Transcript Chapter08a
Air Pressure and Winds I Review: precipitation types Sample weather map (Fig. 13.11) Fig. 11.18 Snow Drizzle Sleet Freezing rain Fog Atmospheric pressure P Atmospheric pressure and density decrease with altitude exponentially!!! force weight of the air P area area Units: 1 bar=1000 mbar 1 Standard atmosphere: 1013 mbar Ideal Gas Law • temperature, and the density of an ideal gas. A relationship between the pressure, the • Ideal gas: a simplified physical model for a gas. It neglects: ♦ the volume of the individual molecules ♦ the interaction between the molecules • the air at room temperature. The ideal gas model is a very good approximation for Ideal Gas Law • • P T r C The pressure P of an ideal gas is proportional to its temperature T and density r. C is a constant of proportionality – air gas constant. Examples: ♦ T increases, r constant -> P increases (tea kettle) ♦ r increases, T constant -> P increases (blow a balloon) ♦ T decreases, r decreases -> P decreases (climb a mountain) ♦ P constant -> T increases, r decreases (example in the book: Fig. 8.2 (a) and (b)) Simple model of atmospheric pressure • Column of air molecules • • • Assumptions: ♦ Constant density ♦ Constant width Atmospheric pressure P is simply due to the weight of the column. P decreases with height because there are less molecules remaining above. • • • From high to low pressure Equal surface pressures in cities 1 and 2 result from ♦ Cold dense air in city 1 ♦ Warm, less dense air in city 2 At higher altitudes the pressures are different (L vs H) The air flow (due to the pressure gradient force) is from High to Low -> expect to see the pressure dropping as the air temperature increases P T r C Daily pressure variations • • How do we measure pressure? Mercury (Hg) barometer. The weight of the Hg column is balanced by the weight of the atmosphere above the open air surface. • • Can we measure the atmospheric 1 atmosphere = 76 cm.Hg = 29.92 in.Hg pressure with a water barometer? Altitude Corrections • Pressure decreases with height. • Altitude adjustment: • • ♦ Why: to compare pressure readings from stations at different altitudes. ♦ Convert all P readings to the pressure at the Mean Sea Level: sea-level pressure. ♦ For every 100 m add 10 mbar ♦ This is a rough correction. Sea-level pressure chart Height surface: surface of constant height ♦ Pressure maps on constant height surfaces show the horizontal variation of the pressure -> isobars • • Sea-level pressure chart Elements: isobars, high (H) and low (L) pressure regions It is an example of a constant height chart (sea-level) Constant height charts • Pressure variations are plotted at a fixed altitude • Atforhigher altitudes, no need altitude correction: what you measure is what you plot • Typical values for the atmospheric pressure at various altitudes ♦ Sea-level: 1000 mb ♦ 3 km: 700 mb ♦ 5.6 km: 500 mb Isobaric charts • Constant height chart: we fix the altitude and plot the pressure: the map shows lines of constant pressure (isobars). • Isobaric chart: we fix the pressure and plot the altitude where it is found: the map shows lines of constant height (contour lines). • High pressure <-> High height on the isobaric chart • Low pressure <-> Low height on the isobaric chart The two types of pressure charts • Surface map (constant height chart) • ♦ Anticyclones (H) – centers of high pressure ♦ Cyclones (L) – centers of low pressure Upper-air chart (isobaric chart) ♦ Pressure contour lines are parallel to the isotherms ♦ Winds flow parallel to the pressure contour lines • • Flying on a constant pressure surface Airplanes measure altitude based on pressure readings They move on constant pressure surfaces • High to Low, Look Out Below This is a problem when T changes. The altimeter needs to be calibrated often with actual altitude measurements.