Patterns of pressure and wind: from the horizontal to the vertical Mt. Washington Observatory ATMOSPHERIC PRESSURE •Weight of the air above a given area = Force (N) •p = F/A (N/m2 = Pascal, the SI unit of pressure) •Typically Measured in mb (1 mb=100 Pa) •Average Atmospheric Pressure at 0m (Sea Level) is 1013mb (29.91” Hg) •A map of sea level pressure requires adjustment of measured station pressure for sites above sea level Your book PRESSURE 100 km (Barometer measures 1013mb) PRESSURE 500mb pressure 1013mb pressure As you go up in altitude, there’s less weight (less molecules) above you… …as a result, pressure decreases with height! Pressure vs. height p = p0Exp(-z/8100) Pressure decreases exponentially with height This equation plots the red line of pressure vs. altitude Barometers: Mercury, springs, electronics can all measure atmospheric pressure precisely CAUSES OF PRESSURE VARIATIONS Universal Gas Law PV = nRT Rearrangement of this to find density (kg/m3): ρ=nM/V=MP/RT (where M=.029 kg/mol for air) Then P= ρRT/M •Number of Moles, Volume, Density, and Temperature affect the pressure of the air above us •If temperature increases and nothing else changes, either pressure or volume must increase •At high altitude, cold air results in lower pressure and vice- versa Temperature, height, and pressure • Temperatures differences are the main cause of different heights for a given pressure level SEA LEVEL PRESSURE VARIATIONS - RECORDS Lowest pressure ever recorded in the Atlantic Basin: 882 mb Hurricane Wilma, October 2005 Highest pressure ever recorded in the world: 1086 mb Mongolia, December 2001 Pressure Differences cause Air to move PRESSURE GRADIENT FORCE EARTH’S ROTATION and Coriolis Force •Coriolis force results from Earth’s rotation •Causes rightward deflection in NH, leftward in SH •It depends on latitude and speed of movement •It only is important when it can act for a long time -5 •Fcoriolis=2vΩsinφ where Ω=7.3x10 Can you identify the equator? Why have there been no hurricanes there? ROTATION OF THE EARTH ROTATION OF THE EARTH ROTATION OF THE EARTH ROTATION OF THE EARTH ROTATION OF THE EARTH CORIOLIS EFFECT CORIOLIS EFFECT Geostrophic balance • The balance develops gradually. As PGF increases velocity, coriolis force gets stronger until they reach equal strength and wind is parallel to isobars What other force affects winds near surface? How will this new force affect wind direction? Jetstream in cross-section • The jetstream occurs where the strongest pressure gradient exists • This pressure gradient is caused by the overall temperature difference SUMMARY UP TO THIS POINT 1) Differences in atmospheric pressure across the globe lead to winds across the surface of the planet 2) The PRESSURE GRADIENT FORCE pushes air from high to low pressure (WIND) 3) The BIGGER the difference between high and low pressure the STRONGER the wind 3) As air moves away from high pressure and towards low pressure, the CORIOLIS EFFECT deflects it to its right (NH) 4) The above leads to air circulating COUNTER-CLOCKWISE around LOW PRESSURE and CLOCKWISE around HIGH PRESSURE (NH) 5) South or North winds will result in higher or lower temps WHAT ABOUT THE CLOUDS? Remember that air converges towards low pressure and air diverges away from high pressure and FRICTION due to the surface of Earth “enhances” this WHAT ABOUT THE CLOUDS? So, as the air moves towards the low pressure it has nowhere to go but UPWARDS WHAT ABOUT THE CLOUDS? So, as the air moves away from high pressure it must be replaced by air from above that is moving DOWNWARDS Covergence/Divergence is different at high altitude •High altitude divergence: air diverges and pulls up air from below. diverging, rising air creates colder air and lower pressure and plentiful cloudiness •High altitude convergence: air piles up and is forced to sink. Piling, sinking air creates warmer air and higher pressure and clear skies What pressure level is this? Where are the highs and lows? Where are the strongest winds? What pressure level is this? Where are the highs and lows? Where are the strongest winds? Low pressure is clearly associated with clouds and precipitation. High pressure is associated with clear skies Fronts are located in “troughs” of low pressure •How can you identify a trough or ridge? •Troughs are relative minima in pressure: They look like troughs, sagging downward •Ridges are relative maxima in pressure: They look like ridges, bulging upward Identifying troughs and ridges: pressure or height? Different methods to identify a front on a map.