Chapter 3

Explain why the warmest time of the day is usually in the afternoon, even though the sun's rays are most direct at noon.

Around noon, the sun's rays are most intense. However, even though incoming solar radiation decreases in intensity after noon, it still exceeds outgoing heat energy from the surface for a time. This situation yields an energy surplus for two to four hours after noon and substantially contributes to a lag between the time of maximum solar heating and the time of maximum air temperature several feet above the surface

Diurnal Cycle

Day and Night Measuring temperature Air temperature changes rapidly in the first few centimeters 2 m temperature; sheltered - The diurnal and annual ranges of temperature are greater in dry climates than in humid ones. Near the equator, the annual range of temperature is small.

Why is the largest annual range of temperature normally observed over continents away from large bodies of water?

For one thing, solar energy reaching land is absorbed in a thin layer of soil; reaching water, it penetrates deeply. Because water is able to circulate, it distributes its heat through a much deeper layer. Also, some of the solar energy striking the water is used to evaporate it rather than heat it. It takes a great deal more heat to raise the temperature of a given amount of water by one degree than it does to raise the temperature of the same amount of land by one degree. Water not only heats more slowly than land, it cools more slowly as well, and so the oceans act like huge heat reservoirs. Thus, mid-ocean surface temperatures change relatively little from summer to winter compared to the much larger annual temperature changes over the middle of continents.

Relative Humidity Diurnal Cycle

Note the temperature and humidity cycles ◦ Not in phase ◦ Lower temperature, higher humidity and vice versa

When the relative humidity is given, why is it also important to know the air temperature?

Relative humidity is always given as a percent. Air with a 100 percent relative humidity is saturated because the air is holding as much water in the vapor state that it can hold (at that temperature). Air with a 50-percent relative humidity is only holding 50 percent of its capacity for water vapor.

Explain why the daily range of temperature is normally greater in dry regions than in humid regions

The largest diurnal range of temperature occurs on high deserts, where the air is fairly dry, often cloud-free, and there is little water vapor to radiate much infrared energy back to the surface. By day, clear summer skies allow the sun's energy to quickly warm the ground which, in turn, warms the air above to a temperature often exceeding 160°F. At night, the ground cools rapidly by radiating infrared energy to space, and the minimum temperature in these regions occasionally dips below 45°F, thus giving an extremely high daily temperature range of more than 55°F. For example, in humid regions, the diurnal temperature range is usually small. Here, haze and clouds lower the maximum temperature by preventing some of the sun's energy from reaching the surface. At night, the moist air keeps the minimum temperature high by absorbing the earth's infrared radiation and radiating a portion of it to the ground.

Factors that affect relative humidity

When the amount of water vapor in the air remains constant, cooling the air raises the relative humidity and warming the air lowers it.

heat index

a measure of how hot it feels to an average person for various combinations of air temperature and relative humidity.

Weak absorption

leads to greater temperature difference

Minimum Temperature

few minutes before sunrise takes long time for long wavelength to go back to atmoshpere

Wet Bulb temperature (Tw)

lowest temperature at which water will evaporate Tw is a good measure of dryness

Explain how radiational cooling at night produces a radiation temperature inversion.

As the night progresses, the ground and the air in contact with it continue to cool more rapidly than the air a few meters higher. The warmer upper air does transfer some heat downward, a process that is slow due to the air's poor thermal conductivity. Therefore, by late night or early morning, the coldest air is next to the ground, with slightly warmer air above. This measured increase in air temperature just above the ground is known as a radiation inversion because it forms mainly through radiational cooling of the surface. Because radiation inversions occur on most clear, calm nights, they are also called nocturnal inversions

On a calm, sunny day, why is the air next to the ground normally much warmer than the air just above?

As the sun rises in the morning, sunlight warms the ground, and the ground warms the air in contact with it by ground conduction. However, air is such a poor heat conductor that this process only takes place within a few centimeters of the ground. As the sun rises higher in the sky, the air in contact with the ground becomes even warmer, and, on a windless day, a substantial temperature difference usually exists just above the ground. This explains why joggers on a clear, windless, hot summer afternoon may experience an extreme air temperature of over 120°F at their feet and only 95°F at their waists

Controls of Temperature

The main factors that cause variations in temperature from one place to another; includes: amount of solar radiation determined by length of daylight and intensity of incoming radiation → a function of: - Land - Land and Water Distribution - Ocean Currents - Elevation

How do Ocean currents play a role in the average worldwide distribution of temperature?

by transporting warm and cold water from one region to another. Warm water flowing northward along the coastline of Eastern Europe helps to keep winters in coastal cities much warmer than cities located in the middle of the continent.

Explain why the daily range of temperature is normally greater on clear days than on cloudy days.

clouds (especially low, thick ones) are good reflectors of incoming solar radiation, and so they prevent much of the sun's energy from reaching the surface. This effect tends to lower daytime temperatures. If the clouds persist into the night, they tend to keep nighttime temperatures higher, as clouds are excellent absorbers and emitters of infrared radiation—the clouds actually emit a great deal of infrared energy back to the surface. Clouds, therefore, have the effect of lowering the daily range of temperature. In clear weather, daytime air temperatures tend to be higher as the sun's rays impinge directly upon the surface, while nighttime temperatures are usually lower due to rapid radiational cooling. Therefore, clear days and clear nights combine to promote a large daily range in temperature.

Explain two ways the relative humidity may be changed.

if the air temperature increases (and the content of water vapor in the air remains the same), the relative humidity will drop. Conversely, if the air temperature decreases (again, with no change in water vapor content), the relative humidity will rise. So a change in air temperature will bring about a change in relative humidity because the air's capacity for water vapor changes relative to the air's actual water vapor content. Another way to change the air's relative humidity is to increase or decrease the amount of water vapor in the air. At a constant temperature, adding water vapor to the air raises the relative humidity; removing water vapor from the air lowers it.

Maximum temperature

lags 2-4 hours during clear days because radiation conversion takes time

Strong absorption

leads to less temperature difference

Maximum solar radiation takes place

local noon

Dewpoint temperature (Td)

lowest temperature at which water vapor will condense Td is a good measure of moisture content - high dew points indicate high water vapor content - low dew points indicate low water vapor content. - In very cold weather, low dew points indicate dry air. If the dew point is close to the air temperature, the air is nearly saturated and the relative humidity is close to 100 percent.

Air Temperature

the air temperature is controlled mainly by the input of energy from the sun and the output of energy from the surface. On a clear, calm day, the surface air warms, as long as heat input (mainly sunlight) exceeds heat output (mainly convection and radiated infrared energy). The warmest part of the day is usually in the afternoon because, until this time, incoming solar energy usually exceeds outgoing energy from the surface. The surface air cools at night as long as heat output exceeds input. Because the ground at night cools more quickly than the air above, the coldest air is normally found at the surface, where a radiation inversion usually forms. The coldest nights of winter tend to occur on clear, calm, dry nights.

Why is cold polar air described as "dry" when the relative humidity of the air is very high?

the desert air (with a higher dew point) must contain more water vapor. So even though the polar air has a higher relative humidity, the desert air that contains more water vapor has a higher water vapor density, or absolute humidity, and a higher specific humidity and mixing ratio as well.

At night

the earth's surface cools, mainly by giving up more infrared radiation than it receives—a process called "radiational cooling."

Relative Humidity

◦ Need two kinds of temperature: air (T) and wet bulb (Tw) Lowest temperature in which water can evaporate into the air if T=Tw then RH = 100% RH = (water vapor cont/water vapor cap)* 100