| Weather is the current state of the Atmosphere in terms of AIR PRESSURE, WIND, TEMPERATURE, AND MOISTURE. The interactions of AIR, WATER, AND SOLAR ENERGY CAUSE WEATHER. Clouds often indicate that changes are taking place in the Atmosphere. Clouds can provide a clue to the Weather ahead. The appearance of DARK GRAY Clouds on the Horizon, Alerts people to the possibility of Heavy Rains. The Formation and Movement of Clouds indicate how MOISTURE in the Air affects our day-to-day Weather. Changes in Earth's Water, The Water Cycle 1. Almost all SUBSTANCES can exist as a SOLID, LIQUID OR GAS. Water is one of the few Substances commonly found in all THREE forms at Normal Earth Temperature. 2. Most of the Earth's Water is LIQUID, including OCEANS, LAKES, RIVERS, AND GROUNDWATER. 3. GLACIERS contain most of the Earth's SOLID form of WATER - ICE. Glacial Ice several Km thick covers most of the rock of Greenland and Antarctica 4. The remainder of the Earth's Water is a GAS - WATER VAPOR. It occurs mostly throughout the LOWER part of the TROPOSPHERE, even in the Desert Air. 5. The total amount of Water on Earth changes very little over time. Water, However, is always moving from place to place and is always CHANGING FORM. EVAPORATION: 1. EVAPORATION is the process in which a LIQUID Changes to a GAS. HEAT ENERGY is needed to change molecules of LIQUID WATER into molecules of WATER VAPOR. The LIQUID takes in this ENERGY from its surroundings. Evaporation is considered a COOLING Process because Liquid takes HEAT AWAY from its SURROUNDING. 2. A Process related to Evaporation is TRANSPORATION. During Transpiration, PLANTS release WATER VAPOR Through their LEAVES. Much of the water collected by plant roots is TRANSPIRED into the AIR. CONDENSATION 1. When WATER changes from a GAS to a LIQUID, it releases ENERGY to its SURROUNDINGS. The in which a GAS changes into a LIQUID is called CONDENSATION. Warm Air contains more WATER VAPOR than COLD Air. When Warm, Moist Air is COOLED some of the VAPOR condenses to LIQUID. 2. Condensed Water Vapor on COOL SURFACES is DEW. During the NIGHT when Temperatures DROP, WATER VAPOR CONDENSES on the grass, flowers, and other surface that have become COOL. 3. When Temperatures are below FREEZING, FROST forms instead of DEW. FROST IS NOT FROZEN DEW, WATER VAPOR CHANGES DIRECTLY INTO CRYSTALS OF ICE. WATER CIRCULATES IN THE WATER CYCLE 1. Earth is a water Planet. In fact, over 70 percent of Earth's surface is covered by water. 2. The HYDROSPHERE, Earth's water environment, includes OCEANS, LAKES, RIVERS, AND OTHER BODIES OF WATER, GROUNDWATER, ICE FROZEN IN GLACIERS, SNOW, AND WATER VAPOR IN THE ATMOSPHERE. 3. Water Circulates among these different parts of the Hydrosphere. Like circulation of air, the WATER CYCLE IS POWERED BY THE SUN. 4. The Earth's Water is Continuously changing forms through EVAPORATION AND CONDISATION. These changes help to move Water between the SURFACE AND ATMOSPHERE. THIS CONTINUOUS MOVEMENT OF WATER IS CALLED THE WATER CYCLE. THE WATER CYCLE: A. The Sun provides the Energy needed to Evaporate Water. B. Global Winds then carry the water vapor through the TROPOSPHERE. C. When the Air COOLS, Water Vapor CONDENSES into tiny water droplets, which From CLOUDS. D. The Water in CLOUDS may fall to the Earth's surface as RAIN OR SNOW. E. Some of the Water Flows into RIVERS, LAKES, OCEANS, or the GROUND. F. EVAPORATION CONTINUES THE CYCLE. 2. The Water Cycle has no true Beginning, IT IS CONTINUOUS. THE WATER CYCLE IS BALANCED. The Amount of water entering the AIR is about EQUAL to the amount of Water that LEAVES IT. WATER VAPOR IN AIR - RELATIVE HUMIDITY 1. Air has MASS and VOLUME; High Atmospheric Pressure areas may form when Air is COOLED. In Cool Air, the molecules move CLOSER Together, causing Air to become DENSER. The DENSE Air creates a HIGH-PRESSURE AREA. 2. LOW-PREASURE AREAS may form when Air is WARMED, causing Molecules to move FARTHER APART and Air to become LESS DENSE. 3. TEMPERATURE affects the ability of Air to HOLD MOISTURE. Moisture in the Atmosphere is called HUMIDITY. 4. WATER VAPOR MOLECULES ARE HELD IN SPACES BETWEEN OTHER GAS MOLECULES. As Air is HEATED, gas molecules move FARTHER Apart. There is then MORE SPACES Available for WATER VAPOR. When Air is COOLED, gas molecules move TOGETHER, and there is LESS SPACES for WATER VAPOR. 5. WARM AIR CAN HOLD MORE MOISTURE THAN AN EQUAL VOLUME OF COLD AIR. 6. RELATIVE HUMIDITY IS A MEASURE OF THE AMOUNT OF WATER VAPOR IN A GIVEN VOLUME OF AIR, COMPARED TO THE TOTAL AMOUNT OF MOISTURE THAT THAT VOLUME OF AIR COULD HOLD AT A GIVEN TEMERATURE. 7. EXAMPLE: At 20 degrees C, a cubic meter of air can hold a total of twelve grams of water vapor. If only four grams are present, the Relative Humidity is 33percent. 8. When Air holds ALL the MOISTURE it can at a given Temperature, the Air is said to be SATURATED, AND THE RELATIVE HUMIDITY IS 100 PERCENT. 9. At 100 Percent Relative Humidity, Water Vapor may CONDENSE, or CHANGE FROM A GAS TO A LIQUID. 10. The Temperature at which CONDENSATION OCCURS IS THE DEW POINT. 11. On A clear night, (Clouds Hold in Heat), a Land Surface will LOSE HEAT RAPIDLY into SPACE. A MOIST Layer of Air near the Surface will COOL and become SATURATED. DEW THEN FORMS AT THE SURFACE AS THE WATER CONDENSES, 12. IF THE SURFACE AIR TEMPERATURE IS BELOW FREEZING, FROST FORMS DIRECTLY FROM THE SATURATED AIR. CLOUDS 1. A CLOUD IS A COLECTION OF WATER DROPLETS (OR SOMETIMES ICE CRYSTALS) FLOATING IN THE AIR. THE WATER IN CLOUDS COMES FROM THE WATER VAPOR IN THE AIR. WHEN AIR COOLS, THE WATER VAPOR CONDENSES INTO MILLIONS AND MILLIONS OF TINY LIQUID WATER DROPLETS. THE RESULT IS A CLOUD. 1. The Water Vapor produced by Evaporation and Transpiration Rises and Slowly COOLS. Condensation occurs if the Air is SATURATED and if DUST PARTICLES are present. 2. During Condensation, Water Vapor COLLECTS around the DUST, SMOKE or SALT PARTICLES FROM OCEAN SPRAY, called CONDENSATION NUCLEI, and form Tiny Droplets of Water. 3. CLOUDS are a Collection of Tiny Droplets of Water Suspended in the AIR. 4. Droplets may range from 0.002 to 0.1 mm in Diameter. In Clouds where LARGE Amounts of MOISTURE are present, Water Droplets may JOIN. When these Droplets REACH 2.0 to 6.5 mm in Diameter, They may FALL AS RAIN. 5. PRECIPITATION, A WORD USED TO DESCRIBE ANY KIND OF RAIN OR SNOW. PRECIPITATION OCURRS WHEN WATER DROPLETS FLOATING WITHIN CLOUDS COMBINE AND BECOME SO HEAVY THAT THE AIR CAN NO LONGER HOLD THEM. 6. RAIN IS A FORM OF PRECIPITATION. There are several different types of rain. When Raindrops are very small they're called DRIZZLE OR MIST. Rain that STARTS and STOPS suddenly is called a SHOWER. Rain that contains POLLUTION IS CALLED ACID RAIN, and it can be very destructive to plants, rivers, and lakes. 7. SNOW, HAIL, AND SLEET ARE ALSO FORMS OF PRECIPITATION. 8. HAIL forms when droplets (RAINDROPS) of Water FREEZE in Layers AROUND a Small Nucleus of ICE. HAILSTONES GROW LARGER AS THEY ARE TOSSED UP AND DOWN BY RISING AND FALLING AIR CURRENTS THAT OCCUR DURING SEVER THUNDERSTORMS. 9. SLEET FORMS when Raindrops FALL Through a Layer of Air BELOW -3 Degrees C. SLEET usually falls during the WINTER. In SUMMER, Sleet MELTS and Falls as RAIN. 10. SNOW Forms when WATER VAPOR CHANGES DIRECTLY TO A SOLID. SNOWFLAKES are usually SIX-SIDED CRYSTALS occurring in many different patterns. TEMPERATURE IN THE AIR MUST BE BELOW FREEZING FOR SNOW TO FORM. BELIVE OR NOT, MOST PARCIPITATION BEGINS AS SNOW. THE REASON FOR THIS IS THAT EVEN IN THE SUMMER, CLOUDS CAN HAVE TEMPERATURES WELL BELOW FREEZING POINT, 32 DEGREES F, OR 0 DEGREES C. SNOW comes in many different forms, LIGHT SNOW Falling for a short period of time is called SNOW FLURRIES OR SNOW SHOWERS. A Sever Storm, with Drifting and Blowing Snow, is called a BLIZZARD. 12. PRECIPITATION IS SOMETIMES A MIX OF BOTH RAIN AND SNOW. SOME EXPAMPLES OF MIXED PRECIPITATION ARE SLEET, ICE PELLETS, AND FREEZING RAIN. 13. CLOUDS are Classified according to their SHAPES and ALTITUDES (HEIGHT ABOVE THE GROUND). 14. THE FOUR BASIC TYPES OF CLOUDS ACCORDING TO SHAPE, ARE CIRRUS, CUMULUS, STRATUS, AND NIMBUS. 15. A CIRRUS CLOUD IS A HIGH, WHITE, FEATHERY CLOUD USUALLY ASSOIATED WITH FAIR WEATHER. This type of Cloud is composed of ICE CRYSTALS or Supercooled Water, and may sometimes indicate that BAD WEATHER WILL OCURR IN THE NEAR FUTURE. 16. CUMULUS CLOUDS ARE THICK, PUFFY MASSES THAT LOOK LIKE HEADS OF CAULIFLOWER WITH FLAT BOTTOMS, COMMONLY SEEN ON SUMMER AFTERNOONS. Cumulus Clouds usually develop during the DAY over LAND when columns of MOIST AIR are forced ALOFT and COOLED to the DEW POINT Temperature. 17. CUMULUS CLOUDS usually form less than a mile (1.6 km) above the ground. CUMULUS CLOUDS THAT FORM AT HIGHER ALTITUDES - 1 TO 3 MILES (1.6 TO 4.8 km) ABOVE THE GROUND-ARE CALLED ALTOCUMULUS (ALTO MEANS "HIGH"). THESE CLOUDS APPEAR SMALLER THAN CUMULUS CLOUDS, BUT THIS IS SIMPLY BECAUSE THEY ARE FARTHER AWAY. 18. STRATUS CLOUDS ARE THE DULL, GLOOMY CLOUDS YOU SEE ON OVERCAST DAYS. THEY OCCUR IN LAYERS AND OFTEN COVER THE WHOLE SKY. Stratus Clouds usually look DARK because they Block out the Sun- often they appear to cover the entire sky. Stratus Clouds usually are only a Few Hundred Meters Thick, but may Extend over Thousands of Square Km. 19. CLOUDS THAT PRODUCE PRECIPITATION ARE NIMBUS CLOUDS (NIMB MEANS "RAIN"), NINBUS CLOUDS ARE DARK GRAY CLOUDS THAT HAVE RAGGED EDGES. RAIN OR SNOW Falls continuously from the BOTTOM of these Clouds. Spotting these Clouds is one way to predict the Weather, appearance of these clouds usually means that THUNDER, LIGHTNING, AND RAIN ARE COMING. 20. FOUR LEVELS OF CLOUD HEGHT ARE RECOGNIZED. 21. HIGH CLOUDS HAVE BASES ABOVE 6000 M. 22. MIDDLE CLOUDS OCURR BETWEEN 2000 AND 6000 M. 23. LOW CLOUDS FORM BELOW 2000 M. 24. Clouds that have their BASE in the LOW HEIGHT Range but EXTEND UPWARD into the MIDDLE or HIGH ALTITUDES are called VERTICAL CLOUDS. 25. VERTICAL CLOUDS ARE OFTEN ASSOICIATED WITH THUNDERSTORMS. 26. A STRATUS CLOUD CLOSE TO THE GROUND IS CALLED FOG. Fog is nothing more than a Cloud that forms at Ground Level. When it's Foggy outside, you can literally "walk with your head in the clouds." FOG FORMS WHEN AIR NEAR THE GROUND COOLS, CAUSING WATER VAPOR TO CONDENSE. 27. FOG is especially common in Marine Climates. Near the Sea, Air always contains LARGE AMOUNTS of MOISTURE. At NIGHT, it the Temperature FALLS BELOW the DEW POINT and the Air is STILL, Water Vapor Condenses in the Air and forms FOG. 28. STRATUS CLOUDS AND FOG MAY ALSO OCURR WHEN WARM, VERY MOIST LAYERS OF AIR FLOW ACROSS A COLD SURFACE. AIR MASSES 1. Weather includes many Short-Term Changes in PRESSURE, TEMPERATURE, WIND DIRECTION, AND HUMIDITY. These Changes are generally related to the development and movement of AIR MASSES. People can FORCAST the Weather by Studying Weather Patterns. 2. When a LARGE Part of the TROPOSPHERE STOPS OR MOVES over a uniform Land or Water Surface, an AIR MASS FORMS. 3. An AIR MASS is a BODY of AIR that has the SAME PROPERTIES as the SOURCE REGION over which it Develops. Air Masses extend over Thousands of km. An Air Mass is a LARGE Body of Air that may extend over a large part of a Continent or Ocean. It is Fairly UNIFORM in TEMPERATURE AND HUMIDITY AT A GIVEN ALTITUDE LEVEL. 4. An Air Mass Acquires its properties of TEMPERATURE AND HUMIDITY FROM ITS SOURCE REGION, THE LAND OR SEA SURFACE OVER WHICH IT ORGINATES. Air Masses are Classified according to their SOURCE REGION: a. Air Masses that develop over Continental and are relatively dry are identified by "c" for CONTINENTAL AIR MASSES. b. Air Masses that develop over Oceans are MOIST and are indicated by an "m" meaning MARITIME AIR MASSES. c. Air Masses that develop over High Latitudes are COOL, They are labeled "P' for POLAR AIR MASSES OR "A" FOR ARTIC DEPENDING ON WHETHER THEY FORM OVER LAND OR WATER. d. Low Latitude Air Masses are WARM and are labeled "T" for TROPICAL AIR MASSES. 1. On a Weather Map, TWO SYMBOLS are used in combination to describe the TEMPERATURE AND HUMIDITY OF AN AIR MASS. 2. EXAMPLE: THE SYMBOL Mt is used for an air mass that is WARM AND MOIST. The POLAR and ARTIC CONTINENTAL Air Masses (cP and cA) are cold and dry, of the two, the Arctic Air Mass is the Coldest. They originate from Hudson Bay to Alaska. Their influence is Greatest in the WINTER, when they bring COLD WAVES too much of the country. POLAR MARITIME Air (mP) is of Oceanic Origin, the north Atlantic, and is COLD and MOIST, bringing Drizzle to the Northeastern US in the warmer seasons and often heavy snowfalls from storms called NORTHEASTERNS IN THE WINTER. 3. If you could see AIR, an AIR MASS would appear as a large PILE of AIR over the Earth's SURFACE. This pile of Air would produce a HIGH-PRESSURE AREA CALLED AN ANTICYCLONE. 4. In an Anticyclone, air circulates AWAY from the CENTER in a CLOCKWISE motion in the Northern Hemisphere. An Anticyclone creates areas of FAIR WEATHER. 5. As Air Spirals out from an Anticyclone, it enters AREAS OF LOW PRESSURE, CALLED CYCLONES. Cyclones appear as Depressions or Basins in an Air Mass. The Air Flow TOWARD THE CENTER of a Cyclone in a COUNTERCLOCKWISE Motion. A CYCLONE MAY BE AN AREA OF STORMY WEATHER. WEATHER FRONTS 1. Although Air Masses are modified by the surface over which they move, they tend to KEEP their UNIQUE PROPERTIES FOR A LONG TIME. When TWO Air Masses MEET, they tend NOT TO MIX. Instead, they form a BOUNDARY CALLED A WEATHER FRONT. STORMY WEATHER IS OFTEN ASSOCIATED WITH WEATHER FRONTS. 2. THE MOVEMENT OF ONE AIR MASS INTO A REGION OCCUPIED BY ANOTHER IS OFTEN MARKED BY MAJOR CHANGES OF WEATHER. FOUR TYPES OF FRONTS MAY DEVELOP: A. WARM FRONT. A Warm Front develops when a WARM AIR MASS MEETS A COLD AIR MASS. The Warm Air, because it is LESS DENSE, SLIDES UP OVER A COLD FRONT. The FIRST Sign of this front is the presence of HIGH CIRRUS CLOUDS. A Long period of CLOUDINESS and PRECIPITATION may Follow. STRATUS Clouds form as the Front continues to move, NIMBOSTATUS Clouds may develop and produce RAIN OR SNOW. As the Warm Front passes, the Temperature RISES Gradually and the WINDS Shift to the SOUTH OR WEST. B. COLD FRONT. A Cold Front forms when a Cold Air Mass invades a Warm Air Mass. The Cold air FORCES the Warm Air aloft along a Steep Front. Cumulus and Cumulonimbus Clouds tend to form along the front, producing RAINSHOWERS AND THUNDERSTORMS. COOLER Temperatures follow the passage of a Cold Front. Cold fronts advance Faster in the Winter than in the Summer. The Weather Clears Shortly after a Cold Front Passes. THE PASSAGE OF A COLD FRONT USUALLY BRINGS A SHIFT IN WINDS, A RISE IN AIR PRESSURE, A DECREASE IN RELATIVE HUMIDITY, AND A DECREASE IN TEMPERATURE. C. STATIONARY FRONT. A Stationary Front forms when EITHER a Warm Front or a Cold Front STOPS MOVING FORWARD. This type of Front may remain in the same place for several days. Weather Conditions include SLUGGISH WINDS AND PRECIPITATION ACROSS THE ENTIRE FRONTAL REGION. D. OCCLUDED FRONT. An Occluded Front results when TWO COOL AIR MASSES MERGE, forcing Warmer Air Between them to RISE. HIGH WINDS AND HEAVY PRECIPITATION USUALLY ARE ASSOCIATED WITH AN OCCLUDED FRONT. |