Lesson 105 - Atmospheric Pressure

Front 1

Define: Atmospheric Pressure

Back 1

Force per unit area exerted by the weight of the air at a specific level

105.1.1

Front 2

Define: Station Pressure

Back 2

Actual atmospheric pressure at the elevation of the observing station

105.1.1

Front 3

Define: Mean Sea Level Pressure

Back 3

Atmospheric pressure reduced to mean sea level

105.1.1

Front 4

Give 3 units of pressure measurement, and state the standard atmospheric pressure value for each

Back 4

- Hectopascal (hPa), 1013.0, used on weather charts
- Kilopascal (kPa), 101.3, used for general public
- Inches of Mercury ("Hg), 29.92, used for altimeters

105.1.2

Front 5

Explain the relationship between station pressure and elevation

Back 5

The higher the station elevation, the lower the pressure

105.1.3

Front 6

Describe what mean sea level pressure represents on surface weather maps

Back 6

Station pressure plus an imaginary column of air extending from station level down to sea level

105.1.4

Front 7

Define: Isobar (4 points)

Back 7

- lines joining points of equal pressure at MSL
- measured in hPa
- analyzed on met charts to identify pressure systems
- spaced every 4 hPa above and below the 1000 hPa reference point

105.2.1

Front 8

State the isobaric interval on Canadian surface weather maps

Back 8

Drawn at 4 hPa intervals above and below the value of 1000 hPa

105.2.2

Front 9

Define: Highs, lows, troughs, and ridges and state how they are labelled on weather maps

Back 9

Low - region of relatively low pressure, pressure values decreasing towards the center, labelled "L"
High - region of relatively high pressure, pressure values increasing towards the center, labelled "H"
Trough - valley of low pressure area, a bulge in the side of a low pressure area
Ridge - ridge of high pressure area, a bulge in the side of a high pressure area

105.2.3

Front 10

Describe the horizontal extent of low and high pressure areas

Back 10

- high pressure systems always hundreds of km in diameter
- low pressure systems vary in size from a couple of meters to hundreds of km

105.2.4

Front 11

Describe the character of air movement within high and low pressure areas and the general associated weather conditions

Back 11

High pressure area - clockwise circulation, usually clear weather

Low pressure area - counter clockwise circulation, usually cloudy weather

105.2.5

Front 12

Define pressure gradient

Back 12

Rate of change of pressure with horizontal distance

105.2.7

Front 13

Identify areas of weak/strong pressure gradient on a surface map

Back 13

Strong gradient - isobars are close

Weak gradient - isobars are far apart

105.2.8

Front 14

Define pressure tendency

Back 14

Rate of change of pressure with time at a station

105.3.1

Front 15

List the terminology used when the central pressure is changing in time with a low or high pressure system

Back 15

- deepening
- filling
- weakening
- building

105.3.2

Front 16

List the three causes of pressure variations over time

Back 16

- movement of pressure systems
- variation in intensity of systems
- diurnal variations in pressure

105.3.3

Front 17

Describe the usual displacement of pressure systems

Back 17

West to east

105.3.4

Front 18

Describe diurnal pressure variations and state their basic cause and their typical range

Back 18

- due to day-night atmospheric temperature fluctuation
- two pressure max at 10am/10pm
- two pressure min at 4am/4pm
- can be +/- 2.5 hPa

105.3.5