1 Geostrophic Balance In the Northern Hemisphere with the wind straight at your back: –the coriolis force acts from your left to right. –The Pressure gradient force acts from your rt. to left –1. Low Pressure is at your left –2. The closer the isobars the stronger the Pressure Gradient Force and wind –3. In Northern Hemisphere winds around a Low blow CCW and blow CW around a High –4. Geostrophic balance does not occur in small scale circulations: Tornadoes, Thunderstorms, sea breezes etc. Ch 4 Sec E

2 Geostrophic Balance Ch 4 Sec E

3 Estimating Winds from Isobars and Contours Geostrophic balance is best observed on a 500mb analysis chart because friction effectswith the surface of the Earth are reduced. Lines on a constant pressure chart are contours of equal altitude. The difference between Pressure Altitude and True Altitude is called “D”. D=TA-PA. Ch 4 Sec E

4 Estimating Winds from Isobars and Contours The difference between Pressure Altitude and True Altitude is called “D”. D=TA-PA. Remember PA = Elevation+(29.92-AS)*1000 Where AS is the Altimeter Setting So D=TA –(29.92-AS)*1000ft – elevation If you’re on the ground D= (AS-29.92)*1000ft The bigger the |D|  the bigger the wind If D is negative you're near a LOW(Bad Wx)‏ Simply: if AS is less than your in a LOW Ch 4 Sec E

5 Friction Effects on Wind Surface friction has greatest effect on the first 2000 AGL. This is called the boundary layer. AGL Above Ground Level. Anemometer Level is usually 10m AGL (Book says 30ft really closer to 33ft)‏ At this level the wind blows more toward the low. The angle between the wind and the isobars can range from 10 o to 45 o At 2000ft wind is more parallel to isobars Ch 4 Sec F

6 Friction Effects on Wind Ch 4 Sec F

7 Charateristics of Wind 1. Wind increases with altitude 2. Wind is more aligned with isobars with higher altitude. 3. In higher winds, winds are more turbulent as you get closer to the ground in the boundary layer. 4. The Boundary is layer is deeper in the day and in the warmer months of the year. 5. In a strong wind the boundary layer is thicker over rough terrain. Cyclones are CCW spirals in the Northern Hemi. Storms may mask all of the above generalizations. Ch 4 Sec F

8 Other Effects on Wind Wind follows the contour of the land Near Mountains the wind goes up and down up and over the mountain. A Microburst is a sudden very intense downpour of rain. It may produce intense downward wind. Famous L1011 DFW Fl191 crash in 1985 Tornadic Winds: Tornadoes and Dust Devils Ch 4 Sec F

9 Surface Analysis Charts Usually Computer Generated Includes Surface Reports in a graphical format Shows High, Lows, Isobars, Fronts Surface analysis began in the 19 th Century Ch 4 Sec F

10 Surface Analysis Charts Usually Computer Generated Includes Surface Reports in a graphical format Shows High, Lows, Isobars, Fronts Surface analysis began in the 19 th Century Ch 4 Sec F

11 Surface Analysis Charts Usually Computer Generated Includes Surface Reports in a graphical format Shows High, Lows, Isobars, Fronts Ch 4 Sec F

12 Surface Analysis Charts Ch 4 Sec F

13 Winds are named by where they blow from. Ch 4 Sec F N degrees NNE 22.5 degrees NE45.0degrees ENE 67.5 degrees E 90.0 degrees ESE degrees SE degrees SSE degrees S degrees SSW degrees SW225.0 degrees WSW degrees W270.0 degrees WNW degrees NW315.0degrees NNW degrees  NE NW SW NNE N S E W SE

14 Surface Analysis Charts Upper Clouds TempSLP Wx O Press Change Trend Dew PtPrecipitation WindBarb Lower Clouds See Page D14 of Text Ch 4 Sec F

15 Surface Analysis Charts Ch 4 Sec F FEW Scattered: SCT Broken: BKN Overcast: OVC Clear: CLR 3 or 4 1 or 2 5, 6, or 7 eighths

16 Surface Analysis Weather Ch 4 Sec F

17 Surface Analysis Charts Ch 4 Sec F