Lecture 9

THE ATMOSPHERE

HEATING THE ATMOSPHER

I. Weather and Climate

A. Weather is the word used to denote the state of the atmosphere at a particular place for a short period of time. Weather is constantly changing-hourly, daily, and seasonally.

B. Climate is best described as an aggregate or composite of weather. The climate of a place region is a generalization of the weather conditions over a one period of time.

C. Eventhough weather and climate are not identical the nature of both is expressed in terms of the same elements, those quantities or properties that are measure regularly are air temperature, humidity, type of cloudiness, type of precipitation, air pressure, and speed/direction of wind.

II. Composition of the Atmosphere

A. Air is a mixture of many gases in varying quantities. The composition of air is not constant. Clean dry air is composed of 78% nitrogen, and 21% oxygen. The remaining 1% is made up of argon and carbon dioxide.

B. Ozone is another component of the atmosphere. It is a form of oxygen that combines three oxygen atoms into each molecule. The ozone is concentrate in the stratosphere. Ozone absorbs harmful UV radiation from the sun.

C. An ozone problem exists due to chlorofluorcarbons (CFCs). Over the decades CFCs have been used for air conditioning cleaning solvents, propellants for aerosol sprays, among other things. The sunlight breaks up CFCs and releases chlorine atoms, which break up ozone molecules.

III. Structure of the Atmosphere

A. Pressure

• Atmospheric pressure is the weight of air above.
• At sea level the air pressure is 14.7 pounds per square inch. Thus, the pressure at higher elevation is less.

B. Temperature

• The atmosphere is divided into 4 layers based on temperature.
• The troposphere is the bottom layer in which we live. Temperature decreases with an increase altitude.
• Stratosphere is beyond the troposphere. The temperature in the stratosphere remains constant to a height of about 20 km and then begins a gradual increase that continues until a height of about 50 km above the Earth's surface. The reason for the increase temperature in the stratosphere is that the ozone is concentrated in this layer.
• Mesosphere is the third layer and here temperature like in the troposphere begins to decrease with height until at the mesopause approximately 50miles above the surface.
• thermosphere is the fourth layer that extends outward from the mesopause and has no well-defined upper limit. This layer contains a minute fraction the atmosphere's mass. In the air of this layer temperature again increases due to absorption of very short-wave, high energy solar radiation by atoms of oxygen and nitrogen.

IV. Earth-Sun Relationships

A. The Earth's two principal motions

• Rotation is the spinning of the Earth about its axis. The axis is an imaginary line running through the poles. The Earth rotates once every 24 hours, which produces day and night. The line separating the dark half from the lighted half is the circle of illumination.
• Revolution refers to the movement of the Earth in its orbit around the sun. The Earth revolves around the sun at 107,000 km per hour.

B. Seasons

• Seasons are due to the Earth's orientation to the sun continually changing as it travels along its orbit. The Earth's axis is tilted 23.5 degrees from the perpendicular. This is called the inclination of the axis. If the axis were not inclined we would have no seasonal changes. Also, because the axis remains pointed toward the same direction (North Star) the orientation of the Earth's axis to sun's rays is constantly changing.
• On one day each year the axis is such that the Northern Hemisphere is leaning 23.5 degrees toward the sun. Six months later, when the Earth has moved to the opposite side of its orbit, the Northern Hemisphere leans 23.5 degrees away from the sun.
• Solstice occurs on June 21 or 22 when the Earth's position is such that the north end of its axis is tilted 23.5 degrees toward the sun. At this time the vertical rays of the sun strike 23.5 degrees north latitude (Tropic of Cancer). This is the summer solstice for the Northern Hemisphere.
• Six months later on Dec. 21 or 22 the Earth is in the opposite position with the sun's rays striking 23.5 degrees south latitude (Tropic of Capricorn). For the Northern Hemisphere this is the winter solstice.
• Midway between the solstices are the equinoxes. September 22 or 23 is the date for the autumnal equinox in the Northern Hemisphere and March 21 or 22 is the date for the Spring equinox.

V. Mechanisms of Heat Transfer

A. All forms of matter, whether solid, liquid, or gas, are composed of atoms or molecules that are in constant motion.

B. All matter is said to contain thermal energy.

C. Temperature is described as the "hotness" or "coldness" of an object.

D. Conduction is the transfer of heat through matter by molecular activity. The energy of molecules is transferred through collisions from one molecule to another, with the heat flowing from the higher temperature to the lower temperature.

E. Convection is the transfer of heat by the movement of a mass or substance from one place to another. It can take place only in liquids and gases.

• Advection is the term reserved for horizontal convective motions, such as winds.
• Convection is the term reserved for vertical convective motions in the atmosphere.

F. Radiation is the only mechanism of heat transfer that can transmit heat through the relative emptiness of space.

• Radiation or electromagnetic radiation transmits energy through the vacuum of space at 300,000 km per second and only slower through our atmosphere.
• Visible light is the only portion of the spectrum we can see. Visible light is made up of a mixture of colors. Visible light can be separated out into its various colors using a prism.
• Infrared radiation, we can not see this form of radiation.

G. Paths Taken By Incoming Solar Radiation

• About 25% of solar radiation penetrates directly to Earth's surface without some sort of interference by the atmosphere. The remainder is scattered or reflected back to space.
• Scattering is the process in which solar radiation travels in a straight line and the gases and dust particles in the atmosphere redirect the energy. About 30% of the solar energy reaching the outer atmosphere is reflected back to space.
• The fraction of the total radiation that is reflected by a surface is called its albedo. The albedo for the Earth is 30 %.
• Gases are selective absorbers, meaning that they absorb strongly in some wavelengths, moderately in others and only slightly in still others. For the atmosphere as a whole, none of the gases are effective absorbers of visible radiation. Thus, this explains why most visible radiation reaches Earth's surface and why we say that the atmosphere is transparent to incoming solar radiation.

VI. Temperature Measurement and Data

A. Changes in air temperature are probably noticed by people more often than changes in any other element of weather.

B. The daily maximum and minimum temperatures are bases for many of the temperature data compiled by meteorologists.

• The daily mean temperature is calculated by adding the maximum and minimum temperatures and then dividing by two.
• The daily range of temperature is computed by finding the difference between the maximum and minimum temperatures for a given day.
• The monthly mean is calculated by adding together the daily means for each day of the month and dividing by the number of days in the month.
• The annual mean is an average of the twelve monthly means.
• The annual temperature range is computed by finding the difference between the highest and lowest monthly means.
• Mean temperatures are useful for making comparisons, whether on a daily, monthly, or annual basis.

VII. Controls of Temperature

A. A temperature control is any factor that causes temperature to vary from place to place and from time to time.

B. Land and water, altitude and geographic position all control temperature.

C. The heating of the Earth's surface directly influences the heating of the air above. Different land surfaces absorb varying amounts of solar energy, which in turn causes temperature to vary.

D. Land heats more rapidly and to higher temperatures than water and cools more rapidly and to lower temperatures than water.

E. Two cities may be near the equator, but yet both display different temperatures. One city may be at a higher elevation than the other; thus, it may be cooler than the city of lower elevation. This is an example of how altitude affects temperature.

F. The geographic setting may influence the temperatures experienced at a locale. For example, a windward coastal location subject to prevailing ocean winds experience considerably different temperatures from a coastal location where the prevailing winds are directed from the land toward the ocean.

THE ATMOSPHERE IN MOTION I. Measuring Air Pressure

A. Atmospheric pressure is measured in millibar units. Standard sea level pressure is 1013.2 millibars.

B. The media, however, uses inches in mercury when presenting atmospheric pressure. The inches in mercury uses mercury for measuring air pressure. The device is called a mercury barometer.

C. The need for smaller and more portable instruments for measuring air pressure led to the development of the aneroid (without liquid) barometer.

II. Factors Affecting Wind

A. When air moves horizontally it is called wind. Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. Solar energy is the ultimate driving force of wind, because solar energy is responsible for generating pressure differences due to unequal heating.

B. Pressure data are shown on a weather map using isobars. That is, lines that connect places of equal air pressure. The spacing of isobars indicates the amount of pressure change occurring over a given distance, which is expressed as the pressure gradient. Closely spaced isobars indicate a steep pressure gradient and high winds, whereas widely spaced isobars indicate a weak pressure gradient and light winds.

C. Coriolis effect prevents wind from crossing the isobars at right angles as the pressure gradient directs it to do. Instead, winds are deflected to the right of their paths of motion in the Northern Hemisphere and to the left of their paths of motion in the Southern Hemisphere.

D. Frictional effect on wind is important only within the first few kilometers form Earth's surface. Friction acts to slow air movement and alter wind direction. Eventually, the Coriolis effect will balance the pressure gradient force and the wind will blow parallel to the isobars. Upper air winds generally take this path and are called geostrophic winds.

E. The most prominent feature of the upper level flow are the jet streams. These fast moving "rivers of air" travel 120-240 km/hr in west to east direction. One such stream is situated over the polar front, which separates the cool polar air from the warm subtropical air.

III. Cyclones and Anticyclones

A. Cyclones or lows are center of low pressure and anticyclones or highs are high pressure centers. Friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

B. The cyclones or low pressure centers produce bad weather in any season. These lows move from west to east.

IV. Global Circulation

A. Near the equator, the rising air is associated with the pressure zone know as the equatorial low. A region marked by abundant precipitation. As the upper level flow from the equatorial low reaches 20-30 degrees latitude, north or south, it sinks back toward the surface. This subsidence and adiabatic heating produce the hot, arid regions in this latitude. The center of this zone of subsiding dry air is the subtropical high, which encircles the globe at about 30 degrees latitude, north or south.

B. At the surface, air flow is outward from the center of the subtropical high. Some of the air travels equatorward and is deflected by the Coriolis effect, producing the trade winds.

C. The remainder of the winds travel poleward and is also deflected, generating the westerlies of the mid-latitudes.

D. As the westerlies move poleward, they encounter the cool polar easterlies in the region of the subpolar low. The interaction between the warm and cool air produce the stormy polar fronts. The source region for the polar easterlies is the polar high.

V. Local Winds

A. Local winds are small scale winds produced by a locally generated pressure gradient.

B. A sea breeze develops, because cooler air over the water (higher pressure) moves toward the warmer land (lower pressure). The sea breeze begins to shortly before noon and generally reaches its greatest intensity during the mid-to late afternoon.

C. The land breeze develops at night when the land cools more rapidly than the sea.

D. During the daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air is less dense, it glides up along the slope and generates a valley breeze.

E. After sunset, the pattern above may reverse. Rapid radiation cooling along the mountain slopes produces a layer of cooler air next to the ground. The cool air is denser than warm air, it drains down slope into the valley. This movement is called mountain breeze.

F. Chinooks are when warm, dry winds, common off the eastern slopes of the Rockies.

G. Santa Ana winds are chinooklike winds that occur in southern California.

VI. How Wind is Measured

A. Winds are always labeled by the direction from which they blow. North wind blows from the north toward the south.

B. A wind vane is commonly used to determine the wind direction. This instrument always points into the wind.