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Properties of the Atmosphere Composition Structure and Function Air Pressure 5/4/2017 1 What Elements are in our Atmosphere? 5/4/2017 2 Find this graphic in your ESRT now. 5/4/2017 3 5/4/2017 4 Layers of the Atmosphere • 4 main layers--from lowest to highest – Troposphere – Stratosphere – Mesosphere – Thermosphere What is the upper limit of each of these layers? 5/4/2017 5 Each layer of the Atmosphere absorbs energy and protects the Earth from harmful radiation. 5/4/2017 6 Air Pressure • Caused by the weight of the air above you – Greatest at Earth’s surface – Lower at higher elevation 5/4/2017 7 Air Pressure • Because air pressure is exerted in all directions we are seldom aware of it • Can be measured with one of two types of barometers 5/4/2017 8 Mercury Barometer • Uses a column of mercury in a vacuated tube 5/4/2017 9 Aneroid Barometer • Uses a thin metal airtight box 5/4/2017 10 Air Pressure • Measured in millibars • Standard sea level pressure is 1013.2 mb – From your ESRT 5/4/2017 11 • Why do Air Pressure people often use oxygen tanks when they climb high mountains? 5/4/2017 12 Factors that affect air pressure • Temperature • Moisture • Altitude 5/4/2017 13 Temperature as a factor • Cold air –Molecules closely packed • Weighs more • Warm air –Molecules further apart • Weighs less 5/4/2017 14 How does pressure affect temperature? What happens when air is pressurized? What happens when air is expanded? 5/4/2017 15 Increase temperature – decrease pressure PRESSURE T What weighs more???? Water vapor replaces other air molecules that are heavier MOIST AIR WEIGHS LESS Moisture as a factor Increasing water vapor makes air less dense –Nitrogen weighs more than the water vapor that replaces it 5/4/2017 21 Dry Air 5/4/2017 Moist Air 22 Air Pressure • Air with most pressure is cold and dry • Air with least pressure is warm and moist 5/4/2017 23 Pressure and Density Decrease with ALTITUDE Properties of the Atmosphere Wind Coriolis Force Clouds 5/4/2017 25 Air Pressure • In general, the poles are areas of high pressure and the equator is an area of low pressure (ESRT) • Air moves from the poles to the equator then back to the poles – Cools the tropics and warms the poles 5/4/2017 26 Wind • Wind velocity is measured with an anemometer • Is named by the direction it is coming from 5/4/2017 27 Wind • Caused by the uneven heating of Earth’s surface • Creates areas with different air pressures 5/4/2017 28 Wind • Air always moves from high pressure areas to low pressure areas – Just like air in a balloon • Warm, rising air creates low pressure areas – Less dense • Cold falling air creates high pressure areas – More dense 5/4/2017 29 Wind--sea breezes • During the day the land is heated more than the water causing low pressure over the land • Remember-air moves from high pressure to low pressure 5/4/2017 30 Wind--land breezes • Just the opposite happens at night – The land cools down more than the water causing high pressure over the land 5/4/2017 31 5/4/2017 32 Wind speed is DIRECTLY related to the pressure gradient Closer the isobars gradient Greater the pressure Greater the wind speed Prevailing winds and ocean currents • This is what air movement would look like if the Earth were not spinning 5/4/2017 38 Actual Prevailing Winds and Ocean Currents • Due to the Earth spinning. What is the name we give motion due to the earth’s rotation? 5/4/2017 39 Coriolis Force • Because Earth is spinning wind curves to the right in the northern hemisphere and to the left in the southern hemisphere – As seen from space 5/4/2017 40 Coriolis Force Air moves away from a high pressure area like this 5/4/2017 Air moves into a low pressure area like this 41 WIND MOVES CLOCKWISE OUT OF THE HIGH WIND MOVES COUNTERCLOCKWISE INTO THE LOW 5/4/2017 44 Wind –Wind always blows from areas of high pressure to areas of low pressure 5/4/2017 45 Wind • High pressure areas are zones of divergence (ESRT) – Where air pushes away • Low pressure areas are zones of convergence (ESRT) – Where air moves together 5/4/2017 46 Wind--zones of convergence and divergence 5/4/2017 47 5/4/2017 48 Air pressure systems 5/4/2017 49 3 convection cells due to rotation, and unequal heating due to tilt WINDS AND CONVECTION CELLS CAUSED BY DIFFERENCES IN AIR PRESSURE WINDS NAMED FOR THE DIRECTION FROM WHICH THEY COME Isobars • Isobars are drawn on maps to show atmospheric pressure – Isobars are isolines that connect points of equal atmospheric pressure – Similar to contour lines on a topographic map • The closer the isobars, the greater the difference in pressure and therefore the stronger the wind 5/4/2017 52 Isobars of a hurricane Fast wind Slow wind 5/4/2017 53 5/4/2017 54 Evaporation of Water • Three factors that affect the rate at which water evaporates (vaporizes) – Wind – Heat – Surface area 5/4/2017 57 Measuring the Atmosphere’s Moisture Dew Point Relative Humidity 5/4/2017 58 Measuring moisture in the atmosphere • Can be measured 2 ways –Dew point –Relative humidity 5/4/2017 59 Moisture in the Atmosphere • Saturated – When air is holding as much moisture as it can • The warmer the temperature, the more moisture air can hold • For every 10 °C rise in temp. the air can hold approx. twice as much moisture 5/4/2017 60 Dew point • When air temperature is lowered it reaches a point of saturation – Where it can hold no more water vapor • This is known as the dew point • When a mass of air is cooled, the air cannot hold as much water vapor. This water vapor “falls” out of the air as dew. 5/4/2017 61 Write a concise statement or draw a graph that describes how temperature affects the amount of water air can hold: 5/4/2017 62 Dew point • Dew point temperature is determined with a sling psychrometer – A wet thermometer and a dry thermometer 5/4/2017 are used 63 Dew point • You need 2 measurements: – Wet bulb temp – Dry bulb temp • Find the difference • Use the dew point temperature chart in your ESRT – Read the chart! – Follow the lines! 5/4/2017 64 Dew point • The difference between the dry bulb and wet bulb temperatures is known as the wet bulb depression 5/4/2017 65 DPT & Relative Humidity Charts Handy Dandy Earth Science Reference Tables Page 12 The “Dry Bulb” • Don’t let it fool you. It is just a thermometer. • It measures the air temperature. • Duh! 20°C The “Wet Bulb” • Has a little wet booty tied to the bottom. • Gets cool when water evaporates. Wet Booty 12°C A Dry Day… • A lot of moisture will evaporate. • The wet bulb will be a lot cooler than the dry bulb. 20°C Difference between wet bulb & dry bulb is 12 °C. 20°C 14°C 8°C A Humid Day… • A little bit of moisture will evaporate. • The wet bulb will not be much cooler than the dry bulb. 20°C Difference between wet bulb & dry bulb is 6 °C. 14°C Page 12 of your Handy Dandy Earth Science Reference Tables • Warning #1: Be sure to READ the correct chart: DPT or RH • Warning #2: Dew Point Temperature IS NOT “Difference between wet bulb and dry bulb”. • Warning #3: The wet bulb temp IS NOT the DPT. Dry-bulb temperature is your air temperature. 20 °C Subtract (the difference) between the dry bulb and wet bulb Put it all together 20°C 8°C 20-8=12 Relative Humidity = 11% Dew point Your dry bulb temperature is 10°C and your wet blub depression is 3°C. What is the dew point? 5/4/2017 76 Dew point Using a sling psychrometer, you measure the dry bulb temperature to be 20°C and the wet bulb temperature to be 15°C. What is the dew point? 5/4/2017 77 Dew point If the dew point is below zero, what happens when the dew point is reached? 5/4/2017 78 5/4/2017 79 Relative Humidity • Relative humidity expresses how full of moisture the air is • Compares how much moisture the air is holding compared to what it could hold 5/4/2017 80 Relative Humidity • Relative humidity is also determined by comparing the dry bulb temperature with the difference between the wet and dry bulb temperature • You use a different chart in your ESRT. – Be careful. Read the title! 5/4/2017 81 5/4/2017 82 Relative Humidity Your dry bulb temperature is 23°C and your wet bulb is 20°C. What is the relative humidity? 5/4/2017 83 Draw a line to complete this graph: 100 Where on this line would you expect rain? 0 Relative Humidity Relative Humidity vs. wet bulb depression 0 5/4/2017 Wet Bulb Depression 84 The influence of temperature on relative humidity • Raising the temperature without adding more water vapor makes the relative humidity go down 5/4/2017 85 The influence of increasing moisture on relative humidity • Adding more moisture without increasing the temperature will also cause a rise in relative humidity 5/4/2017 86 What causes precipitation? • Cloud formation 5/4/2017 87 Ingredients Required for Clouds: Water vapor (water as a gas) Conditions favoring the change of state (from gas to liquid or ice) CONDENSATION A surface for water vapor to condense on (condensation nuclei) 5/4/2017 88 What causes cloud formation? • When a parcel of air rises it expands • When air expands it cools – Called adiabatic cooling • When air cools it reaches the dew point – 100% relative humidity 5/4/2017 89 Cloud Formation • When air reaches the dew point it condenses – Forming clouds if conditions are right • The water vapor has to have something to condense on – Called condensation nuclei • The rate at which a parcel of air cools as it rises is known as the adiabatic lapse rate 5/4/2017 90 Condensation Nuclei • Raindrops are typically millions of times larger than their condensation nucleus – Condensation nucleus – Raindrop 5/4/2017 91 Cloud droplets can survive by latching onto microscopic solid particles, or condensation nuclei in our atmosphere. These solid particles can be dust, smoke, and salt particles. From volcanoes From Forest Fires The Ocean Pollution 5/4/2017 (First three pictures are not by the author) Salt water droplets from the ocean are carried by updrafts into the atmosphere. When the water evaporates, the salt is left behind. 92 Clouds--what makes air rise? • Air moving over mountain ranges –Orographic effect • When low density air is pushed up by high density air –Happens when a cold front pushes up a warm mass of air 5/4/2017 93 5/4/2017 94 Air rising and cooling to the dew point by expansion (adiabatic cooling) By forced lifting—such as when air is forced over a mountain: Pictures from the National Audubon Society Field Guide to Weather 5/4/2017 95 Air is forced to rise over mountains and cools on the windward side and warms on the leeward side (rain shadow) Clouds • Names are based on shape and altitude • Can tell the observer something about atmospheric conditions – Ex. Cirrus clouds indicate high pressure and clear conditions 5/4/2017 100 Cloud Height • You can find the elevation of the base of a cumulus cloud by knowing – Dew point – Dry bulb temperature 5/4/2017 102 • The lapse rate chart is used to find the altitude of the bottom of cumulus clouds 5/4/2017 103 The second way to get air to rise is for less dense air to be force up and over more dense air. • This happens at the boundaries between AIR MASSES • AIR MASS: A region of air that has similar temperature and moisture. 5/4/2017 106 What is an Air Mass? • Air masses are large bodies of air which have similar temperature and moisture characteristics. • Air masses form when air stays over a region (called the source region) for several days. • Air masses that form over water will be moist. • Air masses that form over land will be dry. 5/4/2017 107 AIR MASSES • Air takes the characteristics of the surface over which it formed. • An air mass that forms over the ocean will be moist. These air masses are called MARITIME air masses • An air mass that forms over land will be relatively dry. These air masses are called CONTINENTAL air masses 5/4/2017 108 Air Masses • An air mass that forms over a cold portion of the Earth is called a POLAR air mass. • An air mass that forms over a warm portion of the Earth is called a TROPICAL air mass. • An air mass that forms over an extraordinarily cold portion of the Earth 5/4/2017 109 is called an ARTIC air mass. Open your ESRT to page 13! Air Masses Air Mass Classification Source Region Identifiers: • A for Arctic, • P for Polar, • T for Tropical; Moisture Content Identifiers: • c for continental (meaning the air is relatively dry), • m for maritime (meaning the air is relatively moist); 5/4/2017 110 • • • • cT cP mT mP Continental Tropical – dry & warm Continental Polar – dry & cold Maritime Tropical – humid & warm Maritime Polar – humid & cold • cA Continental Artic – very dry & very cold 5/4/2017 111 5/4/2017 112 5/4/2017 113 What happens when air masses meet? • The boundary between two air masses is called a FRONT. 5/4/2017 114 Fronts • Occur when different air masses collide • Types include: – Cold front – Warm front – Stationary front – Occluded front 5/4/2017 115 Cold Front • Where cold, dense air pushes beneath warm, less dense air – Where powerful storms occur – Cold fronts move quickly 5/4/2017 116 5/4/2017 117 5/4/2017 118 Warm Front • Where a mass of warm air slides over a retreating mass of cold air – Often producing slow steady rain – Warm fronts move slowly 5/4/2017 119 5/4/2017 120 Occluded Front • When one front catches up to another • Produces large areas of rainy, unsettled weather 5/4/2017 121 Stationary Front • Front does not move • Winds blow in opposite directions 5/4/2017 122 Causes of Precipitation • LOW PRESSURE • FRONTS • MOUNTAINS 5/4/2017 123 5/4/2017 124 Precipitation Types • Rain: water droplets leave cloud and fall to earth – Drizzle, mist, etc. • Snow: water droplets freeze inside cloud and fall to earth • Hail: water droplets fall, are swept up and freeze forming a ball of ice that falls when the wind is not strong enough to support it • Sleet: water droplet leaves cloud, moves to cold area and freezes • Freezing rain: water droplet freezes on contact with frozen surface 5/4/2017 125 Synoptic Weather Maps • Symbols are used on weather maps to show current weather conditions –Allows you to predict future weather 5/4/2017 126 Synoptic Weather Maps • Symbols for present weather – Called a station model – This graphic is in your ESRT! 5/4/2017 127 Synoptic Weather Maps • Symbols used for fronts: 5/4/2017 128 5/4/2017 129