GEO 211 1st Edition Exam 4 Study Guide Lectures 20 26 Lecture 20 October 22 How do we measure air pressure Barometers Aneroid barometer contains no liquid an aneroid cell either expands or contracts depending on a slight shift in air pressure Higher the reading the more clear the sky is Altimeter calibrated to indicate altitude Barograph recording barometer Mercury barometer long glass tube open at one end and closed on the other removing air from the tube and covering the open end allows the mercury in a dish below to rise to the pressure of the air surrounding it Why does air pressure decrease with height Because there are fewer air molecules as height increases Most of our atmosphere is crowded close to the Earth s surface which causes air pressure to decrease with heat rapidly at first then more slowly at higher altitudes What is standard pressure at sea level 1013 25 mb 1013 25 hPa 29 92 inches Hg 760 millimeters Hg 76 centimeters What is considered sea level air pressure Station pressure observations are adjusted to a level of mean sea level which represents the average surface of the ocean The adjusted reading is called sea level pressure 1013 25 mb Why does the wind blow Horizontal differences in atmospheric pressure causes air to move and thus wind to blow pressure gradient force What triggers wind As the sun warms the Earth s surface the atmosphere warms too Some parts of the Earth receive direct rays from the sun all year and are always warm Other places receive indirect rays so the climate is colder Warm air which weighs less than cold air rises Then cool air moves in and replaces the rising warm air This movement of air is what makes the wind blow Lecture 21 October 24 What is the pressure gradient force The force due to differences in pressure within the atmosphere that causes air to move and thus the wind to blow It is directly proportional to the pressure gradient the amount of pressure change that occurs over a given distance Force that arises from changes in pressure over distance o Shown on weather maps by spacing of isobars Isobars line on a map conveying places with the atmospheric pressure Widely spaced isobars indicate gentle gradient weaker winds Closely spaced isobars indicate steeper gradient stronger winds Pressure gradient force generates wind Always directed from higher pressure towards lower pressure What is the Coriolis force The deflective force resulting from the Earth s rotation Deflection is to the right in the northern hemisphere and to the left in the southern hemisphere Deflection is absent at the equator but increases with latitude pressure The deflection is more when wind speed is greater Coriolis force doesn t influence wind speed The stronger the wind the greater the deflection What is friction force A resistive force caused by wind blowing over Earth s surfaces Friction reduces the wind speed which in turn reduces the Coriolis force What are geostrophic winds A theoretical horizontal wind blowing in a straight path parallel to the isobars or contours at a constant speed The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force The isobars are straight and evenly spaced and the wind speed is constant Speed is directly related to the pressure gradient How does geostrophic winds differ with surface winds The surface of the Earth exerts a frictional drag on the air blowing just above it with high pressure Lecture 22 October 27 Describe how the wind blows around the highs and lows aloft and near the surface in the northern hemisphere and southern hemisphere Northern Hemisphere winds blow 1 Into low pressure center counter clockwise in spiral 2 From high pressure center clockwise out spiral Southern Hemisphere winds blow 1 Into low pressure center clockwise in spiral 2 From high pressure center clockwise out spiral Cyclones have a low pressure center anticyclones have a high pressure center Describe the type of vertical air motions associated with surface high and low pressure areas If the air is moving towards the center of low pressure converging then the air will rise at the center and diverge into the air If there is high pressure the air will converge higher in the atmosphere sink then diverge when it hits the surface Explain how on a 500 mb chart you would be able to distinguish a trough from a ridge The trough will be the dip in the lines which means the air is cold while the ridge will look like a giant hill or bends and turns which means the air is warm Ridge contours bend pole ward Trough contours bend equator ward Lecture 24 October 31 What are the various scales of motion and examples of each Microscale smallest scale of atmospheric motions form by convection or by the wind blowing past obstructions and are usually short lived lasting only a few minutes at best Ex dust devils smoke coming out of a chimney simple gusts Mesoscale the scale of meteorological phenomena that range in size from a few kilometers to about 100 km 62 miles Ex land and sea breezes thunderstorms tornados mountains valley winds Synoptic scale weather map scale dominate a large region hundreds to thousands of kilometers usually last for days and sometimes even weeks Ex cyclones hurricanes Macroscale largest wind pattern ranging over the entire globe Ex westerlies trade winds Lecture 25 November 3 How is wind measured Wind vane indicates wind direction Cup anemometer indicates wind speed is mounted on a vertical shaft the difference in wind pressure from one side of the cup to the other causes the cups to spin around the shaft the rate at which they rotate is directly proportional to the speed of the wind may be read from a dialer or recorder or a system of gears Aerovane skyvane indicates both wind speed and direction is a bladed propeller that rotates at a rate proportional to the wind speed when attached to a recorder a continuous record of both wind speed and direction is obtained Describe all wind systems we have covered in class Sea breeze a coastal local wind that blows from the ocean onto the land The leading edge of the breeze is termed a sea breeze front A type of thermal circulation Blows at the surface from the sea towards the land During daytime Land breeze a coastal breeze that blows from land to sea At night time Flows from land toward the water Valley breeze warmer air glides up the mountain slope during the day Heated during the day becomes heated and moves to the less dense air of the same