tank. See the chart below - and here's how the Institute of Gas Technology (IGT) explains it in their 1993 document:
Natural gas (or any gas) when confined in a fixed space will increase in pressure as the ambient temperature rises or decrease in pressure as the temperature falls. This is true for natural gas in a vehicle's storage tank as well as the air in the vehicle's tires. The magnitude of the change in pressure is a function of the size of temperature change, starting pressure, and the gas composition. Since the term "natural gas" represents a broad range of gas compositions as distributed in the US, IGT used a mean natural gas composition.
The temperature-pressure relationship figure presented (below) can be used to determine the change in pressure corresponding to a change in temperature for natural gas. Six reference lines have been plotted corresponding to six different starting gas pressures at 70 deg F. As an example, point A represents natural gas in a storage container at 3,000 psig and 70 deg F. Should the gas temperature drop to 40 deg F, one can expect the gas pressure in the tank to drop to 2,596 psig (point B)... It is important to note that the change in pressure does not represent a change in the amount of gas (energy) within the tank. The gas simply contracted or expanded as a result of the change in temperature.
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