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Atmospheric Moisture

Atmospheric Moisture. Chapter 24.1 and sample problems to help with the worksheet!. Phases of Matter. Solid Lowest energy state Ice: water molecules in rigid crystal structure, atoms very close together Liquid Intermediate energy state

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Atmospheric Moisture

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  1. Atmospheric Moisture Chapter 24.1 and sample problems to help with the worksheet!

  2. Phases of Matter • Solid Lowest energy state • Ice: water molecules in rigid crystal structure, atoms very close together • Liquid Intermediate energy state • Water: water molecules further apart, moving faster, but molecules still bonded together (water flows in a stream when poured) • Gas Highest energy state • Water vapor: water molecules moving MUCH faster, and they are so far apart, water vapor is invisible!

  3. Transitions between phases • In order to move from one phase to another, energy either needs to be added or removed. We call this energy LATENT HEAT. • LATENT means “hidden” • Latent heat ADDED or absorbed to allow water to move to a higher energy state • Latent heat RELEASED to allow water to move to a lower energy state

  4. LATENT HEAT ABSORBED WHEN: • Solid > Liquid: MELTING • Ice at room temperature will melt into liquid water. In doing so, it absorbs heat from surrounding air, which cools the air. • Liquid > Gas: EVAPORATION • If you wet the back of your hand and blow on it, your skin will feel cooled. The liquid water is absorbing latent heat from your skin, becoming energized, and evaporating into the air. • Most water vapor enters atmosphere via evaporation. More evaporation happens closer to the equator (it’s warmer there) • Solid > Gas: SUBLIMATION • Doesn’t happen often, but ice and snow CAN sublimate and go directly to the water vapor phase. • When it starts to snow, sometimes drier air lower in the atmosphere causes the snow to SUBLIMATE before it reaches the ground • that’s what is happening right now, at 8:30 am on Monday, February 28th, as we enjoy our 3rd consecutive snow day, and wait for the first flake of this storm to hit the ground!

  5. LATENT HEAT RELEASED WHEN • GAS > LIQUID: CONDENSATION • This is how dew forms on grass, or bathroom mirror fogs up when you are taking a shower. THIS IS ALSO HOW CLOUDS FORM! Latent heat is given off and warms the air. • LIQUID > SOLID: FREEZING • Water freezes to form ice, and latent heat is released • GAS > SOLID: DEPOSITION • This is what causes frost to form on windows and car windshields in cold weather. Ice crystals form directly on surfaces directly from the air.

  6. Water vapor in the air • HUMIDITY: the amount of water vapor in the air • SATURATION POINT: the maximum amount of water vapor that can be in the air Warm air can have MORE water vapor in it Cooler air can’t have as much water vapor in it…not enough energy available. Remember, HEAT IS ENERGY

  7. RELATIVE HUMIDITY (RH) • RH is a RATIO: How much water vapor is in the air How much water vapor air CAN hold at maximum capacity (saturation point) So, if air has 5 grams/m3 of water vapor, but it can have as much as 20 grams/m3 in the air: RH= 5 g/m3 = ¼ = 25% relative humidity 20 g/m3

  8. A little review on fractions • The number on top is the NUMERATOR • The number on the bottom is the DENOMINATOR • With RH • Numerator = amount of water actually in the air • Denominator = saturation point of the air Remember: Saturation point depends on temp of air higher temp means more water vapor can be in the air lower temp means less water vapor can be in the air.

  9. Reading a Saturation Curve • This is the graph on p 480 in the book, or the link I sent you where temperature is on the X axis and “water in g/kg” is on the Y axis • NOTE: if you are using the saturation curve from the link I sent, the water vapor is NOT in g/m3, as on the sheet, but as g/kg. Just use what units are on the graph, and be sure to include them on your worksheet • ALWAYS specify what your units are!

  10. ALGEBRA TIE-IN • Many of you have been working with DOMAIN and RANGE. Domain is the X variable, and range is the Y variable. • You find that the value of Y changes, depending on what X is. • Each point on the saturation curve is an (x,y) pair of points • Notice this is not a straight line, but a curved line. • If you know X, you can find Y; if you know Y, you can find X!

  11. Reading the Sat Curve • You know the temp of the air and you want to know how much water vapor can be in the air at that temperature (the SATURATION POINT). • Simply locate the temperature on the X axis, and trace that line straight up till you hit the saturation curve. • Now, trace over to the left, and THAT’S how much water can be in the air at that temperature!

  12. TRY IT WITH ME! • If the air is 30 deg C, what’s the saturation point of this air? • ON X AXIS: find 30 deg C. Now, follow this line straight up to where it hits the curve. • Trace over to the Y axis and read off the corresponding value. • TEXT: 30 g/m3 • Online Chart: 28g/kg

  13. When Temp goes down…. • Now see how much water vapor can be in the air at 20 deg C: • Text: about 17 g/m3 • Chart: about 15.5 g/kg How come the answers aren’t the same? Different units are being used (m3 vs. kg) and, the info sources are different. Just please include units in your answer so I know which graph you used.

  14. So, this worksheet: • For numbers 1 & 2, you don’t need any graphs at all. If there are 4 g/m3 water vapor in the air and the saturation point of the air is 16 g/m3, what’s the relative humidity? 4 g/m3 = 25% relative humidity 16 g/m3 For number 2, simply substitute the values given in the denominator (amount of water in air at saturation). What is the new percentage? So, if air above had instead a saturation point of 20 g/m3: 4 g/m3 = 20% relative humidity 20 g/m3

  15. Relative humidity depends on air temperature • See how as saturation point increases, the RELATIVE humidity decreases? • It’s because the number in the denominator is increasing. When that happens, the whole fraction value gets smaller. ½ > 1/3 > ¼ > 1/5 > 1/6 • If the denominator gets SMALLER, the whole fraction increases in value: 1/5 > ¼ > 1/3 …etc. • And since saturation point increases with temperature (warm air holds more water vapor than cold air) relative humidity is dependent on temperature of the air.

  16. #3 • Dew point: see paragraph on p. 483. It’s defined as the point at which dew begins to form on grass…when the air becomes so saturated with moisture it begins condensing on surfaces. • “Saturated” means totally soaked, or full. When the air can’t hold any more water vapor, the vapor HAS to start condensing into liquid droplets. • So…what’s the relative humidity of air which is SATURATED? No more room for any more water vapor? Hmmm? SATURATED AIR…….

  17. #4 • You need to use the graph on p 480 or the saturation curve. Here’s a sample: • There’s 7 g/m3 water vapor in the air, which is at 20 deg C. What’s the RH? • OK, I find 20 deg C, and trace up to the curve. Turns out, about 17g/m3 is the saturation point at that temperature. • ALrighty then, the RH = 7g/m3 = about 41% 17g/m3 For b and c, substitute the saturation point at the new temperatures into the denominator, and recalulate the percentage (you may need a calculator for some of them)

  18. #5 • Weeeelllll, what do you think the relative humidity of the air is when it’s RAINING outside? Hmmm, liquid water, falling from the sky. Guess it would be pretty humid out…hmmmm…well, how humid can it GET? (hint: if you turn in a perfect test paper, what percentage did you get right?) You will use the SATURATION curve graph (p 480 or the link online) to find the answer! Thinkthinkthinkthinkthink…..hmmmmmmmmmm…

  19. #6 • The air has 15 g/m3 of water vapor in it. The thermometer outside your window says the air temperature is 28 degree C. If the amount of water vapor in the air does NOT change, how will the air temperature need to change in order for it to start raining? How do you know? • OK, you will need the saturation curve for this one again. Find 28 degrees C, trace up to find where the curve crosses that line, and read across to find out that the saturation point is….oh, about 28 g/m3. • At what point will it start raining? Oh, I’d say when the atmosphere is saturated with water vapor. So at what point would that be? Well…at what temperature would the air be saturated, with 15 g/m3 in it? Look on the Y axis, find 15 g/m3, follow across to the saturation curve, now THIS time trace DOWN to find the corresponding temperature! • That’s how cold it will have to get, for the air to be saturated if there is 15 g/m3 of water vapor in it…about 18 degrees C for the text graph, closer to 19.5 deg C for the online saturation curve.

  20. #7 Still using the same graph! • Just look for the temperature on the X axis, trace up to cross the saturation curve, and read across to find the corresponding Y value.

  21. #8 OK, now we use p. 481 THIS IS A SAMPLE PROBLEM TO SHOW YOU HOW TO DO 8A You got your very own sling psychrometer for your birthday from Aunt Sue! Your very first measurements are: dry bulb temp = 24 degrees C, wet bulb temp = 20 degrees C. Ok, if you don’t have your book, this is where that other psychrometer chart I sent the link to works; you can go to this link for a quick, illustrated description of this instrument http://www.usatoday.com/weather/wsling.htm How you read the tables: the dry bulb temp is the air temperature, plain and simple. It’s found on the left-hand side of the text chart or across the top of the online version. You have to determine the DIFFERENCE between wet and dry bulb readings. The difference runs along the top of the chart in the book, down the left side for the online version. Whichever you are using, find 24 deg C in the dry bulb temps, and then find the column/row in the “difference dry-wet bulb temp” which says “4” because 24-20 = 4…where the column/row meets, you get your answer: 69% is the relative humidity.

  22. On to 8b • OK. You know the dry bulb temp = air temp, which is 24 deg C, and you know the RH = 69%. • What can the graph on 480 tell you? (saturation curve for you online people) It can tell you how much water vapor is in the air at saturation, or 100% relative humidity, for a given temperature. • What’s the saturation point at 24 deg C? About 22g/m3? • 100% is to 69% as 22g/m3 is to ?g/m3, or .69 = x Now just find x. (use “cross multiplication”) 1.00 22g/m3 like I did below: (.69)(22 g/m3) = (1.00)(x) or, 15.18 g/m3 = x. Since we had to estimate using the graph to get the 22 g/m3, how about we just round the 15.18 down to 15 g/m3?

  23. 8C How would the relative humidity of the air change if the air temperature were to drop to 20 degrees C? Well, since the starting temp in MY sample problem is 24 deg C, and I know that cooler air has a lower saturation point than warmer air…what would the RH do, increase or decrease? Remember, this is like a fraction where the denominator is getting smaller and the numerator is staying the same…what happens to the value of the fraction?

  24. #9 • Your wet bulb temperature reads 21 degrees C and the relative humidity of the air you are measuring is 77%. • What’s the dry bulb temp? • OK. This is one for the psychrometer chart. What info do we have? RH and wet bulb temp. • Look in the table for “RH = 77” and you will find two places it occurs…which one is it?

  25. Two possible answers, let’s reason it out • The first possibility is that the dry bulb reading is 10 deg C, and the wet bulb is 2 deg less than that. • The second possibility is that the dry bulb is 24 deg C, and the wet bulb is 3 degrees less than that. • OK, we know that the wet bulb reading is 21 degrees. So that eliminates the first possibility…because 10 (dry bulb)-2 (wet bulb) = 7 • The second works, though…24-3=21. • So answer is “24 degrees C” • Now…you try it for the numbers in 9A.

  26. On to 9B Approximately what amount of water vapor, in g/m3, is in this air? OK…again, what do you know? The RH is 77% and you just discovered the air temp is 24 deg C. Go back to the saturation curve. For 24 deg C, when RH is 100% (totally saturated), there is about 23 g/m3 water vapor. So, 77% of 23 g/m3 is…17.71 g/m3. Since we can only approximate the 23 from the graph, shall we round off the 17.71 and call it 18 g/m3? This keeps our “significant figures” in order.

  27. 9C The air temperature increases by 5 degrees. What is the approximate relative humidity now? Well, my answer to 9B tells me there is 18 g/m3 of water vapor in the air. The air temp is 24 deg. C. If the air temp increases to 29 deg C, using the saturation curve: find 29 on the temp axis and trace up to see that at saturation, 29 g/m3 water vapor is in the air. SO, 18g/m3 = 62% is the relative humidity of the air at 29g/m3 29 degrees C.

  28. 9D • What if the temp decreases by 5 degrees, instead of increasing as in 9C? OK, again, we start with 18 g/m3 water vapor and an air temp of 24 deg C. Now we DECREASE the temp to 19 deg C, using the saturation curve, how much water vapor can be in the air at that temp? Looks like about 17, maybe 18 g/m3. What’s the relative humidity? 18 g/m3 = 100% RH…the air is completely saturated, so 18 g/m3 condensation is going to start taking place.

  29. So how come… • Sometimes you watch the weather and it’s snowing out, but the relative humidity is less than 100%. How come? • Remember RH can vary with altitude. At higher altitudes, the temp is colder, so air may be saturated and snow develops, and starts to fall. • But just like this morning, the snow is falling from the clouds but it’s not hitting the ground. Where’s it going? Right now the air closer to the ground is warmer and drier…enough so that as the flakes fall, they SUBLIMATE and go back into the water vapor state before they ever reach the ground • Remember, latent heat is absorbed to allow the water to go from a solid (snowflake) back to water vapor…so this is why the air temperature will fall when it starts to snow!

  30. Watch the weather today! • Right now, at 9:20 am 2/28/05, the Weather Channel says the air temp in Baltimore is 33 degrees F, and the relative humidity of the air is 66%. • The snow is falling from the clouds but sublimating before it hits the ground. • Watch throughout the day…I hypothesize that, as snow sublimates on the way down, this will cool the air temperatures AND relative humidity will increase…in part because the air is cooler but also due to the sublimation of the snowflakes, which adds more water vapor to the air. • Finally, the RH will rise enough so that the snowflakes will stop sublimating, and they will make it all the way to the ground.

  31. Something to try • Http://scifun.chem.wisc.edu/WOP/HomeMeteor.html • This is a simple experiment you can do at home to determine the dew point, or saturation point, of the air in your house. Try it in a steamy bathroom, then try it other places in the house. • Remember, even if you don’t have your book, go to www.hrw.com, the text website. Follow their links to SciLinks and create a profile for yourself to go onto the site. SciLinks has a lot of neat information (including the experiment above) and it’s a great way to access additional info about the material we are studying, if you aren’t the sort who likes to ask for extra help from a teacher.

  32. I see snowflakes! • It’s 10:06 am on 2/28/05, about 35 minutes since I last checked the Weather Channel. • Air temp still 33 deg C • RH has risen from 66% to 72%...all those sublimated snowflakes have increased the amount of water vapor in the air! • The first flakes are floating down past the window. • They’ve downgraded snow amounts to 5-9 inches. Maybe we will have school tomorrow. (But, I personally doubt it) • Stay safe and warm today!

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