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Land and water
Land and soil conditions are yet another factor that can make a big difference in temperature from place to place, even if other atmospheric conditions are equal. For example, if you're on a ship in the middle of the ocean, you won't notice much of a difference between daytime temperatures and nighttime temperatures because the water doesn't heat up or cool off much during the day and the air doesn't absorb much radiation by itself. However, we won't be forecasting for any ships during this contest.
Coastal stations experience the same sort of effect, whenever the wind is right. An "offshore" wind blows from land to sea, and an "onshore" wind blows from sea to land. When the wind is onshore, the air reaching a coastal station has passed over lots of water and very little land. Its temperature will be close to the water temperature, whether it's daytime or nighttime. The dew point also tends to be close to the water temperature. However, when the wind is offshore, the air at the station will have passed over lots of land and little water, and the temperature will behave as usual: rising in the morning and falling in the evening.
Soil moisture
Believe it or not, moist ground can have an effect similar to open water. Although moist ground can easily absorb radiation, some of the energy is taken up in evaporating water. So the ground does not get as hot as it would if it were dry, and neither does the air. You may have noticed that effect around here during the summertime: if we've had recent widespread thunderstorms, the temperature does not get exceptionally hot for a few days. But if it's been dry, the century mark (100 F) is quite attainable.
Cities and the heat island effect
An opposite sort of effect is caused by cities. The large amounts of concrete and asphalt in cities are excellent at absorbing radiation and storing heat. Also, they tend not to be moist, so all the radiation they absorb gets converted into heat. Thus, cities tend to be a few degrees warmer than rural areas during the daytime, and as much as ten to fifteen degrees warmer than rural areas at night. This effect, called the "urban heat island" effect, is strongest for especially large cities, like New York, Chicago, and Snook.
(Well, maybe not Snook.)
When does it matter for the forecast?
At night, if it's cold enough for dew to form, you may have to consider soil type. If the soil at a station easily absorbs water, the dew point (and the temperature) may drop by several degrees during the night. Another good surface for lowering the dew point is snow. It's very easy for water vapor to condense onto ice crystals, so the dew point will frequently drop by many degrees on calm, clear nights over a blanket of snow. If it's cloudy, though, or windy, dew or frost will not form and this effect will not come into play.
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