City air is drier but rainfall is higher

City streets and squares are paved. This is the first and most obvious difference between the urban and rural environments. It means that rain does not soak into the ground, but is removed by storm drains. When the rain ceases the streets dry very quickly. Rainwater is removed too quickly for much of it to evaporate and there are fewer areas of standing water— ponds and lakes—than there are outside the city. Consequently, there is less evaporation in the city than there is in the countryside. There is also less transpiration, because there are fewer plants to move water from the ground into the air.

Combined with the warmer temperature, which lowers the relative humidity by about 6 percent compared to rural air (see the sidebar on page 152)—or sometimes by up to 30 percent on warm nights—reduced evaporation and transpiration suggest the urban climate should be drier than

Humidity

The amount of water vapor air can hold varies according to the temperature. Warm air can hold more than cold air. The amount of water vapor present in the air is called the humidity of the air. This is measured in several ways.

The absolute humidity is the mass of water vapor present in a unit volume of air, measured in grams per cubic meter (1 g m-3 = 0.046 ounces per cubic yard). Changes in the temperature and pressure alter the volume of air, however, and this changes the amount of water vapor in a unit volume without actually adding or removing any moisture. The concept of absolute humidity takes no account of this, so it is not very useful and is seldom used.

Mixing ratio is more useful. This is a measure of the amount of water vapor in a unit mass of dry air—air with the water vapor removed. Specific humidity is similar to mixing ratio, but measures the amount of water vapor in a unit mass of air including the moisture. Both are reported in grams per kilogram. Since the amount of water vapor is always very small, seldom accounting for more than 4 percent of the mass of the air, specific humidity and mixing ratio are almost the same thing.

The most familiar term is relative humidity. This is the measurement you read from hygrometers, either directly or after referring to tables—and it is the one you hear in weather forecasts. Relative humidity (RH) is the amount of water vapor in the air, expressed as a percentage of the amount needed to saturate the air at that temperature. When the air is saturated the RH is 100 percent (the "percent" is often omitted).

the rural climate—but it is not. Scientists are not sure why this is so, but it may be that the dustier air over the city produces a higher concentration of cloud condensation nuclei (see the sidebar "John Aitken and cloud condensation nuclei" on page 125). In calm weather with clear skies, city streets tend to trap warm air, and the air remains moist because the surfaces of streets and buildings are too warm for excess moisture to be deposited as dew.

Whatever the reason, there are six or seven more rainy days a year in European and North American cities than there are in the nearby countryside, suggesting the urban rainfall is up to 10 percent higher. Summer thunderstorms and hail are also more frequent in some areas, especially within and up to 25 miles (40 km) downwind of cities in the Midwestern states.

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