Things to look for on the weather maps Visible and IR satellite images (& radar too): Look at cloud movements and locations - do they correlate with what you would expect from the surface or 500mb pressure patterns? How can you distinguish between high and low level clouds and between deep and shallow clouds? VIS IR

Surface: Look for locations of high and low pressure centers, warm and cold fronts, regions of high winds, rain, snow, drylines, or other significant weather. Things to look for on the weather maps

850mb: Use this map to look at temperature gradients, and to find regions of warm or cold air advection. It is also important to look at the moisture field and advection at this level if available (low-level moisture). Things to look for on the weather maps

500mb: Where are the troughs and ridges? Where are the troughs and ridges in relation to the high and low pressure systems on surface maps (Positive Vorticity Advection & Negative Vorticity Advection)? Things to look for on the weather maps

300mb: Use this map to look for jet streaks, or elongated pockets of very strong winds embedded in the jet stream. Jet streaks generally occur above regions of strong temperature gradients that you would find on the 850mb map. Why? Things to look for on the weather maps

Forces govern the atmospheric motions Pressure gradient force Coriolis force Gravity Friction Forces Momentum equations

Coriolis Force O B * reference point (fixed star) O * O v1v1 d x x<<d v 2  d  Deflect to the right on the northern hemisphere, and to the left on the southern hemisphere. ball

Coriolis Force O v1v1 d x x<<d v 2  d  Given Coriolis acceleration a, the distance is Travel time:

Coriolis Force If is that due to the earth ’ s rotation about a local vertical axis, such that Then (= Coriolis force if M = 1 kg)

Forces govern the atmospheric motions Pressure gradient force Coriolis force Gravity Friction Forces Momentum equations

Pressure Gradient & Pressure Gradient Force Why do we want to know sea level pressure (SLP)? Why do we want to know pressure gradient? 960mb1000mb 960mb 1000mb Positive pressure gradient but negative pressure gradient force in the x direction. Pressure gradient Pressure gradient force Negative pressure gradient but positive pressure gradient force in the x direction.

s: along the wind direction n: perpendicular to the wind direction, positive to the left. Nature coordinate x y s n The nature coordinate can be obtained by rotating the Cartesian Coordinate until the x direction is along the wind direction. Then, x is in the s direction and y is in the n direction. And u=V and v=0 V

P 1 (x o,y 1 ) P 2 (x o,y 2 ) O P2 P1 Po L H (x 0,y o ) Estimate Pressure Gradient How to calculate pressure gradient at point O? P 1 (x 1,y o ) P 2 (x 2,y o )

Pressure Gradient & Pressure Gradient Force How to calculate real distance 1 o lat = ? km ReRe yy  lat = lat2 – lat1  y=  lat x 111 km per degree

Pressure Gradient & Pressure Gradient Force xx  long = long2 – long1  x=  long x 111 km x cos  How to calculate real distance r ReRe 1 o long = ? km

Pressure gradient at point O: Pressure Gradient & Pressure Gradient Force P 1 (x 1,y o ) P 2 (x 2,y o ) O P2 P1 Po L H (x 0,y o )

Pressure gradient at point O: Pressure Gradient & Pressure Gradient Force P 1 (x o,y 1 ) P 2 (x o,y 2 ) P2 P1 Po L H (x 0,y o ) O

Or using the nature coordinates Pressure Gradient & Pressure Gradient Force P 1 (x 3,y 3 ) P 2 (x 4,y 4 ) O x1x1 x2x2 y2y2 y1y1 O P2 P1 Po L H (x4-x3) (y4-y3) (x 0,y o )

Horizontal pressure gradient vs. vertical pressure gradient Pressure Gradient Vertical pressure gradient force is much greater than the horizontal one, but is mostly balanced out by gravity. So, the motion in the atmosphere is dominated by horizontal winds.

Geostrophic flow (V g ) O C P P P P C C.... Not happen often in the real world. If it does, this is seen in large scale. The horizontal pressure gradient force (P) is balanced by the Coriolis force (C). rest

Gradient wind flow (V G ) C > P P > C C C P P This is for large scale flow since Coriolis force is important. Subgeostrophic fV G > => V G > V g Supergeostrophic If one uses geostrophic wind to approximate gradient winds, what happens? V G < V g fV g =

500 mb Wind Vectors Gradient wind flow (V G )

Surface Weather Map Any difference from the 500-mb one? L Because of what?Friction!

Friction Friction is proportional to the roughness of the Earth’s surface and wind speed, and is opposite to the wind moving direction geostrophic balanceunbalanced flow balance with friction Wind turns toward to the low pressure side !!! P C P C FF P C F P C

Flow with friction This is for large scale flow too since Coriolis force is important. Convergence Stormy weather Divergence Good weather but potentially bad for air quality

Cyclostrophic wind flow P This is for small scale when Coriolis force is not important! This flow can exist only around a low pressure, and the force needed to change the wind direction is provided by the pressure gradient force. So, dust devils and tornadoes can turn either cyclonically or anticyclonically. P