Weather

1. 7th Grade Science Weather

2. Why should we study weather? • Weather has an incredible influence on our lives. • Rainy days may cancel a soccer game. • Hail may damage crops. • Hurricanes have killed many people and may cause millions of dollars in property damage. • Despite our modern scientific achievements, we cannot control the weather. • Most of man’s efforts in studying weather is focused on learning to predict its behavior.

3. Job 36 • 27 “He draws up the drops of water,    which distill as rain to the streams[c]; 28 the clouds pour down their moisture    and abundant showers fall on mankind. 29 Who can understand how he spreads out the clouds,    how he thunders from his pavilion? 30 See how he scatters his lightning about him,    bathing the depths of the sea. 31 This is the way he governs[d] the nations    and provides food in abundance. 32 He fills his hands with lightning    and commands it to strike its mark. 33 His thunder announces the coming storm;    even the cattle make known its approach.

4. Weather • Weather comes from the interaction of three things---heat, air, and water vapor. • Differences in the amount of solarradiation reaching a certain spot on earth eventually produces winds. • Weather may be described using atmosphericconditions such as temperature, air pressure, wind speed, wind direction, cloud cover, humidity, and precipitation. • A change in weather involves a change in atmosphericconditions.

5. Weather • “Reading the signs” • Weather developments are often described using meteorology. • Meteorology- the science of weather • Meteorologist- a scientist who studies weather • Meteorology involves learning how one atmospheric condition is affected by others and helps a scientist make an accurate forecast. • A wind shift may cause an increase or decrease of cloud cover. • A fall in air pressure often signals the coming of a storm.

6. Weather • Weather and climate are not the same. • Weather is defined as the atmospheric conditions at a certain time and place. • After years of closely monitoring the weather patterns of an area, it becomes clear that weather goes through a yearly cycle. • Temperature may be consistently colder in winter. • Rain may happen most often in the spring. • Climate is the weather of a location averaged for a long period of time.

7. Seasons • Autumnal equinox • This is the official beginning of fall when both day and night have an equal length • It currently occurs on September 22 • Winter solstice • During this 24 hour period, we experience the shortest day and the longest night. • The winter solstice is the official start of winter. • It currently occurs on December 21st

8. Seasons • Vernal equinox • On March 20th, the period of day and night are of equal length • The vernal or spring equinox is the date when spring begins • Summer solstice • On June 21st, we experience the longest day and shortest night of the year • The summer solstice marks the start of summer • The dates on this presentation do not match the dates given in the book for the spring equinox and winter solstice. This happens about every 70 years due to leap year.

9. Seasons • Every 24 hours the earth completes one full rotation on its axis • Rotation • Spin of the earth on its axis • The seasons arise from the tilt of the earth’s axis • Axis • The imaginary line running through the center of the earth from pole to pole • The earth’s axis is tilted at a slight angle is relation to its path around the sun

10. Seasons • The earth’s axis is tilted at an angle of 23 ½ degrees. • The earth takes 365 ¼ days or one solar year to complete a revolution around the sun. • Revolution • Completion of an orbit such as the sun circling around the sun • No matter where the earth is in its revolution, it’s axis always points to the polestar, Polaris (also known as the North Star) • During our summer the northern hemisphere is tilted to the sun • During our winter, the northern hemisphere is tilted away from the sun

11. Seasons • In the southern hemisphere, the seasons are reversed • They experience winter during our summer and summer during our winter • Longer days during the summer lead to hotter days • Longer nights during the winter lead to colder days • The tilt of the earth causes a change in the angle that the sun’s rays strike the atmosphere

12. Seasons • Why do places like Hawaii have long summers while Antarctica has little or no summer • The earth is encircled by imaginary lines called parallels of latitude • The equator marks 0º latitude while the south pole is 90º south, and the equator is struck directly by the sun on the equinoxes. • The Tropic of Cancer marks 23.5º north and is struck directly by the sun’s rays on the Summer Solstice. • The Tropic of Capricorn marks 23.5º south and is struck directly by the sun’s rays on the Winter Solstice.

14. Seasons • Within the Arctic circle, the land experiences 24 hours of no sun on the Winter Solstice • On the Summer Solstice, those within the Arctic Circle experience 24 hours of daylight • Bordering the sunlit side of the earth, there is a band of twilight. This band of refracted light is what causes there to be some light before sunrise and after sunset. • Zenith- the highest point in the sky • If the sun seems close to the zenith, that portion of the earth is receiving more direct sunlight.

15. Wind • Wind • Horizontal movement of air • Currents • Vertical movements of air • Updrafts are upward moving currents and are said to be ascending (rising) • Downdrafts are currents that are said to be subsiding or sinking • Air is heated by infrared energy that is radiated from the earth • Air is also heated by conduction with the ground and other air molecules. • Conduction is the transfer of heat or energy by direct contact.

16. Wind • Air moves by a process called diffusion. • Diffusion • A movement from an area of high concentration to an area of low concentration • If a tire is punctured, air from the area of high pressure in the tire will flow outward to the low pressure area outside of the tire and leave you with a flat • We find this happening when a balloon is popped by a pin, or if a soda is opened and air escapes with a “hiss”.

17. Wind • Warm air molecules are more spread apart than colder air, so warm air has lower air pressure than cold air.

18. Breezes to Know • Sea breeze • During the summer, cooler air(high pressure) above water will move to land to replace rising warm air (low pressure) above land • Forest breeze • Cooler air moving under trees will move in to replace warmer air above an open space • Valley breeze • During the day, mountains warm up faster than valleys, so air in the valley with higher pressure flows up the mountain to the area with lower pressure

19. Breezes • Mountain breeze • When air in the valley is warmer than the air on a nearby mountain, colder mountain air with a higher pressure will flow into the low pressure valley • Lake breeze • Cooler air above a lake will move to land to replace rising warm air above land (This is the same as a sea breeze, but it involves a lake instead of an ocean or sea.) • Katabatic Winds • Cold wind that moves from an area of high elevation to an area of low elevation.

20. Breezes • Chinook • Dry, gusty wind that is seen in the American Rocky Mountains • It forms when stormy air passes over a mountain range • On the windward side (the side of the mountain facing the wind), the stormy air is forced up the mountain where it cools and produces precipitation and clouds • When the air passes over the mountain to the leeward side (side away from the wind), the air is very dry and warm and may melt any existing snow cover

21. Chinooks and foehn • A Foehn is very much like a chinook, but it occurs in the European Alps

22. Global winds • The sun does not shine with equal intensity on all parts of the earth. • The solar rays hit the equator nearly straight-on. • The rays hit the polarregion at a slant. • Rays that hit the poles must travel further through the atmosphere than waves that hit the equator. • Air at the poles is colder than air at the equator, so air moves from the poles(high pressure) to the equator(low pressure)

23. Global winds • Solar rays striking the Earth • Air flow without rotation (spinning)

24. Global winds • Global winds are further influenced by the rotation of the Earth • There are three wind zones in each hemisphere [Look at p213 in your book.] • Tropical zone • 0º latitude to 30º • These winds are northeasterly in the NorthernHemisphere • They are commonly called trade winds because they helped trade ships get to their destination.

25. Global Winds • Temperate zone • 30º latitude to 60º • These winds are southwesterly in direction • The prevailing winds are called the westerlies • Polar zone • 60º latitude to 90º • These winds are southeasterly in direction in the Northern Hemisphere • The prevailing winds are called the Polar Easterlies

27. Global winds • The trade winds of the Northern and Southern Hemispheres converge [come together] near the equator. • The place where the winds meet is called the Intertropical Convergence Zone (ITCZ) • Sailors called the ITCZ the doldrums because there was little to no wind, and it was very difficult to sail in this region. • In the ITCZ, air is converging and rising (updrafts)

28. Global Winds • Horse latitudes • Point around 30º where there is little to no wind because air movement is vertical • According to tradition, because ships might get stuck in this area of no wind, many horses kept on the ship died because of starvation • Subtropical highs • Region of high pressure found in horse latitudes

29. Global winds • When two winds such as the westerlies carrying warm air collide with the easterlies carrying cold air from the poles, the two winds do not combine • The boundary between the two winds is a polar front

30. Global winds • Polar highs • High pressure areas in the polar regions • Inertia • The tendency of a motionless object to remain at rest and the tendency of a moving object to continue moving in the same direction unless acted upon by a force (such as gravity) • When a warm front and cold front collide, there is often some precipitation.

31. 1/24 and onward material • Test 2 Material Starts Here

32. Global wind • Coriolis Force • The apparent force from the Earth’s rotation that causes all wind in the Northern Hemisphere to curve to the right. • This is not a real force but just an effect of the earth’s rotation.

33. Global winds • Air circulation within the tropics forms a complete circuit. • Air near the ITCZ (doldrums) rises and moves to 30 degrees latitude where it subsides (sinks) • Trade winds carry that air from near the horse latitudes back to the ITCZ • Hadley Cell • Circulation cycle of tropical air

35. Global winds • In all parts of the globe, the movement of air in the upper troposphere is westerly • Jet stream • Long narrow tube of westerly wind just below the tropopause (top of the troposphere) • It is commonly used to aide the travel of planes traveling east • Each hemisphere has two jet streams • Subtropical jet stream • Polar front jet stream

36. Global winds

37. The cloud factory • The sun is the basic cause of all weather • It’s influence causes seasons, winds, clouds, lightning, rainbows, hurricanes, blizzards, and more • Air always has a small amount of water vapor • It ranges from a trace to 4% of the air in the atmosphere • Humidity • The amount of water vapor in the air • Steaming air of a rainforest is more humid than the dry air of a desert.

38. The cloud factory • The amount of water vapor that air can hold decreases as air temperature decreases • Warm air can hold more water than cold air • If air holds as much water vapor as it can, the air is saturated. • Relative humidity measures the percentage of the possible water vapor in the air. • It is humidity relative to saturated air. • The relative humidity for Dothan on 1/25 was projected to be 95%

39. The cloud factory • Dew point • The temperature at which air becomes saturated • Dew point depends on how much water is in the air • The lowest temperature at which air keeps all water vapor • 2 conditions encourage the formation of dew • Clear skies • They allow the earth’s energy to escape to space • Little or no wind • Wind will not allow air to stay in contact with the ground long enough for dew to form

40. The cloud factory • Frost is not frozen dew • Frost is frozen water vapor. • What are clouds? • Clouds are made of water droplets that have condensed from water vapor • It takes over one million of those droplets to form a rain drop • Lapse rate • Rate at which air cools as you ascend the atmosphere

41. The cloud factory • Air cools (temperature decreases) as it rises through the troposphere • In the stratosphere (above the troposphere), the air warms (temperature increases) • Temperature inversion is when warmer air rests on top of cooler air • Air in the stratosphere is warmer than the troposphere because the ozone layer in the stratosphere is responsible for absorbing much of the sun’s heat.

42. 4 steps to forming a cloud • Water evaporates becoming water vapor and mixes with air. • Air warms and rises with the water vapor • The water vapor eventually rises to a place so cold that it reaches dew point. • Water vapor goes through condensation (turns into a liquid) and forms a cloud • Water needs a surface in order to condense. • Condensation nucleus • Any object on which water may condense (dust, exhaust from planes, or even sea salt)

43. The cloud factory • Clouds have different appearances that depend on how they were formed • Thermals are rising columns of warm air • Little white, puffy clouds are often formed at the top of a thermal • Clouds will grow as long as they stay in contact with a thermal • Unstable air occurs when thermals cool more slowly than surrounding air (hotter than surrounding air) • Unstable air often causes very tall clouds.

44. The cloud factory • Stable air is found when a thermal cools more rapidly than the lapse rate (it is cooler than the surrounding air.) • Stable air will not rise very high because of how quickly it cools • Mountain clouds form when air is pushed up as it moves over a mountain

45. 2/2/2011 Agenda • Review • Quiz • Take notes • Go outside and identify clouds if the weather permits

46. Clouds and precipitation • Meteorologists recognize 4 basic cloud groups. (read p 233) • Cumulus • Stratus • Cirrus • Lenticular

48. Clouds and precipitation • Cumulus clouds • Puffy and bulgy clouds produced by rising thermals or a warm air mass violently colliding with a cold air mass • The primary factor of forming a cumulus cloud is vertical lifting • Cumulus clouds come in a variety of sizes • Cumulonimbus clouds or thunderheads are said to resemble a giant anvil and produce many violent storms

49. Clouds and precipitation • Stratus clouds • These clouds get their name from the Latin word for layer (stratum) • Stratus clouds may look like a flat blanket • Cirrus clouds (means wisp) • Thin clouds that float near the top of the troposphere • In some extremely cold altitudes, cirrus clouds contain tiny ice crystals because of the low temperatures • Because of the wispy appearance that ice crystals give cirrus clouds, they are called “mare’s tails”

50. Clouds and precipitation • Lenticular clouds • Clouds shaped like a lens • Stacks of these clouds may look like a stack of pancakes • Combination clouds • 3 Basic types of combination clouds • Cirrostratus • Cirrocumulus • Stratocumulus