Atmospheric Properties As atmospheric scientists what properties of the atmosphere do we care about? How do these properties vary in space and time? A key goal of this class, and other ATOC core classes, is to explain why these properties vary in this manner. TemperatureHorizontal Variations in Temperature Surface Air Temperature - January Surface Air Temperature - JulyAtmospheric Pressure Horizontal and vertical variations in pressure give rise to the atmospheric motions, which are one focus of ATOC 5050. What is atmospheric pressure? A molecular perspective… SI Units for pressure Pressure is a force per unit area exerted by a fluid From Newton’s second law: F=ma Force has units of kg m s-2 (or Netwons – N) Force per unit area has units of: kg m-1 s-2 = N m-2 (or Pascals – Pa) Other commonly used units of pressure: 1 hPa = 100 Pa 1 millibar (mb) = 1 hPaIn the atmosphere the observed pressure is also equal to the weight of air per unit area above the observation point. At the surface of the earth the pressure, ps, is given by: € ps=ρgdz0∞∫ Neglecting variations in g this becomes: € ps= g0ρdz0∞∫= mg0 where € m =ρdz0∞∫ is the mass per unit area of the overlying air and g0 = 9.807 m s-2 Therefore, pressure decreases with increasing altitude in the atmosphere.Surface pressure – the actual air pressure measured at the surface of the earth What is the rate of decrease of pressure in the lower atmosphere? Example: Estimate the surface pressure in Boulder, CO. Sea-level pressure (SLP) – surface pressure interpolated to zero elevation (sea level) Example: Comparison of weather maps of surface and sea level pressure Why do meteorologists use SLP instead of surface pressure?Horizontal Variations in Pressure Sea Level Pressure - January Sea Level Pressure - JulyWind Using the following coordinate system: the horizontal wind can be given as the vector: €  V ≡ u i + v j where: €  i is a unit vector directed towards the east €  j is a unit vector directed towards the north u is the east/west or zonal component of the wind v is the north/south or meridional component of the wind By convention meteorologists indicate the direction of the horizontal wind vector as the direction the wind is coming from. What sign would the horizontal wind components have for: An east wind? A south wind? A northwest wind?Horizontal Variations in Wind \What is the relationship between surface winds and SLP? Cyclonic – winds rotating in the same sense as the Earth’s rotation Anticyclonic – wind rotating in the opposite sense as the Earth’s rotationVertical Variations in Temperature Standard Atmosphere What features of the standard atmosphere temperature profile are notable? Lapse rate – rate of decrease of temperature with increasing height € Γ ≡ −∂T∂z What is the lapse rate in the troposphere? What is the lapse rate in the stratosphere? Inversion – a layer in which temperature increases with height What is the significance of a temperature inversion?Atmospheric layers: • Troposphere Tropo – turning or changing • Stratosphere Strato - layered Why does the temperature decrease with height in the troposphere? Why does the temperature increase with height in the stratosphere? Tropopause - boundary between troposphere and stratosphere Tropo – turning or changing Pause – limit of The height of the tropopause varies in both space and time. What impact does the height of the tropopause have on weather?Zonally Averaged Temperature Cross-Section - January Zonally Averaged Temperature Cross-Section - JulyVertical Variations in Wind Zonally Averaged Zonal Wind - January Zonally Averaged Zonal Wind - July Jetstream - narrow band of strong windsThe Jetstream 300mb Wind Speed - January 300mb Wind Speed - JulyHow does the position of the jetstream change between winter and summer? How does the speed of the jetstream change between winter and summer? What is the relationship between the strength of the jetstream and the horizontal temperature gradient? Surface Weather MapsDew point temperature – the temperature to which a small volume of air must be cooled at constant pressure in order for the air to become saturated Dew point temperature indicates the amount of moisture contained in the air. What units are used for temperature and dew point temperature reported on surface station models? Significant weather Cloud cover Wind direction – the direction the wind is coming from Wind speed Units for wind speed SI: m s-1 Meteorological convention: Knots (kts) 1 kt = 0.51 m s-1 Station model wind speed plotting convention: - Wind speed symbols shown in figure are additive - Actual wind speed is within ±2 kts of plotted wind speedSea level pressure Decoding sea level pressure reported on station models If coded SLP is greater than 500: Put a 9 in front of the 3 digit coded SLP Insert a decimal point between the last two digits Add units of mb Example: coded SLP = 956 Decoded SLP = 995.6 mb If coded SLP is less than 500: Put a 10 in front of the 3 digit coded SLP Insert a decimal point between the last two digits Add units of mb Example: coded SLP = 052 Decoded SLP = 1005.2 mb The normal range of sea level pressure is 950 to 1050 mb. Record high sea level pressure: 1086 mb (Tosontsengel, Mongolia) Record low sea level pressure: 870 mb (Typhoon Tip, western Pacific)Time and weather observations UTC – Universal time coordinate GMT – Greenwich Mean Time Z – Zulu time Quick facts about Universal Time Coordinate (UTC) UTC is based on a 24 hour clock (so add 12 to any times after 12:59PM) 6AM UTC would be written as 06 UTC 12 noon UTC would be written as 12UTC 6PM UTC would be written as 18UTC If UTC time is given as both hours and minutes it looks like this: 2:15AM UTC would be written as 0215 UTC 12:00 noon UTC would be written as 1200 UTC 10:20PM UTC would be written as 2220 UTC UTC never switches from standard time to daylight savings time This means we need to change how we convert between UTC and Mountain time depending on whether we are on standard time or daylight savings time How do I convert from UTC to MST or MDT? MST = UTC - 7 hours MDT = UTC - 6 hours How do I convert from MST or MDT to UTC? UTC = MST + 7 hours UTC = MDT + 6