Weather and Climate

Weather and Climate

Composition of the Atmosphere The atmosphere is a mixture of gases, liquids, and suspended solids that surrounds our planet • Gases-include nitrogen, oxygen, carbon dioxide, water vapor and trace gases • Mostly nitrogen (78%) and oxygen (21%) • Even though the air thins with altitude, the proportion of gases remains the same through the first Km • Water vapor- enters the air by evaporation from the ocean & other water sources • Amount of water vapor varies from 0 to 4% (depends on location, season, and time of day) • Clouds, fog, and dew are caused by water vapor in the air • Suspended solids- include dust, sea-salt, smoke, pollen, bacteria, and chemicals from industry & transportation • Cause hazy skies • Act as condensation nuclei, where water vapor condenses on them to form clouds, fog , and raindrops

Structure of the Atmosphere The atmosphere is divided into layers based on changes in temperature • Troposphere- Begins at sea level and extends approximately 8 to 18 kilometers depending on location and season • Stratosphere- Extends from the tropopause to an altitude of about 50 Km • Mesosphere- Extends from the stratopause to about 80 Km above the Earth’s surface • Thermosphere- Begins after the mesopause, but has no definite upper limit

Troposphere • Begins at sea level and extends approximately 8 to 18 kilometers depending on location and season • Contains about 75% of the gases in the atmosphere and most of the dust & water vapor • Layer where weather occurs • Since the atmosphere is mainly heated from the Earth’s surface, temperature decreases with altitude

Stratosphere • Layer of the atmosphere extending outward from the tropopause to about 50 km • Strong eastward winds, called the jet stream, blow in a narrow band in this layer • A special form of oxygen, called ozone, is found in this layer • Most is located in the ozone layer • Absorbs harmful ultraviolet radiation from the sun • Temperature increases with altitude due to the absorption of solar energy by ozone

Mesosphere • Atmospheric layer that extends from the stratopause to about 80 Km above the Earth’s surface • Protects the Earth from meteoroids • Temperatures drop with altitude to about -100°C, making it the coldest zone in the atmosphere

Thermosphere • Begins at about 80 Km, after the mesopause, but has no definite upper limit • Temperatures increase rapidly with height due to the absorption of solar energy • Subdivided into two layers: • Ionosphere-(lower layer) • Extends from 80 to 550 Km above Earth’s surface • Layer of electrically charged particles that reflect radio waves and produce auroras • Exosphere-(upper layer) • Begins 550 Kmabove the surface and extends for thousands of kilometers • Artificial satellites orbit the Earth in this layer • Layer in which solar winds take place

Measuring Air Temperature • Air temperature- A measure of the average speed of the random motion of air molecules • The higher the temperature, the faster the molecules move • Thermometer- Instrument used to measure temperature • Works on the principle that most materials expand with increasing temperatures • As the temperature rises, liquid in the bulb expands into the stem of the thermometer • Temperature scales- Air temperature can be measured in Fahrenheit (°F), Celsius (°C), or Kelvin (°K) • Fahrenheit to Celsius °C = 5/9 (°F - 32°) • Celsius to Fahrenheit °F = 9/5 °C + 32° • Celsius to Kelvin °K = °C + 273°

Heating the Atmosphere • Almost all of the energy heating our atmosphere comes from the sun (called radiant energy) • Not all of the sun’s radiant energy reaches Earth’s surface; some is reflected back into space, scattered, or absorbed by gases and clouds in the atmosphere • Solar energy which reaches the Earth’s surface heats the atmosphere in one of three ways: radiation, conduction, or convection

Radiation • Radiation- The transfer of energy by electromagnetic waves such as light • Can travel through a vacuum • When the sun’s radiant energy is absorbed by earth, it is changed into heat

Conduction • Conduction- The direct transfer of heat energy from one substance to another • Earth’s surface is warmed by radiant energy • Air coming in contact with Earth’s warm surface is heated by conduction • Due to conduction, air temperatures are usually higher the closer you are to the ground

Convection • Convection- The transfer of heat energy in a fluid ( gas or liquid) • Causes warmer fluids to rise in colder ones • Convection currents in the atmosphere are caused by the unequal heating of the Earth by the sun • Most of the heat energy in the atmosphere is transferred by convection currents

Global Temperature Variations • The temperature of the air can be affected by many factors • Amount of sunlight • Wind • Latitude • Altitude • Proximity to oceans and other water bodies

Seasons on Earth • The different seasons are caused by the Earth being tilted 23 ½ degrees on its axis • The tilt of the Earth on its axis, in conjunction with it’s rotational path around the sun, causes an unequal distribution of light and heat at the surface

Seasons on Earth

Solstices • Solstice refers to the time when the sun seems to stop moving higher in the sky each day • On the solstice the Earth’s axis is pointing toward the sun in one hemisphere and away from the sun in the other • Summer solstice- First day of summer in the Northern Hemisphere (June 20th or 21st) • Northern Hemisphere tilted toward the sun • Direct rays located at 23 ½ ° north latitude (Tropic of Cancer) • Longest day of the year in Northern Hemisphere • Winter solstice- First day of winter in the Northern Hemisphere (December 21st or 22nd) • Northern Hemisphere tilted away from the sun • Direct rays located at 23 ½ ° south latitude (Tropic of Capricorn) • Shortest day of the year in Northern Hemisphere

Equinoxes • Equinoxmeans equal night • On the equinox, the Earth’s axis is not tilted toward the sun in either hemisphere • At the equinoxes, day and night are of equal length all over the world • Direct rays of the sun located at the equator • Autumnal equinox- First day of fall (September 22nd or 23rd) • Vernal equinox-First day of spring (March 20th or 21st)

Mapping Temperatures Isotherms- Lines drawn on weather maps to show places that have the same temperature at a given time • One isotherm is usually drawn on a map for each 10° difference in temperature • Isotherms are similar to contour lines in that they each represent a unique measurement and as a result won’t touch or cross

How Isotherms are Shown on Weather Maps Isotherms showing 10° differences in temperature Creating color bands between isotherms

Atmospheric Pressure • Air pressure- Measure of the force of the atmosphere pressing down on Earth’s surface • The pressure at any point on the Earth is equal to the weight of the air directly above that point • Air pressure depends on the density of the air - the denser the air, the higher the pressure • Differences in air pressure cause winds and changes in the weather

Factors Affecting Air Pressure Air pressure is affected by three factors • Temperature • Since the gas molecules in warm air are farther apart than in cold air, warm air is lighter (less dense) than cold air • When warm air replaces cold air, the pressure decreases • If cold air replaces warm air, the pressure increases • Water vapor content (humidity) • The more water vapor the air contains, the lighter it is • Humid air weighs less (has less pressure) than dry air • Elevation (altitude) • As elevation increases, the air becomes thinner (less dense) • Air pressure decreases with elevation

Measuring Air Pressure • Barometer- Instrument used to measure air pressure (two types) • Aneroid barometers- consist of a thin flexible can with a pointer attached • Mercury barometers- use a glass tube filled with mercury • The average air pressure at sea level is 30.00 inches of mercury • Barometric readings drop about 1 inch for every 1000 feet above sea level

Air Pressure Readings on Weather Maps Weather maps use isobars to show areas that have the same air pressure at a given time • Pressure gradient- the rate of change between two isobars • Isobars close together represent a steep gradient (expect winds) • Isobars far apart show a weak gradient (expect calm air)

Air Pressure & the Weather • Barometers are used to forecast the weather • Air pressure increases when large masses of air come together in the upper atmosphere • Prevents warm, moist air from rising • Expect fair skies with no precipitation • Pressure decreases when large air masses in the upper atmosphere move apart • Causes warm air to rise • Expect cloudy, rainy weather • In general: • Low pressure- warm, humid air • High pressure- cool, dry air • Falling pressure- expect bad weather • Rising pressure- expect good weather

Winds • Winds result from the unequal heating of the atmosphere by the sun • This causes pressure differences • Winds occur when air moves from areas of high pressure to areas of low • The bigger the difference in pressure (the larger the gradient), the stronger the winds

Wind Direction • Wind Vane- Device used to determine wind direction • Points in the direction the wind is blowing from • Windsock- Cloth bag-like device used to determine wind direction and speed • The end of the windsock points in the direction the wind is blowing to • Winds are named for the direction from which they come • A northerly wind would blow from the north to the south

Coriolis Force Coriolis Force- an effect produced by Earth’s rotation which causes objects in motion to be deflected • Deflection is to the right in the Northern Hemisphere and to the left in the Southern • The further from the equator, the greater the deflection • Cause surface winds to blow at an angle to the isobars The Coriolis Force has no effect on the way water swirls when you flush a toilet

Wind Speed • Anemometer- Instrument used to measure wind speed • Factors that effect wind speed • Pressure gradients- The steeper the gradient, the stronger the winds • Friction with the Earth’s surface- Winds travel faster over smooth, obstacle free surfaces • Season- Wind speeds are generally greater in winter than in summer

Wind Chill Factor • Wind Chill Factor- Perceived temperature to the human body based on both wind speed and air temperature • The stronger the wind, the colder it feels

Types of Winds There are two types of winds- local & global • Local winds- Movement of air caused by differences in the heating and cooling rates of land and water • Global Winds- Large scale movement of air caused by the unequal heating of the earth’s surface • Convection currents in the atmosphere cause cold, dense air from areas near the poles to sink and move along the surface, forcing warm air near the equator aloft • Create a worldwide system of wind and ocean currents

Local Winds • Sea breeze- Winds occurring during the day when cool, dense air from over the water flows inward and forces the warm air over the land to rise • Land breeze- Winds occurring at night when cool air from over the land flows seaward and forces the warm air over the ocean to rise

Local Winds • Monsoons- A major land and sea breeze that reverses direction based on seasonal temperature changes • During one season winds blow from the ocean onto the land and during another they blow from the land back over the water • Warm, moist air blowing from the ocean results in a rainy season

Global Winds • Major wind patterns produced by the unequal heating of Earth’s surface in combination with its rotation • Winds blow from areas of high pressure to those of low pressure • Types of global winds: • Trade Winds-Blow toward the equatorial low (0° latitude) from the subtropical highs (30° lat.) • Prevailing Westerlies- Blow from the subtropical highs (30° lat.) to the sub-polar lows (60° lat.) • Polar Easterlies- Cold, dense air mass which moves from the polar highs (90° lat.) to the sub-polar lows (60° lat.)

Global Wind Patterns

Relative Humidity • Humidity- Water vapor or moisture in the air • Relative humidity- Percent of moisture the air is holding relative to the amount that it can hold at a particular temperature % Relative humidity =Amount of water vapor in the air Amount of water vapor the air can hold at that temperature • Saturated air- Air with a relative humidity of 100%

Measuring Relative Humidity • Hygrometer- Instrument used to measure relative humidity • Psychrometer- A hygrometer that determines relative humidity based on cooling that occurs due to evaporation • Consists of two thermometers: a wet bulb and a dry bulb • Once the temperature is determined on both thermometers, a chart is then used to determine the relative humidity

Condensation and Dew Point • Condensation- The formation of liquid drops from water vapor. • It is the process which creates clouds • Necessary for rain and snow formation • Condensation nuclei- Tiny particles of salt or dust that water vapor in the air condenses around • Dew point- The temperature at which saturation occurs • This is the temperature that a sample of air would need to be cooled so that its relative humidity would be 100%

Four ways that air can be cooled to its dew point • Contact with a colder surface • Radiating heat • Mixing with colder air • Expanding when it rises

Forms of Condensation • Dew-Water droplets forming on surfaces when the air is saturated and temperatures are above freezing • Frost- Ice crystals that form on surfaces when the air is saturated and temperatures are at or below freezing

Forms of Condensation • Fog- A suspension of tiny water droplets in the air, at or near the Earth’s surface • Occurs when a surface layer of air is cooled below the dew point • Basically a stratus cloud that has its base at or near the ground

Forms of Condensation • Clouds- A suspension of tiny water droplets in the air • Form when moisture in the air condenses on small particles of dust or salt • Classified based on shape and altitude • There are three main types of clouds: • Cumulus • Stratus • Cirrus

Types of Clouds • Cumulus- • Fluffy, white clouds with flat bottoms • Usually indicate fair weather • Stratus- • Smooth gray clouds that cover the entire sky • Usually associated with rain or drizzle • Cirrus- • Feathery clouds made of ice crystals • Often indicate that precipitation will occur within several hours

Precipitation • Precipitation- Any form of water that falls from the atmosphere to Earth • Forms when water vapor changes back to liquid water (condensation) or to ice crystals (sublimation) • The form of precipitation that reaches the ground depends on the temperature conditions it encounters on the way and the amount of impurities in the air • The more impurities, the colder the air must be in order to form ice crystals

Types of Precipitation • Drizzle- Fine rain with small, slowly falling water droplets • Rain- Large water droplets that form when water vapor condenses in the clouds then falls to the ground • Snow- Flat, six-sided ice crystals that form when water vapor changes directly into ice crystals (sublimation)

Types of Precipitation • Hail- Irregularly shaped balls or lumps of ice and snow that form in cumulonimbus clouds • Form from ice pellets, trapped in updrafts, that get repeatedly coated with water droplets • Associated with summertime thunderstorms

Types of Precipitation • Sleet- Grains of ice or frozen raindrops that form when rain passes through a cold layer of air and freezes into ice pellets • Raindrops fall through a layer of air which is 32°F or colder • Occurs most often in the winter when warm air is forced over a layer of cold air Sleet is not the same as freezing rain. Freezing rain falling as a liquid but freezes upon contact with cold (below 32°) surfaces

Storms • Storms are violent disturbances in the atmosphere marked by sudden changes in air pressure and rapid air movements • Types of storms: • Rainstorms & Snowstorms- results in extended periods of precipitation • Thunderstorms- heavy rain storms accompanied by thunder and lightning • Hurricanes- storm with violent winds (greater than 75 mph) and heavy rain that forms over tropical oceans during late summer and early fall • Tornadoes- A whirling, funnel-shaped cloud that develops in cumulonimbus clouds and produce winds of up to 300 mph

Factors to consider when trying to make a weather forecast • Sequence of cloud formation- Cirrus clouds changing to low level stratus clouds warn of upcoming precipitation • Wind direction- Winds from the NE, E, and S usually bring unsettled air • Changes in air pressure- A rising barometer usually means fair weather, but falling readings are known for stormy conditions • Air Temperature- Will determine the type of precipitation that falls

Weather and Climate