Announcements

• We will finish discussing chapter 1 this week

• We will discuss chapter 2 during the week of 26 January

• HW1 is due by the end of class on Tuesday 2 February

• Remember to send an e-mail with your answers to all class participation questions at the end of every class

Review

• Overview of surface and upper air weather maps• Scientific units – base units and derived units• Temperature: Units (K, °F, °C), typical values, vertical distribution,

troposphere and tropopause• Pressure: Units (Pa, mb), molecular perspective of pressure

Pressure• Horizontal and vertical variations in pressure give rise to all of the

atmospheric motions we will study this semester

• What is atmospheric pressure?• Pressure is a force per area• Molecular perspective on pressure

• What are the units for pressure?• kg m-1 s-2 = N m-2 (or Pascals – Pa)• 1 hPa = 100 Pa• 1 millibar (mb) = 1 hPa

Pressure

• Pressure is proportional to the weight (or mass) of air above a given point in the atmosphere• Pressure always decreases

with height in the atmosphere• What is the rate of change of

pressure decrease with altitude in the lower portion of the atmosphere?

Sea level pressure

• What is the sea level pressure at the locations marked by the black and red dots?

Sea level pressure vs surface pressure

Why doesn’t the surface pressure map (left) look like the sea level pressure map (right)?

Surface Weather Maps

• Surface station model• Temperature• Dew point

temperature• Sea level pressure• Wind speed and

direction• Cloud cover• Significant weather

• What are the units for each of these variables?

Surface weather maps – sea level pressure

• Decoding sea level pressure from station models• If coded pressure is >500

• Add a 9 in front of 3 digit coded SLP• Add decimal point between last two digits• Add units of mb

• If coded pressure is

Surface weather maps - time

• All weather observations are reported using UTC or Z time• UTC is based on a 24 hour clock• 6AM UTC would be written as 06 UTC• 6PM UTC would be written as 18UTC

• If UTC time is given as both hours and minutes it looks like this:• 10:20PM UTC would be written as 2220 UTC

• Convert between UTC and MST or MDT• MST = UTC - 7 hours• MDT = UTC - 6 hours• UTC = MST + 7 hours• UTC = MDT + 6 hours

Surface Weather Maps• What significant weather

is being reported on this map?• What is the valid time of

this map in MST?• Decode the station model

at Denver:• Temperature• Dew point temperature• Sea level pressure• Wind speed and direction• Cloud cover• Significant weather

Air masses• Air mass – a large volume of the atmosphere with relatively uniform

temperature and humidity• Continental (c) - dry• Maritime (m) - moist• Polar (P) - cold• Tropical (T) – warm

• Air masses form over areaswith relatively uniformsurface conditions• Air masses will be modified

as they move out of theirsource regions

Fronts

• Front – boundary between differing air masses• Fronts are defined based on the thermodynamic differences across

the front and the direction of movement of the air masses on either side of front

Fronts

• What types of fronts are present on this weather map?• In what direction

are these fronts moving?• How does

temperature change across a cold front?

Upper Air Weather Maps• Upper air weather maps are

plotted on constant pressure surfaces• What causes the height of a

constant pressure surface to change?• Commonly used constant

pressure maps• 850, 700, 500, 300, 250 and 200 mb• What features do we look at on

each of these maps?

Upper Air Weather Maps: Troughs, Ridges and Wind• Trough – region of low

heights on a constant pressure map• Ridge – region of high

heights on a constant pressure map• How does the height of

the 500 mb surface vary from north to south? Why does it vary in this way?• Locate the troughs and

ridges on this map• What is the relationship

between wind and height contours on this map?

Upper Air Weather Maps: Troughs and Ridges

• Where are the lowest heights located on this Northern Hemisphere map?• Locate the troughs and

ridges on this Northern Hemisphere map

Upper Air Weather Maps: Troughs and Ridges

• Where are the lowest heights located on this Southern Hemisphere map?• Locate the troughs and

ridges on this Southern Hemisphere map

Upper Air Weather Maps: Troughs, Ridges and Wind• How does the height of

the 500 mb surface vary from north to south? Why does it vary in this way?• What is the relationship

between wind and height contours on this map?

• What wind direction should we expect in the mid-latitudes if high heights are found in the tropics and low heights are found in the polar regions?

Upper Air Weather Maps: Station Models

• Upper air station model• Temperature• Dew point temperature• Geopotential height• Wind speed and direction

• How do you decode geopotential height?

• What are the units for each of these variables?

Upper Air Weather Maps: Station Models

• What is the valid time of this map in MST?• Decode the station

model at Denver:• Temperature• Dew point

temperature• Geopotential height• Wind speed and

direction

Extra-tropicalCyclones• Extra-tropical

cyclone characteristics:• Size• Pressure• Winds

• Polar front: Boundary between warm, tropical air masses and cold, polar air masses

Life cycle of extra-tropical cyclones

• What causes the initial low to form?• What causes the low to strengthen or weaken?• Convergence – accumulation of atmospheric mass• Divergence – removal of atmospheric mass

Where is the surface low pressure center located relative to the location of the 500 mb trough?

Will the surface cyclone strengthen or weaken as a result of its position relative to the upper level low?

Life cycle of extra-tropical cyclones

• What is the direction of circulation around an area of low pressure?• How does this circulation alter the temperature distribution?• What types of fronts form in response to this circulation?• How does the upper level height change as the temperature changes?• What impact does this have on the surface pressure?

Life cycle of extra-tropical cyclones

• Where is the upper level trough (ridge) located relative to the surface low pressure center during the mature stage of the cyclone’s life cycle?• What determines how much the surface pressure decreases (or

increases)?

Extra-tropical cyclones: Structure• Where is warm and cold air

located during the mature stage of the cyclone?• What does this indicate about the

position of upper level troughs and ridges?

• Where is air rising (sinking) in this cyclone?• Where are the clouds and

precipitation located relative to the fronts in this cyclone?

Extra-tropical cyclones:Occlusion• How does the speed of movement of

the cold and warm fronts differ?• What happens when the cold front

catches up to the warm front?

• Occluded front – a boundary that separates two cold air masses at the surface with warm air aloft

• How does the position of the upper level trough relative to the surface low pressure center change once the cyclone has occluded?

• What causes the cyclone to weaken and dissipate?

Where is the surface low pressure center located relative to the location of the 500 mb low?

Why is the 500 mb low located where it is?

Will the surface cyclone strengthen or weaken as a result of its position relative to the upper level low?