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Weather and the Atmosphere Earth Science What is Weather? Weather is the state of the atmosphere at a given time and place. A description of the weather includes: The amount and type of clouds Temperature Air pressure Wind speed and direction Humidity: amount of moisture in the air Meteorology Meteorology is the study of the atmosphere including the weather. It includes the study of : How the atmosphere heats and cools? How clouds form and produce rain? What makes the wind blow? What makes up the atmosphere? What causes changes in the climate? Observing Weather Direct observation includes: the presence of rain, wind speed and direction, and the presence of different types of clouds. Wind direction can be shown by clouds or dust. Wind speed is measured using an anemometer and rated for it’s effects by the Beaufort scale. Other Measurable Observations Temperature can be an important indicator of the weather and during the winter is adjusted by the wind speed according to the wind-chill factor. Increased humidity (water vapor in the air) makes hot temperatures feel hotter and cold temperatures feel colder. Changes in air pressure are most easily detected when changing elevations and ear popping occurs (measured by a barometer). Composition of the Atmosphere Earth’ s lower atmosphere is composed of a mixture of gas (mostly nitrogen and oxygen) called air. Air thins out as altitude increases, however its composition remains the same. The most prominent gas is Nitrogen (N2) at 78% Secondly, oxygen (O2) makes up 21% (20.8%) of air Carbon dioxide, is found in very small amounts of almost 0.04% (400 ppm; was at 350 ppm) & there is 0.93% argon There are also trace amounts of helium, hydrogen, and other gases Water Vapor Air always contains some water vapor. Water vapor enters the air through the process of evaporation. Water vapor varies with location, season, and time of day. Most water vapor is near the surface of the earth and decreases with altitude. Ozone Ozone is a form of oxygen that consists of 3 oxygen atoms bonded together. Ozone forms when UV rays from the sun strike oxygen in the Earth’s atmosphere. Ozone exists mainly in a layer of the atmosphere between 10-50 km up called the ozone layer, primarily over the pole regions. Ozone absorbs approximately 99% of the harmful rays from the sun. The ozone layer thinned over the last several decades due to chlorofluorocarbons found in aerosols but is regaining strength since those chemicals were banned. Dust Dust in the air helps form fog and rain. Water vapor condense around the small particles of dust and forming tiny water droplets. When the droplets become heavy enough, they fall to the Earth as rain or snow. Structure of the Atmosphere Scientists divided the atmosphere into four layers that are based on temperature changes. The layers are: Troposphere Stratosphere Mesosphere Thermosphere Troposphere The troposphere is the lowest layer. It starts at Earth’s surface and continues up to approximately 18 km at the equator and 8 km at the poles. All of Earth’s weather occurs in the troposphere. Temperatures decrease gradually with altitude to approximately -55°C at the top of the troposphere, an area called the tropopause. Stratosphere The stratosphere is the second layer from the bottom. It extends from the tropopause to about 50 km above Earth. The stratosphere is clear and dry with strong, steady winds and few weather changes. The bottom of the stratosphere is cold, but it warms up steadily with altitude due to the absorption of sunlight by ozone. Mesosphere and Thermosphere The mesosphere is the third layer in which temperatures drop again. The fourth layer is the thermosphere where temperatures rise again. The top of the thermosphere is about 500 km from Earth’s surface. Nitrogen and oxygen in the thermosphere absorb sunlight causing the temperature rise. Ionosphere The ionosphere is an area between the heights of 65-500 km above Earth where the air is highly ionized. The ionosphere is responsible for reflecting radio waves back to Earth which makes communication by radio possible. The ionosphere is affected by solar events and produces the auroras or northern and southern lights. Heating of the Atmosphere Energy that causes changes in the weather comes from the sun. Heat energy enters and moves through the atmosphere in three ways: Radiation: The sun radiates energy through short waves that are seen as visible light or felt as infrared Conduction: An object receives heat when it comes into contact with a hotter object, example air that contacts warm ground or ocean Convection: heat transfer through liquids and gases where the heated substances rises and cooler substance sink Heat Balance of Earth and Atmosphere The Earth’s heat budget is balanced when equal amounts of heat are entering and leaving the atmosphere. Incoming solar radiation is called insolation. Conduction from the heated ground warms the lowest layer of air. Convection causes air to rise into higher parts of the atmosphere. Absorption and Greenhouse Effect Earth’s surface radiates infrared waves that warm the atmosphere as they are absorbed by water vapor &CO2 as well as other greehouse gases. The water vapor and CO2 work to trap heat in a process called the greenhouse effect. The greenhouse effect in Earth’s atmosphere is increasing as the level of gases such as CO2 and methane are increasing. If the atmosphere warms too much the Earth will heat causing temperatures to rise and glaciers to melt. Temperature Drops with Altitude As altitude increases temperature drops approximately 1°C per 160 m. The drop in temperature is called the normal lapse rate. Air is warmer at the surface of the earth due to the actions of conduction, convection and radiation. Rising air cools and expands as it travel higher in the atmosphere. Adiabatic cooling describes temperature changes that occurs with the compression or expansion of air particles. Temperature Inversions A temperature inversion occurs when air at the surface is colder than the air above it. Temperature inversions typically form during clear, dry nights. Air on the ground cools quickly and is trapped on the ground because it is heavier than warm air. Seasons and the Sun’s Rays Temperature varies with the seasons because the sun does not heat Earth’s surface evenly. The sun’s rays strike the earth at angles from 0° to 90°. The more vertical the rays the more heat. As the angle decreases, energy is spread out over a larger area and less heat is generated. Also the smaller the angle the larger distance the energy waves must travel before reaching Earth. Warmest and Coldest Hours Varying insolation also changes the temperature during the day. The warmest hour of the day is typically in the afternoon because the air and the ground are receiving more heat than they are losing. The coldest hour of the day is typically just before sunrise after the ground and air have been losing heat all night. The difference between highest and lowest temperatures is called the temperature range. Warmest and Coldest Months In the northern hemisphere, June 21 typically has the strongest sunlight, but July is the hottest month. December 21 has the weakest sunlight and January is the coldest month. Typically, oceans have small temperature ranges and continents tend to have large temperature ranges. Heating of Land and Water Water and land warm up and cool off at different rates. Why? Water absorbs heat for a depth of many meters, land only absorbs in the top few centimeters. Water spreads heat easily because it is a fluid. Water needs more energy than land to raise the temperature the same amount. Solar energy is used up in the process of evaporation. Water also cools more slowly than land. Surfaces that warm up faster also tend to cool off faster. Land surfaces that warm up faster include: dark soils, wet soil and rough soil. Atmospheric Circulation Pressure differences in the atmosphere cause air to move. Air generally moves from the poles to the equator. The poles are areas of high pressure where cold air sinks. The equator is an area of low pressure where warm air rises. The Coriolis Effect The circulation of the atmosphere and the oceans is affected by Earth’s rotation. The Earth has the greatest rotation at its equator and the least at its poles. Therefore, when air moves toward the poles it curves. The curving of the air flow is called the Coriolis effect. The Coriolis Effect cont. The Coriolis effect is altered by speed, latitude and direction. Objects in the Northern Hemisphere are deflected right and in the Southern Hemisphere they turn to the left. The faster an object is moving the greater the Coriolis effect. Global Winds The air flow in the Northern and Southern Hemispheres is divided into three distinct looping patterns called convection cells. Each is located in a wind belt, that is characterized by wind that flows in one main direction. The major global winds are: Trade Winds: flow toward the equator between latitudes of 0° and 30°. Westerlies: between 30° and 60° latitude moving toward the poles Polar Easterlies: located over the poles and flowing in a generally easterly direction Doldrums and Horse Latitudes: areas of high pressure with weak winds that occur at the equator and 30° latitude. Atmospheric Convection Cells Winds and Pressure Shifts As the seasons change due to shifts in the angle the sun’s rays hit the Earth, the pressure and wind belts also shift. Most winds occur in the lower troposphere. The jet streams are narrow bands of high-speed winds that occur in a margin between the troposphere and the stratosphere. Two types of jet streams include the polar jet streams and the subtropical jet streams. The jet streams are typically 100 km wide, 2 to 3 km thick and can reach speeds of 500 km/hr. Global Wind Patterns http://teachingboxes.org/jsp/teachingboxes/weatherEssentials/wind/sequence/lesson4_activity1.jsp Local Winds Local winds are movements of air that are caused by local conditions and temperature variations. Land/sea breezes: Sea breezes – cool sea air moves over warm land during the day Land breeze – cool land air moves over warm sea at night Mountain/valley breezes: Mountain breeze – cold air from the mountains flows downslope at night Valley breeze – warm air from the valley flows upslope during the day