**Introduction to the Hydrologic Cycle**

Figures 6-7 and 6-8 on pages 134 and 135 in your textbook show the saturation curve. These curves are identical to the one in the lecture notes except for the fact that water vapor, represented on the y-axis, is measured in different units. The lecture notes curve (also shown below) measures water vapor as grams of water vapor per kilogram of air, Figure 6-7 measures water vapor as grams of water vapor per cubic meter of air, and Figure 6-8 measures water vapor in millibars. Regardless of the units used to measure water vapor, the relationship between water vapor (on the y-axis) and temperature (on the x-axis) is constant.

The water vapor capacity of the air is the maximum amount of moisture that can remain in a vaporous state in the air at a given temperature. The water vapor capacity of the air represents the amount of water vapor present when the air is saturated. As air temperature increases, the water vapor capacity of the air also increases. Think of the air as a water glass; the larger the glass the more water the glass can hold. Likewise, the warmer the air temperature, the greater the capacity. If you know the air temperature, you can use the saturation curve to determine the capacity.

Example: if you know the air temperature is 10^{o}C, you start at 10^{o}C on the x-axis, go up to the saturation curve, and then left to find out that the capacity is about 8 g/kg.

Example: if you know the capacity is 20 g/kg, you start at 20 g/kg on the y-axis, go over to the saturation curve, and then down to find out that the air temperature is about 25^{o}C.

Use the saturation curve below and the ones in Figures 6-7 and 6-8 in your textbook to answer the following questions.

What is the approximate water vapor capacity of air at 15^{o}C using the lecture graph below?

What is the approximate water vapor capacity of air at 15^{o}C using Figure 6-7?

What is the approximate water vapor capacity of air at 15^{o}C using Figure 6-8?

Your capacity measurements should be approximately 11 g/kg, 12 g/m^{3}, and 15 mb; these all represent approximately the same amount of water vapor measured in different units.

If the water vapor capacity of the air is 30 g/kg, what is the approximate air temperature?

If the water vapor capacity of the air is 42 mb, what is the approximate air temperature?

If the water vapor capacity of the air is 28 g/m^{3}, what is the approximate air temperature?

Your air temperatures should be about 31^{o} or 32^{o}C in all three cases because the three measures of water vapor are approximately equal.

To calculate relative humidity, in addition to knowing the water vapor capacity of the air, you also need to know the water vapor content - the amount of water vapor actually present (as opposed to the maximum amount of water vapor that *could* be present).

Relative humidity = water vapor content divided by water vapor capacity = content/capacity

Example: if the amount of water vapor actually present in the air (the content) is 20 g/kg and the current air temperature is 28^{o}C, what is the relative humidity? Using the curve above, at 28^{o}C the water vapor capacity of the air is about 25 g/kg (25 grams of water vapor per kilogram of air). So, the relative humidity = 20/25=0.80 or 80%.

Use the saturation curve above and the ones in Figures 6-7 and 6-8 to calculate relative humidity.

If the water vapor content of the air is 6 g/kg and the current air temperature is 15^{o}C, what is the relative humidity?

If the water vapor content of the air is 7 g/m^{3} and the current air temperature is 15^{o}oC, what is the relative humidity?

If the water vapor content of the air is 8 mb and the current air temperature is 15^{o}C, what is the relative humidity?

Although your relative humidity calculations will not be identical, they all should be around 53-58% because 6 g/kg, 7 g/m^{3}, and 8 mb all represent the approximately the same amount of water vapor. The point is that regardless of how you measure the water vapor content of the air, the relationship between air temperature and water vapor remains the same.

In addition to using the saturation curve to calculate relative humidity, you can also use it to determine the dew point temperature because the water vapor content of the air is directly related to the dew point temperature: if the content increases, the dew point temperature increases. The dew point temperature is the temperature the air must cool down to for the air to be saturated. This would be like a glass with a content less than the capacity; how much must the glass shrink for it to be full given its content? If you know the content, you can use the saturation curve to determine the dew point temperature. If you know the dew point temperature, you can use the saturation curve to determine the water vapor content.

Example: if you know the dew point temperature is 5^{o}C, you start at 5^{o}C on the x-axis, go up to the saturation curve, and then left to find out that the content is about 6 g/kg.

Example: if you know the content is 15 g/kg, you start at 15 g/kg on the y-axis, go over to the saturation curve, and then down to find out that the dew point temperature is about 19^{o}C.

Thus, on the saturation curve, the y-axis can represent either water vapor capacity or water vapor content of the atmosphere, which can be measured in a variety of ways. The x-axis can represent either air temperature or dew point temperature, both measured in ^{o}C.

There are some sample problems on page 145 in your textbook. If you need more practice with the saturation curve, see the practice lab problems and answers.

K.A. Lemke 8/25/2010