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atMO 170A Class Notes Week 4

by: Savannah Way

atMO 170A Class Notes Week 4 ATMO 170 A1

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Savannah Way

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Class notes cover pressure, surface and upper-air charts, wind direction, and wind speed.
Intro to Weather and Climate
Dr. Mullen
Class Notes
weather, climate
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This 5 page Class Notes was uploaded by Savannah Way on Thursday October 6, 2016. The Class Notes belongs to ATMO 170 A1 at University of Arizona taught by Dr. Mullen in Fall 2016. Since its upload, it has received 3 views.


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Date Created: 10/06/16
WEEK 4 (157-178, 182) ATMOSPHERIC PRESSURE Air pressure: mass of air above a given level; decreases with increasing height  Atmospheric pressure always decreases with increasing height  Pressure = temperature x density x constant  Force exerted by the air molecules over a given area Standard atmospheric pressure: 1013.25 mb = 1013.25 hPa = 29.92 in. Hg. Barometer: detects and measures the pressure change  Atmospheric pressure also referred to as barometric pressure  Bar is a unit of pressure that describes a force over a given area  Millibar is a unit of pressure most commonly found on the surface weather maps o Hectopascal is gradually replacing the millibar as the preferred unit of pressure of surface maps  Evangelista Torricelli, Galileo’s student, invented the mercury barometer in 1643  Aneroid barometer: most common type of home barometer, contains no fluid, o Altimeters are aneroid barometers that measure pressure, but calibrated to indicate altitude o Barographs are recording aneroid barometers that consist of a pen attached to an indicating arm that marks continuous record of pressure on chart paper  Station pressure: after correction of temperature, gravity, and instrument error  Instrument error: built in error caused by the surface tension of the mercury against the glass tube  Altitude corrections: created so that a barometer reading taken at one elevation can be compared with a barometer reading taken at another  Seal-level pressure: the level representing the average surface of the ocean adjusted reading  Isobars: lines connecting points to equal pressure  Surface map: or sea-level pressure chart SURFACE AND UPPER-AIR CHARTS Anticyclones: high pressure mid-latitude cyclonic storms: extratropical cyclones, formed in the middle latitudes, outside of the tropics isobaric maps: constant pressure chart to show height variations along a constant pressure surface  contour lines: lines that connect points of equal altitude above sea level  isotherms are lines of equal temperature  ridges (elongated highs) where the air is warmer and indicating depressions  troughs (elongated lows) where the air is colder NEWTON’S LAW OF MOTION 1. first law: an object at rest will remain at rest and an object in motion will remain in motion (and travel at a constant velocity along a straight line) as long as no force is exerted on the object 2. second law: the force exerted on an object equals its mass times the acceleration produced 3. third law: acceleration is the speeding up, or slowing down, or the changing of direction of an object  F =ma Determinates of wind direction  Pressure gradient force: is the directed from higher toward lower pressure at right angles to the isobars; the force that causes the wind to blow o P = difference in pressure / distance o Steep (or strong) pressure gradient: rapid change in pressure over a relatively short distance o Gentle (or weak) pressure gradient: different in pressure would be small over a relatively large distance  Coriolis force: describes an apparent force that is due to the rotation of the earth o after Gaspard Coriolis, a French scientist who worked it out mathematically o causes the wind to deflect to the right of its intended path in the northern hemisphere and to the left of its intended path in the southern hemisphere o the stronger the wind, the greater the defection o the amount of deflection depends on:  the rotation of the earth  the latitude  the object’s speed o acts at right angles to the wind, only influencing wind direction and never wind speed o present in all motions relative to the earth’s surface o minimal on small-scale winds, such as those that blow inland along coast in summer  Centripetal force: inward-directed force o Geostrophic wind: flow of air  when the flow of air is purely geostrophic, the isobars are straight and evenly spaced, and the wind speed as it flows along  cyclonic flow: the counterclockwise flow of air  anticyclonic flow: the clockwise flow of air o gradient wind: a wind that blows at a constant speed parallel to curved isobars above the level of frictional influence o centripetal acceleration: the gradient wind blowing around the low-pressure center is constantly accelerating because it is constantly changing direction and is directed at right angles to the wind, inward toward the low center o meridional: where the wind flows in large, looping meanders, following a more or less north-south trajectory o zonal: there the wind blows in a west-to-east direction  Friction o Due to surface friction, winds on a surface weather map do not blow exactly parallel to the isobars; instead they cross the isobars, moving from higher to lower pressure o The frictional drag of the ground slows the wind down o Friction layer: atmospheric layer that is influenced by friction, usually extends upward to an altitude near 1000 m or 3000 ft above the surface o Near the surface, friction reduces the wind speed, which in turn reduces the Coriolis force BRIEF REVIEW  atmospheric (air) pressure exerted by the mass of air above a region  a change in surface air pressure can be brought about by changing the mass (amount of air) above the surface  heating and cooling columns of air can establish horizontal variations in atmospheric pressure aloft and at the surface  a difference in horizontal air pressure produces a horizontal pressure gradient force  the pressure gradient force is always directed from higher pressure toward lower pressure and it is the pressure gradient force that causes the air to move and the wind to blow  steep pressure gradients (tightly packed isobars on a weather map) indicate strong pressure gradient forces and high winds; gentle pressure gradients (widely spaced isobars) indicate weak pressure gradient forces and light winds  once the wind starts to blow, the Coriolis force causes it to bend to the right of its intended path in the northern hemisphere and to the left of its intended path in the southern hemisphere WINDS AND VERTICAL AIR MOTIONS  as long as the upper-level diverging air balances the converging surface air, the central pressure in the low does not change  the surface pressure will change if the upper-level divergence and surface convergence are not in balance Hydrostatic Equilibrium: when the upward directed pressure gradient force is nearly exactly balanced by the downward force of gravity  when air is in hydrostatic equilibrium, there is no net vertical force acting on it and so there is no net vertical acceleration  most of the time, the atmosphere approximates hydrostatic balance, even when air slowly rises or descends at a constant speed DETERMINING WIND DIRECTION AND SPEED Wind is characterized by its direction, speed, and gustiness  Onshore wind: wind blowing from the water onto the land  Offshore wind: wind blowing from land to water  Upslope wind: air moving uphill  Downslope wind: air moving downhill Prevailing Wind: the name given to the wind direction most often observed during a given time period  Prevailing winds can greatly affect the climate of a region  Wind rose: indicates the percentage of time the wind blows from different directions o Extensions from the center of a circle point to the wind direction, and the length of each extension indicates the percentage of time the wind blew from that direction Wind Instruments  Wind vane: consists of a long arrow with a tail, which is allowed to move freely about a vertical post. The arrows always point into the wind  Anemometer: measures wind speed consisting of three hemispherical cups mounted on a vertical shaft  Aero vane: indicates both wind speed and direction consisting of a bladed propeller that rotates at a rate proportional to the wind speed


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