Presentation on theme: "Air pressure and atmospheric motion"— Presentation transcript:
1Air pressure and atmospheric motion Explain the forces that generate winds Explain why there is well-developed westerly flow in the upper troposphere Differentiate between high and low pressure systems Compare and contrast surface and geostrophic winds Describe/read maps of constant pressure surfaces Identify and discuss wind types and the forces that generate them Recognize surface and upper-level atmospheric maps and identify general patterns of windflow Explain monsoonal flow and land-sea breezes
2Air pressure and atmospheric motion Q: What makes the wind blow?A: Air pressure differences.
3Air pressureForce exerted by molecules in atm due to gravity and temperatureJuly 11, 2006NY TimesAs the World WobblesLate last November, as a big low-pressure system built over Europe and Asia and high pressure settled in over the Pacific and Atlantic Oceans, the shifts in the atmosphere caused the earth to jiggle ever so slightly, like a hiker adjusting to a shifting load in a backpack.As a result, the North Pole and its southern counterpart moved about four inches by one measure. (There are several ways to define the poles.)Despite its diaphanous appearance, the atmosphere weighs about 5,000 trillion metric tons, and its mass is unevenly distributed. All those ridges and troughs on a weather map reflect differences of billions of tons of gases.Scientists have long known that as the atmosphere shifts, it influences the earth’s rotation. The recent advent of satellites’ global positioning systems made it possible to confirm even the tiniest movements.There were well-known, regularly occurring wobbles in the earth’s rotation that could shift the poles 30 feet over a year or more. These shifts blocked the detection of subtler, quicker movements caused by day-to-day changes in the atmosphere and the oceans.Now, these small shifts are being measured by institutions devoted to tracking the planet’s behavior, including the earth orientation department of the United States Naval Observatory and the “time, earth rotation and space geodesy section” of the Royal Observatory of Belgium.Experts at the Belgian observatory and the Paris Observatory found the November polar shift and a series of other little loops by looking particularly closely at a period from last November through February, when two of the larger regular wobbles in the axis canceled each other out.They reported their analysis in the July 1 issue of Geophysical Research Letters.
9Pressure systems Two types: high and low Low: associated with clouds and instability.High: associated with clear conditions and stability
10Low pressure systems Cyclone Converging rising air at surface Diverging air aloftWinds rotate counterclockwise in NHAreas of “light” atmosphere; air is forced into these locationsUnstable surface conditions
11High pressure system Anticyclone Converging air aloft Diverging sinking air at surfaceWinds rotate clockwise in NHAreas of “heavy” atmosphere; air is forced out of these locationsStable surface conditions
13Is the location for these pressure systems the northern or the southern hemisphere?
14High and low pressure systems Occur on a variety of spatial and temporal scalesSome pressure systems may be stationary for a long period of time, others may migrate rapidly around the planetSome pressure systems are closed, others are more belt-like and open
15Low pressure systemsTypes of low pressure systems: tornados, thunderstorms, hurricanes….
16Low pressure systems….., midlatitude cyclones, the ITCZ, thermal lows
18Another secret of weather forecasting The atmosphere is a heterogeneous collection of pressure systems in three dimensions.Weather forecasting involves looking at surface conditions as well as upper level conditions (aloft, in the upper troposphere)
19“Deep” tropospheric phenomena A midlatitude cyclone is a low pressure at the surface coupled to a low pressure aloft in the upper troposphereA hurricane is a high pressure aloft and a low pressure at the surface
20Midlatitude cyclonesStrong, “deep” interaction between surface and upper levelsMay travel large distances around the globeMidlatitude cyclone
21“Shallow” tropospheric phenomena Thermal low (warm)Thermal high (cold)Weak interaction between surface and upper levelsMay occur on a daily basis or persist over many monthsLH
261. PGF: Pressure gradient force – winds blow from high to low Where are winds the fastest?
272. Coriolis Force (CF)Apparent deflection of the winds due to rotation of the EarthNH winds deflected rightSH winds deflected left
28Coriolis Force (CF)CF is not a true force; it is an apparent force arising from the effect of the Earth’s rotationDeflection is strongest at poles and zero at the EquatorCF acts perpendicular to the direction of motionCF deflects to the right in the Northern Hemisphere
32Two major types of winds Geostrophic (upper troposphere winds)Influenced by PGF and CF onlyWind flow is parallel to isobars/geopotential heightsGeostrophic flow is westerly (west to east) in NHShown on geopotential height maps