– the study of weather Here are some variables that are a part of predicting the weather: 1. Air Temperature – () 2. Air Pressure – the force exerted by the air overhead () 3. Humidity – the amount of moisture in the air () 4. Speed – () 5. Wind Direction – (wind vane or weather vane) 6. Atmospheric Transparency- how clear the air is (ceiling) 7. Visibility – how far ahead you can see (in miles) 8. Wind Chill – how cold it feels (combo of temp. and wind) 9. Dew Point Temperature – the temp. air needs to cool to, in order for condensation to happen (cloud forms)

All of these variables interact in the troposphere.

1 People cannot accurately predict weather after 36 hours, b/c there are too many weather variables working and changing at the same time. The atmosphere gains energy in four ways: 1. Direct absorption of Insolation from the Sun- Only 19% of the atmosphere’s heat comes from direct absorption. 2. Change of Phase reactions- Condensation and Sublimation release energy to the atmosphere. 3. Terrestrial Radiation – the Earth releases heat (Infrared) as it cools off at night. 4. The Greenhouse Effect – Carbon Dioxide and Water Vapor in our atmosphere trap heat that is trying to escape into space. All of the energy that is gained by the atmosphere is distributed throughout the rest of the atmosphere by Convection Cells or Convection Currents.

2 Adiabatic Temperature Change – a change in temperature of the air due to it expanding and contracting, not being cooled or heated. As air rises, it expands (spreads out), it’s volume increases, and it’s temperature decreases after a while of being away from the warmer surface.

As air sinks (falls), it contracts (compresses), it’s volume decreases, and it’s temperature increases after a while of being back on the surface of Earth where it is warmer.

Memorize this!!!!!!!!

3 4 Air (Barometric)Pressure The force exerted by the column of air overhead Air pressure is influenced by: 1. Air temperature 2. Elevation above sea level 3. How much moisture is in the air.

1. As air temperature increases, air density decreases, so air pressure also decreases. Hot air has a lower air pressure.

As air temperature decreases, air density increases, so air pressure also increases. Cold air has a higher air pressure.

5 2. As elevation increases (you get higher above sea level), air pressure decreases, because there is less air over your head. As elevation decreases (you get closer to sea level) air pressure increases, because there is more air over your head.

3. Dry air is made of mostly Nitrogen. As air becomes moister, Water Vapor replaces the Nitrogen. Water vapor is less dense than Nitrogen. So, moist air has a lower air pressure than dry air. Moist air is better and easier to breath.

6 We need to learn how to code and un-code air pressure values in order to use and create weather maps. Meteorologists do not put the real air pressure values on the weather maps because the numbers are too long and take up too much space on the map. They use a abbreviation system. How to read the air pressure numbers on a map: 1. If the air pressure (in millibars) is below 500, put a 10 in front of the number and a decimal between the last two digits. This number is the real air pressure. 2. If the air pressure number is above 500, put a 9 in front of it and a decimal between the last two digits. How to put air pressure numbers onto a weather map: 1. Remove the 9 or 10 and the decimal in front of the the air pressure number. 2. Place that number of the weather map in the correct spot.

7 Wind – the horizontal (parallel) movement of air. Facts about wind: 1. are caused by differences in air pressure between two locations. This difference in air pressure is caused by the differences in temperature between two locations. So, in other words, winds are caused by the unequal heating of the Earth. The larger the difference in pressure, the stronger the wind. 2. Winds blow from areas of High Pressure to Low Pressure. 3. Winds blow from areas of Divergence to Convergence. 4. Winds are named after the direction that they are blowing from, not where they are blowing to. 5. Surface ocean currents (not tides) are caused by wind direction. 1. Wind direction is also influenced by the Coriolis Effect (Rotation). Winds curve to their right in the Northern Hemisphere, and to their left in the Southern Hemisphere.

8 Landbreezes and Seabreezes During the day, the land heats up faster than the water. The air over the land has a lower density than the air over the water. Because of this, the air over the land rises. There is now an empty spot over the land, so the air over the water fills it in. That is why the breeze always comes off of the water during the day. This is a seabreeze.

At night, the land also cools off quicker than the water. This causes the water to have a higher temperature than the land. So, the air over the water now has a lower density and it rises. Now the air over the land must fill the empty spot. This is why the breeze blows from the land to the water at night (it comes from behind). This is a landbreeze.

9 10 . The ideal gas law can be applied to the combination of atmospheric gases or to individual gases.

. The value of gas constant for the particular gas under consideration depends on its molecular weight: . Rgas = R* / Mgas where R*=universal gas constant=8314.3Jdeg-1 kg-1

. The gas constant for dry atmospheric air is: Rair = R* / Mair = 8314.3/28.97 = 287 J deg-1 kg-1 (Mair ≅ 0.80*MN2 + 0.20*MO2 = 0.80*28 + 0.2*32 = 28.8)

. The gas constant for water vapor is: Rvapor = R* / Mvapor = 8314.3/18.016= 461 J deg-1 kg-1

11 12 13 14 15 16 17 Hydrostatic Balance and Atmospheric Vertical Structure

 Since P= ρRT (the ideal gas law), the hydrostatic equation becomes: dP = -P/RT x gdz, dP/P = -g/RT x dz P = Ps exp(-gz/RT), P = Ps exp(-z/H) The atmospheric pressure decreases exponentially with height 18 A Mathematic Formula of Scale Height  The heavier the gas molecules weight (m) ~ the smaller the scale height for that particular gas  The higher the temperature (T) ~ the more energetic the air molecules ~ the larger the scale height  The larger the gravity (g) ~ air molecules are closer to the surface ~ the smaller the scale height  H has a value of about 10km for the mixture of gases in the atmosphere, but H has different values for individual gases.

19 20 21 22 23 24 25 26 27 28 29  Hydrostatic balance tells us that the pressure decrease with height is determined by the temperature inside the vertical column.  Pressure decreases faster in the cold-air column and slower in the warm-air column.  Pressure drops more rapidly with height at high latitudes and lowers the height of the pressure surface.

What Is Air Temperature?

. Air temperature is a measurement of the average internal kinetic energy of air molecules.

. Increase in internal kinetic energy in the form of molecular motions are manifested as increases in the temperature of the body.

30 What Happens to the Temperature?

 Air molecules in the parcel (or the balloon) have to use their kinetic energy to expand the parcel/balloon.

 Therefore, the molecules lost energy and slow down their motions

 The temperature of the air parcel (or balloon) decreases with elevation. The lost energy is used to increase the potential energy of air molecular.

 Similarly when the air parcel descends, the potential energy of air molecular is converted back to kinetic energy.

 Air temperature rises.

31 32 33 34 35 36 37 38 39 40 41 42 • Introduction In Chapter 1, we learned that water vapor is a variable gas, occupying only a small percentage of the volume of the gases in the atmosphere. Although water vapor is around us in only small quantities, it has major consequences, not the least of which include icing, thunderstorms, freezing rain, downbursts, whiteouts, frost, and lightning (Lester, 2006). In this chapter, we look at the basics of atmospheric moisture, a term which is used here to imply the presence of H2O in any one or all of its states: water vapor, water, or ice. We examine the transformation between states and the importance of air temperature in the transformation process. When you complete this chapter, you will understand the causes and effects of state changes, how clouds form and dissipate, and how precipitation is produced (Lester, 2006). 43 Moisture Characteristics Moisture Characteristics 1. State Changes State Changes 2. Vapor Pressure Water vapor – is a colorless, 3. Relative Humidity odorless, tastless gas in which 4. Dewpoint the molecules are free to move Temperature about, as in any gas Clouds . Cloud Formation Water – in the liquid state I. Water Vapor (water), molecules are restricted II. Condensation Nuclei in their movements in III. Cooling IV. Latent Heat and Stability comparison to water vapor at the . Cloud and Visibility same temperature Observations I. Standard Cloud Ice – as a solid ice, the Observations II. Visibility molecular structure is even more III. Cloud Type rigid and the freedom of IV. Other Useful Cloud movement is greatly restricted Observations 44  The processes by which water vapor is added to unsaturated air are evaporation and sublimation Change of state – refers to the transition from one form of H2O to another  Melting – ice to water  Evaporation – water to vapor  Sublimation – ice directly to vapor without water as an intermediate state  Condensation – vapor to water  Freezing – water to ice  Deposition – vapor directly to ice without water as an intermediate state  Latent heat – the amount of heat energy that is absorbed or released when H2O changes from one state to another

 Sensible heat – heat that can be felt and measured when45 the molecules pass to lower energy states Vapor Pressure Boiling – occurs when SVP  Partial pressure – in the equals the total air pressure mixture of atmospheric gases, each individual gas Relative Humidity – it is often exerts a partial pressure useful to determine how close the  Vapor pressure (VP) – the atmosphere is to saturation. This partial pressure exerted by information can help you anticipate water vapor (H2O in the formation of clouds or fog. This gaseous form) is done by measuring the amount  Saturation – occurs when of water vapor in the atmosphere the same amount of in terms of actual VP and then molecules are leaving a estimating SVP from a temperature water surface as are measurement. The degree of returning saturation is then computed by  Saturation vapor taking the ratio of VP and SVP and pressure (SVP) – the multiplying it by 100. The result is vapor pressure exerted by called relative humidity, it the molecules of water vapor expresses the amount of water in this equilibrium condition vapor actually in the air as a  The amount of water vapor percentage of the amount needed to saturate the air largely depends on air required for saturation 46 temperature RH (%) = (VP/SVP) x 100 47 48 • Dewpoint – the temperature at which condensation first occurs when air is cooled at a constant pressure without adding or removing water vapor • dew point temperature is always less than the air temperature, with one exception • when the air is saturated • (RH = 100%), the temperature and dew point are equal • Dew point refers to the temperature to which air must be cooled to become saturated

49 • Temperature-Dew point Spread –

• a very useful quantity that relates RH and dew point

– also called the dew point depression

– it is the difference between the air temperature and dew point

– when the temperature-dew point spread is small, the RH is high

– when the spread is very large, the RH is low.

50 Moisture Variables List of Variables

• Mixing ratio (w)

• Saturation mixing ratio (ws) • Specific Humidity (q)

• Vapor pressure (ev)

• Saturation vapor pressure (es) • Relative humidity (RH)

• Dewpoint (Td) Td • Dewpoint Depression

• Virtual Temperature (Tv)

• Wet-Bulb Temperature (Tw)  Mixing Ratio (w or rv)

• Mixing ratio (rv) – a way to tell how much vapor there is relative to a mass of dry air • It is conserved as long as there is no condensation or evaporation. R • Units : g kg-1   d  0.622 Rv ev  R T e R e r  v  v  v d  v v p d d pd Rv p  ev

Rd T 

 Specific Humidity (q)

• Mass of water vapor per unit mass of moist air v v rv ev ev qv qv      rv   d  v 1 rv p  ev (1) p 1 qv

• But mass of water vapor is very small compare to  the total mass (~1-2% of the total mass) e e • v v rv   p  ev p • , (ev<< p) e r  q  v v p 

 Vapor Pressure (ev)

• ev – partial pressure of vapor in (Pa)

• es – saturation vapor pressure over plane surface of pure water Saturation Vapor Pressure (es)

• Vapor pressure ev - most directly determines whether water vapor is saturated or not.

• ev < es(T) subsaturated, evaporation

• ev = es(T) saturated

• ev > es(T) supersaturated, condensation

• es only depends on temperature.

• es(T) increases with increasing temperature. Relative Humidity e RH  v es(T)

• Subsaturated: RH < 100% • Saturated: RH = 100%  S  RH 100% • Supersaturated: RH > 100%

• Depends on both vapor pressure ev and the air temperature T  Dewpoint (Td)

• Consider the case ev < es(T), we could always reduce es(T) to ev by lowering the temperature. • Dewpoint is the temperature at which moist air became saturated over a plane surface of pure

water by cooling while holding ev constant.

• Only depends on the vapor pressure ev.

es(Td )  ev

 Saturation Mixing Ratio (ws)

• It is the mixing ratio for which air is saturated at specific T and P.

es(T) es(T) ws(T, p)   p  es(T) p

 Saturation Mixing Ratio (ws)

es(T) Rd s(T, p)     0.622 p Rv

• Depend on both temperature and pressure • In units of (g kg-1)   • If we choose P to be 622 hPa, then 0.622e (T)  (T,622hPa)  s  0.001e (T) s 622 s g  s(T,622hPa)  es(T)hPa kg 

 Virtual Temperature (Tv) • To apply ideal gas law to mixture of air and vapor • Moist air equation of state : p  RmT p  p  e  ( R   R )T d v d d v v  M  1 d r  R   R  v   R ( d d v v )T  Mv  d Rm  Rd Rd  1 rv  M   1 d r  M v  v   M   Rd T d 1 r 1 rv   v  M   T  v T v 1 r p  Rd Tv v  Tv  T(1 0.61rv )  T(1 0.61q)

   Critical Levels on Thermodynamic Diagram • The level at which a parcel lifted dry adiabatically will become saturated. • Find the temperature and dewpoint of the parcel (at the same level, typically the surface). Follow the mixing ratio up from the dewpoint, and follow the dry adiabat from the temperature, where they intersect is the LCL. 64 65 66 67 68 69 70 71 72 73 74 Dew – a condensation product that Clouds – forms when the ground or other object (such as the wings of a suspension of water droplets parked airplane) loses heat energy and / or ice crystals in the through night time (nocturnal) atmosphere radiation Cloud Formation – White dew – if the temperature falls below 32 degrees F (0 degrees three requirements for cloud C) after dew is present, it will freeze formation are water vapor, condensation nuclei and Frost --forms when the temperature cooling of the collecting surface is at or below the dew point of the adjacent Water Vapor – air and the dew point is below freezing. Frost is considered cloud development requires hazardous to flight because it spoils adequate water vapor cloud the smooth flow of air over the formation favored in air with wings, thereby decreasing lifting high RH (small temperature 75 capability dew point spread) . Condensation Nuclei . conditions favorable for the formation of Radiation Fog Condensation Nuclei are over a land surface are clear microscopic particles such as skies, little or no wind, and a dust and salt that provide small temperature-dew point spread. surfaces on which water vapor . if the fog is very shallow undergoes condensation to (less than 20 feet deep). it is form water droplets or called Ground Fog. deposition to form ice crystals. . Valley Fog forms when radiational cooling causes cool dense air to pool in a . fog is usually more prevalent valley. River valleys with in industrial areas because of ample supplied of moisture an abundance of are favorable locations for this type of fog. condensation nuclei from . The types of fog that combustion depend on wind in order to exist are advection fog and upslope fog. . Cooling -contact cooling, advection fog often forms in . steam fog is common over coastal areas unfrozen water bodies76 in the cold months of the year Cloud and Visibility Observations -learning to observe clouds and visibility is essential for proper interpretation of METAR and for flight safety. Standard Cloud Observations – sky condition, cloud height, cloud amount, cloud type Cloud Type – Low:-< 6500 (2 km) feet I. (ST) Stratus II. (SC) Stratocumulus III. (NS) Nimbostratus Middle:-6,500 to 20,000 ft (2 km to 6 km) I. (AC) Altocumulus II. (AS) Altostratus, High:-> 20,000 ft (6 km) I. (CC) Cirrocumulus II. (CS) Cirrostratus III. (CI) Cirrus Clouds with vertical development (2 to 12 km) I. (CU) Cumulus 77 II. (CB) Cumulonimbus 78 79 80 Precipitation Precipitation Causes 1. collision / coalescence – large droplets fall faster than smaller particles capturing them as they descend. ice crystal process. 1. condensation / deposition 2. super cooled water droplets are primary cause of aircraft icing. ice crystals grow at the expense of super cooled water droplets in the ice crystal process

81 82 Precipitation Characteristics Types . Drizzle . Rain . rain showers . freezing drizzle . freezing rain . black ice . the presence of ice pellets at the surface is evidence that there may be freezing rain at a higher altitude: ice pellets . Snow . snow showers . Virga . fall streaks

83 84 85 86 87 Wind and Pressure Belts

Areas of Convergence – where the winds collide (0°, 60°N and 60 °S). These latitudes are very wet and have a lower air pressure. Areas of Divergence – where the winds blow away from each other. (30°N, 30°S, 90°N, 90°S) These latitudes are dry (deserts of the world) and have a much higher air pressure. 88 Prevailing Winds – zones on the Earth’s surface where winds tend to blow in the same direction over long periods of time. The prevailing winds in NYS are called the southwesterlies (they blow from the SW). See page 14 ESRT’s.

Jet Streams – a narrow zone of very strong winds in the upper atmosphere (troposphere). The Jet Streams steer weather systems across the world. The Jet Stream in the USA steers weather from West to East. The Jet Stream tends to migrate north in the summer and south in the winter.

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