3 - Air Temperature - Lecture notes 3 PDF

Preview

Summary

Eleazar Horta...

Description

Air Temperature The sun is highest in the sky around noon every day, but this is not normally the warmest part of the day. Why not? As soon as the sun rises in the morning, the incoming solar radiation begins to heat the surface of the earth. We have already learned that sunlight does not heat the atmosphere directly very well. Most of the troposphere is heated from below due to its contact with the surface. Daily Variations The surface heats the thin layer of the troposphere in contact with the surface through the process of conduction. Air is a poor conductor of heat and conduction is only an important heat transfer mechanism within a few inches of the surface. Once this lower layer of air is warmed, convection begins and the rising columns of air, known as thermals, tend to heat the lower atmosphere. On a calm day these thermals are small and do not mix very well and large vertical temperature gradients can exist. On a windy day, the mechanical mixing makes the heat transfer process from the surface to the atmosphere more effective and vertical temperature gradients will usually be smaller. When the sun is highest in the sky at noon, the rate at which the surface is absorbing solar energy is at a maximum. After this time, the heating rate will go down but it is still higher than the rate at which heat is transferred to the atmosphere for several hours. For this reason, the surface temperature will continue to rise into the late afternoon, when the incoming solar energy is equal to the outgoing energy. The outgoing energy is a combination of heat energy transferred to the atmosphere and the infrared longwave radiation emitted by the surface. The typical time of the maximum temperature reading is usually between 3 p.m. and 5 p.m., although this may vary due to local conditions. There are other factors that will affect how warm the daytime temperatures will get. For example, if it is cloudy throughout the day the overall temperature will usually be lower because clouds have a net cooling effect. Clouds, and especially thick clouds, reflect a large amount of solar energy back toward space. The presence of a large body of water with its high heat capacity can affect both the time of the maximum air temperature as well as the maximum temperature itself. Large storms can cause warm

air to move northward and cold air to move southward and the peak temperatures can occur at night as a result. If the composition of the ground is a poor heat conductor, such as sand, heat energy does not easily transfer into the ground. More of this incoming solar energy will then be available to heat the atmosphere. If the soil contains a significant amount of moisture or is covered with vegetation, most of the available solar energy goes to evaporating some of this water, leaving less energy available to heat the air. Radiational Cooling Every object whose temperature is above absolute zero - which means every object continually emits electromagnetic energy. The amount of energy emitted, and the frequency and wavelength of that energy, depend on the temperature of that object. At night, when the sun is no longer available for heating, both the surface and the lower atmosphere continue to emit infrared radiation which will tend to lower their temperatures. This process is known as radiational cooling. Because the ground is a much better radiator than the atmosphere, the ground cools more quickly than the atmosphere. As a result, the atmosphere just above the surface is warmer than the surface itself. This bottom layer of the atmosphere will transfer heat to the surface which then radiates it up toward space. This process continues through the night. By morning, the ground and the air next to the ground are much colder than the air above the surface. This increase in temperature with altitude just above the surface is known as a radiation inversion because the temperature inversion was created due to radiational cooling. Since radiation inversions usually occur on clear and calm nights they are also referred to as a nocturnal inversion. They are sometimes also called surface inversions, because they occur just above the surface. Since the surface is cooling all night due to radiational cooling, the coldest surface temperatures are usually shortly after sunrise when the sun begins to heat the surface again. However, since the sun is still low in the sky for a time after sunrise, the surface temperature may still drop for a little while after the sun has actually risen. The surface temperature will begin to rise once the incoming solar energy exceeds the outgoing longwave radiation. Radiation inversions tend to be strongest when the air is calm, the night is long and the air is very dry. Windy conditions will tend to increase the mixing that occurs, which will reduce the vertical temperature gradient and make the vertical temperature profile more uniform. A long night allows more time for radiational cooling to occur before the sun rises the next morning. Water vapor and clouds tend to intercept the outgoing infrared radiation, so a clear night will allow more of the heat energy to escape into outer space. A thermal belt is a region on a mountainside that will usually remain relatively warm at night because the cool air sinks down into the valley below it. This is why farmers will plant crops that are more sensitive to cold temperatures on the hill above the valley floor.

There are several things that farmers can do to protect crops from extremely cold air temperatures. Smaller plants and shrubs can be covered with plastic or cloth to prevent the ground heat from radiating away at night. Farmers may use orchard heaters on calm nights to heat the air nearest the surface, although on windy nights this will naturally have a lesser effect because the wind will mix in cooler air. Wind machines can mix the colder air near the surface with warmer air above the surface. Farmers can spray water onto crops and tree branches to create a layer of ice on their surfaces. The ice acts like an insulator and will protect the crops and plants from the even colder air temperatures. The latent heat given off when the water freezes into ice also helps keep the crops warmer. Controls of Temperature The controls of temperature are the most important factors that cause variations in temperature from place to place. The major controls of temperature are latitude, the land and water distribution, ocean currents and altitude. Viewing a map of isotherms is a good way to visualize the temperature distribution across a region or across the entire planet. An isotherm is a line that connects places that have the same air temperature. Because surface measurements are usually taken at different altitudes, the temperatures are corrected to read at sea level to make a more valid comparison possible. The effect of latitude on temperature is obvious. Lower latitudes receive more direct sunlight throughout the year so they would be expected to be warmer, and in general they are. The tropics receive approximately the same amount of sun all year, while the amount of sun received at high latitude locations varies considerably between summer and winter. For this reason, the isotherms in the winter are generally closer together than they are in the summer, indicating the stronger temperature gradient. Since these temperature gradients will cause pressure gradients, the winds in the winter season are usually stronger than the winds in the summer season. We will be discussing winds and pressure gradients in much more detail in a later chapter. Even if the amount of solar energy input is the same, land and water regions will warm up and cool down at different rates. There are four major reasons why the land and water will heat up and cool down at different rates. One reason is because the specific heat of water is much higher than the specific heat for soil and rock. The specific heat is the amount of energy that it takes to raise the temperature of one gram of a substance by one degree Celsius. Water has one of the highest specific heats of all known materials. This high value for the specific heat of water is due to hydrogen bonds, which are electrostatic attractions between the oxygen atoms of one water molecule and the hydrogen atoms in neighboring water molecules. Because it takes a relatively large amount of energy input or output to change the temperature of water, large bodies of water have a moderating effect on the

temperature. When the amount of solar heating is high, a large body of water will tend to limit how high the temperature will get. When the amount of solar heating is low, a large body of water will tend to limit how low the temperature will get. Another reason that water heats up and cools down more slowly than the land is that water is mobile. Mixing due to wind, waves and currents will affect the ocean’s water temperature down to approximately 20 feet or more below the surface throughout a day. This heat is transferred from the ocean surface downwards through a combination of conduction and convection. These temperature changes can affect a layer of surface water between 200 and 600 meters thick over the course of the year. In contrast, the heat does not penetrate very deeply into either rock or soil. Rock and soil do not permit convection and they are generally poor conductors, so the heat remains near the surface. Typically, daily temperature changes only penetrate four or five inches into the soil. As a result, the energy absorbed by the water will affect a much larger mass, which will also tend to limit the temperature range experienced by the body of water. Soil is opaque to visible light, and all of the heat that is absorbed must be absorbed right at the surface because sunlight cannot penetrate any deeper. Water is more transparent to visible light than is the soil, so some of the energy is absorbed up to several feet below the surface. Since the heat energy is being spread throughout a greater volume, this will tend to make the ocean heat up more slowly. Some water molecules are always evaporating from the surface. Evaporation is a cooling process and the evaporating water molecules carry away with them the latent heat of vaporization. This will reduce the amount of energy that is available to increase the water temperature. The location of the large body of water also has an effect. If the body of water is upwind of a particular location, that body of water will have a larger effect on the temperature than if the body of water were downwind of that location. For this reason, the climate along the West Coast of the United States is more affected by the Pacific Ocean than the East Coast of the United States is by the Atlantic Ocean. Major ocean currents carry large amounts of heat from low latitudes to high latitudes and return cold water from high latitudes to low latitudes. For example, the Gulf Stream flows northward off of the East Coast of the United States, and carries an incredible amount of heat to the northern Atlantic Ocean. Western Europe is much warmer than it would be otherwise because of the heat carried to it by the Gulf Stream.

Temperature Statistics The daily temperature range is, in general, highest at the earth’s surface and decreases with altitude. Regions that experience a high amount of clouds, or are in general more humid, experience lower daily temperature ranges than clear and dry regions. For this reason, the largest temperature ranges are typically observed in desert areas. Clouds have an overall cooling effect because they reflect incoming solar radiation back to space and this lowers the maximum temperatures that would be reached. Clouds also intercept the outgoing longwave radiation emitted by the Earth and reradiate this in all directions. Some of this radiation is sent down toward the surface, making the surface warmer than it would be otherwise. This is the greenhouse effect that receives so much attention in the media these days. The mean daily temperature is the average of the highest and lowest recorded temperatures for that particular location during that 24-hour period. The “normal” temperature is the average mean daily temperature for a particular location and date over a period of 30 years. The average monthly temperature is the average of the daily mean temperatures during that month. The annual range of temperature is the difference between the average temperature of the warmest month and the average temperature of the coldest month. The mean annual temperature is the average of the 12 monthly averages. Using Temperature Information One common use of this temperature information is the concept of degree-days. In the northern states the amount of heating necessary for buildings is measured by heating degree-days. To calculate the number of heating degree-days accumulated during a particular day, subtract the average daily temperature from 65 degrees Fahrenheit. For example, if the average daily temperature on a particular day is 60 degrees Fahrenheit, then five heating degree-days will accumulate that day. This information is used by building designers and heating professionals who are trying to determine what the heating costs will be over the course of the season. Similarly, a cooling degree-day is accumulated when the daily mean temperature is above 65 degrees Fahrenheit. Designers of air-conditioning systems can estimate the necessary cooling load by estimating the typical cooling degree-days accumulating over an entire summer season. Farmers can use an index called a growing degree-day to help them determine when to plant and harvest their crops. Instead of using 65 degrees Fahrenheit as the standard, the base temperature will vary depending on the particular crop. Experience will tell the

farmer approximately how many growing degree-days it takes for a crop to be ready to harvest. Wind Chill Index The human body cools itself by several mechanisms. Our bodies continually emit radiation, just like every other object. Our skin also heats the air around us through conduction and this energy is carried away by convection. Perspiration evaporating away from our skin also removes heat from the body. On a cold and windy day, our bodies will lose more heat than it will on a warm and windless day. To account for this effect, the wind chill index was developed. The wind chill index represents the equivalent air temperature (with no wind) that would result in as much cooling to the body as exists in the current conditions of wind speed and air temperature. For example, if the actual air temperature is 20 degrees Fahrenheit and the wind speed is 30 miles per hour, the wind chill equivalent temperature will be 1 degree Fahrenheit. This means that the current conditions will have the same cooling effect as a windless day with an air temperature of 1 degree Fahrenheit. It is important to note that an object sitting outside in the wind cannot get colder than the actual air temperature, not matter what the wind speed will be.

The combination of high winds and cold air can remove heat from the body very quickly. If the actual air temperature is below freezing, the skin may actually freeze if exposed to the weather for too long. This condition is known as frostbite. Frostbite usually occurs soonest in the hands and feet because they are farthest from the source of body heat. If the internal body temperature gets too low, a condition known as hypothermia may result. In a severe case, hypothermia may result in death when the internal body temperature drops below about 79 degrees Fahrenheit.

Thermometers There are several different types of thermometers commonly used to measure the air temperature. One familiar type is the liquid-in-glass thermometer. These thermometers usually contain either mercury or, more commonly, alcohol. The liquid will expand and contract as the temperature changes, and the thermometer is calibrated to read these volume changes in terms of temperature. There are two special types of liquid-in-glass thermometers. The maximum thermometer has a small constriction just above the thermometer bulb that prevents the volume from decreasing once the temperature starts to drop. In this way, the thermometer is able to record the maximum temperature that has occurred. The minimum thermometer has a small barbell-shaped marker within the bore. As the temperature drops, the liquid contracts into the bulb and the marker moves toward the bulb. As the air warms, the liquid expands but the index marker does not move, and this permits recording the minimum temperature that was observed. Many modern temperature measurements are made with electrical thermometers. Two common types of electrical thermometers are the resistance thermometer and the thermistor. In each case, the electrical resistance across the sensor is measured and converted to temperature. A radiometer is an instrument that measures the air temperature by sensing the infrared energy emitted by the air. It measures the amount of radiation, and the wavelength and frequency at which the maximum energy is emitted, and converts this to a temperature. A bimetallic thermometer consists of two different metals that are welded together to form a single strip in the form of a coil. As the temperature changes, these metals expand and contract at different rates and the coil either tightens or opens up as a result. As the coil moves, a pointer indicates the temperature along a visible temperature scale. Some bimetallic thermometers are connected to a thermograph, which is an instrument that measures and records the temperature that is read by the thermometer. Thermometers and other meteorological instruments are usually mounted in an instrument shelter. These shelters are typically located about five or six feet above the ground so they are affected by radiational cooling of the surface to the smallest extent possible. They are usually painted white in color, and have louvered sides to allow the air to flow through freely. The idea is that this allows the instruments to measure the actual conditions of the air itself. A thermometer that is exposed to direct sunlight will indicate a temperature that is higher than the actual air temperature, because it will be absorbing some of the incoming solar energy. Instrument shelters are typically installed on grassy areas to ensure consistency of measurements from place to place.

Heat Waves A heat wave is a prolonged period of unusually hot and humid weather that may last from several days to several weeks. These weather extremes put more stress on the body, and elderly people are typically affected to a greater degree than the population as a whole. Heat waves usually affect cities more than they do rural areas due to an effect referred to as the urban heat island. Temperatures within cities tend to be higher than in the surrounding rural areas because the concrete and other building materials tend to absorb and retain heat more than the rural vegetation. Also, precipitation runs off from city streets through the sewer system faster than it does through rural surface areas. The energy required to evaporate this surface water tends to limit the temperature extremes seen in the rural regions....