Lecture 10 March 15, 2010, Monday. Atmospheric Pressure & Wind: Part 1

Transcription

1 Lecture 10 March 15, 2010, Monday Atmospheric Pressure & Wind: Part 1 Speed, Velocity, Acceleration, Force, Pressure Atmospheric Pressure & Its Measurement Ideal Gas Law (Equation of State) Pressure Gradient Force (Horizontal & Vertical) Isobars (horizontal) Hydrostatic Equilibrium (vertical) Isobaric Surface Isobars vs. Height Contours Coriolis Effect & Coriolis Force Frictional Force

2 Speed: distance traveled per unit time, a scalar (magnitude only), positive value only, in units of m s -1, mph, kmph, etc. Velocity (v) : speed & direction, a vector (magnitude & direction), both positive and negative (direction, coordinate), same speed with opposite directions having different velocity, also in units of m s -1, mph, etc. Acceleration (a) : change of velocity (either in speed or in direction) with time, in unit of m s -2, also a vector and can be both positive (acceleration) or negative (deceleration). Gravity (g): a particular acceleration in meteorology, related to the downward gravitational force (G) due to attraction by the solid Earth, 9.8 m s -2 with small changes with latitude & altitude Newton s Second Law: F = ma F is force (a vector, in unit of N, newton), N = kg m s -2 a = F / m, g = G / m, m is the mass of a body (kg) Pressure = Force / Area, Pa (pascal) = N m -2

3 Atmospheric Pressure g x air mass (m) = weight (G) surface area on Earth Atmospheric pressure units in meteorology: 1 mb (millibar) = 100 Pa = hpa (hectopascal) 1 kpa (kilopascal) = 1000 Pa = 10 mb = 10 hpa Other units: inch Hg or mm Hg

4 Mercury Barometer: an inverted tube filled with mercury. Measurement of Atmospheric Pressure The weight of the column of mercury balances the weight of the column of atmosphere on the same area which is the cross section of the tube. Atmospheric pressure = g x density of mercury x height of mercury weight=g x density x volume volume = height x area pressure = weight / area

5 Measurement of Atmospheric Pressure inch Hg= inches of mercury mm Hg= millimeters of mercury Conversion: 1 mm Hg = mb 1 inch Hg = mb Shown in this picture is a Fortin Mercury Barometer located in the Atmospheric Science Laboratory (Brewster C-203) of the Department of Geography at ECU. It is very accurate as it meets the National Weather Service Standard.

6 Range of Atmospheric Pressure at the Mean Sea Level ~ 14.7 pounds per square inch (psi)

7 Aneroid Barometers: a collapsible chamber which compresses proportionally to atmospheric pressure.

8 The Equation of State: Ideal Gas Law Relationships between pressure (p), temperature (T), and density ( ): p = R T R=287 J kg -1 K -1, gas constant of dry atmosphere (excluding water vapor) of the Earth. At constant temperatures, an increase in air density will trigger a pressure increase. Under constant density, an increase in temperature will be accompanied by an increase in pressure. Pressure is exerted in all directions equally, not just downward. Dalton s Law: total pressure = sum of partial pressures exerted by different gas molecules. Vapor pressure is the partial pressure by water vapor molecules and is a humidity index.

9 Vertical and Horizontal Changes in Pressure Compressibility of atmospheric gases causes a non-linear decrease in pressure with increasing altitude. Pressure at P 2 < P 1 can be simply due to pressure decreasing with elevation, thus difficult to evaluate horizontal pressure difference, for example, at the surface. Recorded surface pressure is reduced to sea level pressure equivalents to facilitate horizontal pressure comparisons.

10 Mapping Atmospheric Pressure Isobars: lines of equal atmospheric pressure In this weather map, green lines are isobars of sea level atmospheric pressure distribution. Horizontal pressure differences across space are useful. By analyzing isobaric maps, pressure gradients are apparent: strong or weak pressure gradients indicated by closely or widely spaced isobars, respectively.

11 Forces that drive wind Pressure Gradient Force (PGF) PGF drives air move from high pressure to low pressure. PGF is perpendicular to isobars. Magnitude of PGF increases with pressure gradient. PGF is great in severe weather events (tornadoes, hurricanes).

12 strong pressure gradient: high wind speeds weak pressure gradient: low wind speeds

13 Vertical Pressure Gradients Average vertical pressure gradients are usually greater than extreme examples of horizontal pressure gradients such as hurricane, tornado. However, vertical wind speed is usually much smaller than horizontal wind speed because Hydrostatic Equilibrium The downward force of gravity balances strong vertical pressure gradients to create hydrostatic equilibrium. Forces balance and the atmosphere is held static relative in the vertical direction to Earth s surface. Local imbalances initiate various updrafts and downdrafts.

14 The Role of Density in Hydrostatic Equilibrium Gravitational force is proportional to mass: G = g m Denser atmosphere experiences greater gravitational force. A vertical pressure gradient must increase to offset increased gravitational force to maintain hydrostatic equilibrium. Higher temperature columns of air are less dense than cooler ones due to expansion in volume or increase in depth (height). For warm or cold air, greater depth for the same pressure decrease in the vertical translates into smaller or larger vertical pressure gradients, which leads to or maintains hydrostatic equilibrium.

15 Heating causes a density decrease in a column of air. The heated column contains the same amount of air as the cool one, but has a lower density due to greater depth or height and thus smaller vertical pressure gradient.

16 Upper air heights decrease with increasing latitude Constant pressure surfaces of cooler air will be at a lower altitude (height) than those of warmer air. Height contours indicative of horizontal pressure gradient.

17 Horizontal Pressure Gradient in Upper Atmosphere Upper air horizontal pressure gradients may be determined using height contour distribution for given constant pressure.

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19 Isobaric Surface

20 Isobars Height Contours

21 Coriolis Effect Free moving objects in the atmosphere are influenced by Earth s rotation Path of missile Deflected to the right in Northern Hemisphere

22 Coriolis Force A resulting apparent deflective force Path of missile South Pole Deflected to the left in Southern Hemisphere South Pole

23 The Coriolis force is a sine function of the latitude, maximum at the poles and zero at the equator. Taking place regardless of the direction of motion.

24 Coriolis force increases with speed of moving objects. Overall deflection effect noticeable only on objects with long periods of motion. Only changes direction, DOES NOT change speed

25 Frictional Force A force of opposition which slows air in motion (wind). Initiated at the surface and extend, decreasingly, aloft by atmospheric turbulence. Important for air within 1.5 km (1 mi) above the ground surface which is called the planetary or atmospheric boundary layer (PBL or ABL). Because friction reduces wind speed it also reduces Coriolis deflection. Friction above 1.5 km is negligible, thus atmosphere above 1.5 km is called the free atmosphere (flow aloft).

26 Effects of Frictional Force on Winds near the Surface Northern Hemisphere Balance between PGF, Coriolis force, and frictional force.

27 Effects of Frictional Force on Winds near the Surface Rougher surface (Urban) Strong Friction Low Wind Speed L Smoother Surface (Lakes & Oceans) Weak Friction Medium Wind Speed 1000 mb 1000 mb 1000 mb 1008 mb 1008 mb 1008 mb L Upper Free Atmosphere No Friction Highest Wind Speed L 1012 mb 1012 mb 1012 mb H For the same PGF, lower wind speed means smaller Coriolis force and greater angle between the wind and the isobar. H For the same PGF, higher wind speed means greater Coriolis force and smaller angle between the wind and the isobar. H Wind parallel to the isobar Northern Hemisphere