Transcript 637Lesson02
METO 637 Lesson 2 Forces that Move the Air • Gravitational force • Is directed downward , normal to the ground • Approximately equal to the mass of the air times the gravitational acceleration • Pressure gradient force Fig. 6.4 PRESSURE- GRADIENT FORCE • .IS A FUNCTION OF THE PRESSURE DIFFERENCE AND AIR DENSITY. • . FPC = ΔP / D.σ • WHERE • FPC = PRESSURE GRADIENT FORCE PER UNIT MASS • σ = DENSITY OF AIR • ΔP = PRESSURE DIFFERENCE BETWEEN TWO POINTS • D = DISTANCE BETWEEN THE TWO POINTS • .NEWTON’S LAW STATES FORCE=MASS TIMES ACCELERATION • .FPC IS A FORCE PER UNIT MASS, I.E. IS EQUIVALENT TO ACCELERATION CORIOLIS EFFECT • •WEATHER MAP SHOWS THAT WINDS GO AROUND A LOW PRESSURE AREA, NOT ACROSS THE ISOBARS. • THIS DEVIATION IS A RESULT OF THE EARTH'S ROTATION AND HAS BEEN NAMED THE CORIOLIS EFFECT (CORIOLIS FORCE) • THIS FORCE ALWAYS ACTS AT RIGHT ANGLES TO THE DIRECTION OF MOTION. Coriolis Effect The Mechanism for Geostrophic Flow GEOSTROPHIC FLOW • .EVENTUALLY A BALANCE IS REACHED BETWEEN THE CORIOLIS FORCE AND THE PRESSURE GRADIENT FORCE • .IF THERE IS NO FRICTION THEN THIS OCCURS WHEN THE PARCEL OF AIR IS PARALLEL TO THE ISOBARS. • .AT THIS POINT THERE IS NO NET FORCE ON THE AIR PARCEL - NO ACCELERATION - IT NOW MOVES WITH CONSTANT VELOCITY. • .UNDER THESE IDEALIZED CONDITIONS THE AIRFLOW IS SAID TO BE IN GEOSTROPHIC BALANCE. • .WIND GENERATED IS CALLED THE GEOSTROPHIC WIND • .NORMALLY ONLY APPLIES TO WINDS ALOFT. • .SURFACE WINDS ARE SUBJECT TO FRICTION. Gradient Winds Fig. 6.15 Gradient Winds – including Centrifugal Force Fig. 6.16 GRADIENT WINDS • ·WINDS AROUND CENTERS OF HIGH OR LOW PRESSURE FOLLOW CURVED PATHS IN ORDER TO STAY PARALLEL WITH THE ISOBARS. • ·THESE WINDS ARE CALLED GRADIENT WINDS. • ·LOW PRESSURE CENTERS ARE CALLED CYCLONES - ROTATION IS COUNTERCLOCKWISE - SAME AS THE EARTH • ·CENTERS OF HIGH PRESSURE ARE CALLED ANTI-CYCLONES. • ·IN SOUTHERN HEMISPHERE THE DIRECTIONS OF FLOW ARE REVERSED Effect of Friction at Surface on Geostrophic Balance Fig. 6.18 Gradient winds at the surface Fig. 6.24 SURFACE WINDS • .FRICTION AFFECTS WINDS ONLY CLOSE TO THE EARTH'S SURFACE. • NOW WE MUST BALANCE THREE FORCES - CORIOLIS, PRESSURE GRADIENT AND FRICTION. • .NET EFFECT IS TO INDUCE A NET INFLOW AROUND A CYCLONE, AN EFFECT KNOWN AS CONVERGENCE. • AROUND AN ANTICYCLONE WE GET A NET OUTFLOW, DIVERGENCE Electromagnetic spectrum SOLAR ENERGY • Solar constant – energy from the sun that falls on unit surface normal to the line from the sun, per unit time, at the outside of the atmosphere at the mean solar distance S = 1.368 Kw per meter 2 • S is integrated over all wavelengths • S varies with the sunspot cycle UV-VISIBLE SOLAR SPECTRUM Absorption of Solar Radiation in the Earth’s Atmosphere