Ch 16 Understanding Weather

I. Water in the Air A. Humidity 1.Weather= condition of the atmosphere at a certain time in a certain place a.Short term b.Temp, humidity, precipitation, wind & visibility 2.Humidity= the amount of water vapor in the air a.In the water cycle, water evaporates from ocean, lakes, & plants b.Air’s ability to hold water vapor changes with temperature Temperature is the measure of how fast the molecules of air are moving (kinetic energy) More energy (higher temp) the more water that can be evaporated

1.Relative Humidity= amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temp. a.Given as a percentage b.Air is saturated when it hold all of the water it can possibly hold at a given temp= relative humidity 100% c.Calculate relative humidity using this formula: Actual water vapor content (g/m 3) saturation water vapor content (g/m 3) x100 = relative humidity (%)

Problem: Assume 1m 3 air can hold 24g of water at 25°C. The actual content is measure to be 11g of water. Calculate the relative humidity ;

4.Two factors affect relative humidity: a.Water vapor= the more water vapor in the air the higher the relative humidity b.Temperature= if the amount of water vapor in the air stays the same BUT the temperature changes the relative humidity increases or decreases Temp increases= relative humidity decreases Temp decreases= relative humidity increases

5.Psychrometer measures relative humidity a.1 thermometer covered with a damp cloth (wet bulb) & 1 thermometer without (dry bulb) b.Dry bulb only measures air temp c.Wet bulb measures water vapor in the air d.Difference in temp reading between the wet bulb & the dry bulb indicates the water vapor in the air

6.Wet bulb thermometer works through evaporation a.Air passes over the wet bulb thermometer and evaporated the water from the cloth (cooling the cloth) b.When humidity is low = water evaporates more quickly- greater difference in temp reading c.When humidity is High= water evaporates more slowly- less difference in temp reading

Make a psychrometer: 1.Get 2 thermometers, cloth, tape, water, rubber band 2.Wrap the cloth around the end of one thermometer and secure it with the rubber band 3.Wet the gauze with room temp water 4.Tape the 2 thermometers side by side and fan both watching the change in temp on the wet bulb 5.When the wet bulb temp stabilizes record the temps on both thermometers and find the difference 6.The chart on page 484 to find the relative humidity % of the room

B. Condensation Condensation= the process by which gas is changed into a liquid Occurs when saturated air cools 2.Dew point: the temperature at which a gas condenses into a liquid Condensation needs a surface for the water vapor to condense on Better guide to how it “feels” outside Can NEVER be higher than the temp

Dew point Your perception of how it feels Relative Humidity Temp °F Air temp 90°F

C. Clouds 1.Cloud= collection of millions of tiny water droplets or ice crystals Formed when warm air rises and cools Depending on the air temp water will change into a liquid or a solid

2.Clouds are classified by their form and altitude 3.Cumulus clouds: puffy, white & tend to have flat bottoms a.Normally show fair weather b.Cumulonimbus = produce thunderstorms c.“-nimbus or -nimbo” likely to produce rain

4.Stratus clouds: clouds that form in layers a.Cover large areas of the sky& can block out the sun b.Caused by the gentle lifting of a large body of air into the atmosphere c.Nimbostratus= dark stratus clouds that usually produce light to heavy continuous rain d.Fog= stratus cloud that has formed near the ground

5.Cirrus clouds: thin, feathery, white clouds found at high altitudes a.Form when wind is strong b.Indicate that a change in the weather is coming when they get thick

6.Two altitude groups: a.“cirro-” used to describe clouds and that form at high altitudes b.“alto-” describes clouds that form at middle altitudes

D. Precipitation= water (solid or liquid) that falls to Earth from the air 1.Rain: most common type- water droplet grow to ~100xs the size they are in clouds before they fall 2.Sleet= forms when rain fall through a layer of freezing air- producing falling ice

3.Snow= forms when temps are so cold that water vapor changes directly to a solid Can be single ice crystals or joined as flakes 4.Hail= forms in cumulonimbus clouds when updrafts of air in the clouds carry raindrops high in the clouds and they freeze Process happens over and over layering the hail until its too heavy to be carried back up and falls to Earth

II. Air Masses and Fronts A. Air Masses 1.Air mass= large body of air where temp & moisture content are similar throughout 2.Temp & moisture content are determined by the area where the air mass is formed= source region a.Maritime (m)= forms over water b.Continental (c)= forms over land c.Polar (p)= forms over polar regions –cold d.Tropical (t)= develops over the Tropics- warm

B. Cold air masses- 3 polar air masses that influence cold winter weather 1.Continental polar (cP) formed over northern Canada- cool dry 2.Maritime polar (mP) formed over North Pacific ocean- cool wet 3.Maritime polar (mP) from North Atlantic- cool & cloudy from New England

1.Maritime tropical (mT) develops over warm areas in the Pacific Ocean milder 2.Maritime tropical (mT) developing over the Gulf of Mexico Very hot, humid weather- hurricanes 3.Maritime tropical (mT) developing over the Atlantic Ocean Very hot, humid weather- hurricanes 4.Continental tropical (cT)- forms over deserts of northern Mexico & southwestern US Brings clear, dry hot weather C. Warm Air masses- Four that influence USA

D. Fronts- boundary between air masses of different densities & usually different temperatures Cold air moves under less dense warm air & pushed it up a.Move quickly b.Brings thunderstorms, heavy rain or snow c.Brings cooler direr weather

2.Warm Front= warm air moves over cold, denser air a.Warm air GRADUALLY replaces the cold b.Brings drizzly rain rain followed by warm weather

3.Occluded Fronts= warm air mass is caught between two colder air masses a.Coldest air mass moves and pushes up the warm b.Coldest air mass then moves forward & under the cold, but warmer than the coldest, air mass c.Brings lots of rain or snow & cool temps

4.Stationary Front= forms when a cold air mas meets a warm air mass & neither air mass has enough force to lift the warm up & over the cold Brings many days of cloudy, wet weather

E. Air Pressure & Weather 1.Cyclones- area that have lower pressure than the surrounding areas Air masses come together, converge, & rise

2.Anticyclones- areas that have high pressure a.Air diverges (moves apart) & sinks b.Sinking air causes the high pressure c.Cooler, denser, air moves out from high to low

III. Severe Weather A. Thunderstorms Thunderstorm= small, intense weather systems that produce strong winds, heavy rains, lightening & thunder