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If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases. Atmospheric pressure is measured with a "barometer", which is why atmospheric pressure is also referred to as barometric pressure. Atmospheric Pressure: Force exerted by the weight of the air

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Page 1: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases.

Atmospheric pressure is measured with a "barometer", which is why atmospheric pressure is also referred to as barometric pressure.

Atmospheric Pressure: Force exerted by the weight of the air

Page 2: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Location of high or low pressure centers provides information about:

Wind direction

Wind speed

Cloud cover

Precipitation

We use metric unit for pressure called a millibar.

the average pressure at sea level is 1013.25 millibars.

Page 3: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Differences in pressure from one location to another causes:

1) Horizontal motions (wind)

2) Vertical motions (convection and subsidence)

Location of high or low pressure centers provides information about:

Wind direction

Wind speed

Cloud cover

Precipitation

Page 4: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

The change in pressure measured across a given distance is called a "pressure gradient".

The pressure gradient results in a net force that is directed from high to low pressure.

In Northern HemisphereWinds blow clockwise out of a high pressure center.

Winds blow counterclockwise into a low pressure systemPressure Gradient Force directed

from high to low pressure.

Page 5: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

High Pressure Centers

A high pressure center is where the pressure has been measured to be the highest relative to its surroundings.

That means, moving in any direction away from the "High" will result in a decrease in pressure.

A high pressure center is indicated on a weather map by a blue "H".

Wind strength is proportional to the pressure differences.Strong gradients = strong windsWeak gradients = low wind speeds

Page 6: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Low Pressure Centers

A low pressure center is where the pressure has been measured to be the lowest relative to its surroundings.

That means, moving in any horizontal direction away from the "Low" will result in an increase in pressure.

A low pressure center is indicated on a weather map by a red "L" and winds flow counterclockwise around a low in the northern hemisphere.

Page 7: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

As air flows out of a high pressure center, air from higher in the atmosphere sinks to replace it.

This subsidence produces warming, dissipation of clouds and precipitation

As air converges in a low pressure center, it rises and cools. If moist, cooling will cause moisture to condense and form clouds.

Page 8: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Rising pressure readings: approach of a high pressure center and fair weather.

Falling pressure readings: approach of a low pressure center and stormy weather

Mountain barriers influence the development of high and low pressure centers

Intense low or high pressure systems often occur on lee side of mtn. barriers

Page 9: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Pressure decreases with height

Falling pressure readings: approach of a low pressure center and stormy weather.

Pressure gradient maps = semi-horizontal layer

As surface of constant pressure descends, the pressure measurement rises

Rule of Thumb

10 m height change = 1 mb

Top of Space Needle to Base:

184 m --- 22 mb

Page 10: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases
Page 11: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Fixed standard pressure level maps (rather than heights)

Surface maps, at Earth surface

850 mb - available moisture for precipitation, ~1500m level

500mb - middle of atmosphere, troughs (lows) and ridges (highs), ~5500 m level

300 mb - jet streams at ~9100 m level

[The average atmospheric pressure at sea level is ~1000mb]

Mt. Everest: 8848 m w/ 314 mb pressure at summit

31% of atmosphere is present above this level.

Page 12: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

A line connecting points of equal pressure is called an isobar.

At every point along a given isobar, the values of pressure are the same.

The black numbers are station reports of sea-level pressure in millibars.

The small blue numbers are contour labels, which identify the value of an isobar.

Page 13: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

These isobars have been generated from pressure observations.

For example, pressure reports in Washington State are between 1014 mb and 1024 mb.

An area of relatively high pressure is centered in western NE Nevada, while the pressure increases outward from this region.

An area of lower pressure is located in western Alberta.

Page 14: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Troughs: Elongated low pressure centers

Ridges: Elongated high pressure centers.

The black numbers are station reports of sea-level pressure in millibars.

The small blue numbers are contour labels, which identify the value of an isobar.

Page 15: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Wind Direction, Isobars andsurface maps

Ships on ocean example

Small wave length waves or ripples are called short-wave troughs or ridges.

500 mb forecast for 1/21/1999. Troughs = red; solid lines = 500 mb height contours.

Page 16: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

High and low pressure centers and Wind direction

Primary force = pressure gradient force (differences in pressure are proportional to pressure gradient

Pressure gradient force is directed from high pressure to low pressure, crossing isobars at right angles.

Secondary force = Coriolis force, apparent force from earth’s rotation

Deflects winds at 90° to the right in direction of travel (N. hemisphere)

Above 1000m, the balance of pessure forces cause winds to blow parallel to isobars or pressure height contours

Page 17: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

High and low pressure centers and Wind direction

Path of air parcel starting out from rest in upper levels.

Air flows towards low pressureAir turns to the right by CFWhen CF and PGF balance, winds blow parallel to the isobars.

These winds are called geostrophic winds.

Geostrophic wind blows parallel to the isobars because the Coriolis force and pressure gradient force are in balance.

A. Aloft & B. at the ground

Different wind directions and speeds relative to height contours (aloft) and pressure contours (ground).

Page 18: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

High and low pressure centers and Wind direction

Near Ground surfaceThird force - frictional drag

Slows wind down (fig 5.4b)

Lower wind speeds weaken Coriolis force, causing winds to turn left, across isobars at angles 10° to 35°.

Winds at ground blow more directly into low pressure centers.

Monitor changes in wind direction, can see changes of pressure patterns and movement of high and low pressure centers

Monitoring changes in wind direction.

A. Winds aloft from west, low to the north

B. Several hours later, winds aloft from the south, the low is now driven southward, west of observer.

Page 19: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

General Circulation

Unequal heating between equator and pole causes circulation cells

Location of cells correspond to alternating belts of high and low pressure regions.

Cells also correspond to wind.Easterly winds from equator to 30° latitude (trade winds) and 60° to poles.

Westerly winds from 30° to 60°.

Page 20: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Jet Streams

Strong air currents produced by pressure gradient between poles and equator.

Location, strength and orientation vary with season and day to day.

Summer and Winter positions

Page 21: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Air Masses

Regional scale volume of air with horizontal layers of uniform temperature and humidity.

Form during episodes of high pressure

Note location name = origin

M = maritime

C = continental

T = tropical and P = polar

Page 22: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Surface Maps

Uses available surface data

Shows: Temperature Weather Dew point temperature Cloud cover Pressure Wind direction and speed (wind barb)

Page 23: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases
Page 24: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases
Page 25: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Surface Maps

Use available surface data

Shows:TemperatureWeatherDew point temperatureCloud coverPressureWind direction and speed (wind barb)

Observed Dew Point Temperature

Observed Temperaturestation reporting symbol

Page 26: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Dew Point and RH

When the dew point temperature and air temperature are equal, the air is saturated. Dew point temperature is NEVER GREATER than the air temperature.

If the air cools, moisture must be removed from the air.this is accomplished through condensation.

Condensation is the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.

Relative Humidity can be inferred from dew point values.

When air temperature and dew point temperatures are very close, the air has a high relative humidity.

The opposite is true when there is a large difference between air and dew point temperatures, which indicates air with lower relative humidity.

Page 27: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Dew Point and RH

Dew points indicate the amount moisture in the air.

The higher the dew points, the higher the moisture content of the air at a given temperature.

Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation.

A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure.

Page 28: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Surface Maps

Use available surface data

Shows:TemperatureWeatherDew point temperatureCloud coverPressureWind direction and speed (wind barb)

Cloud Cover:

The the amount of cloud cover observed at the time the observation is taken. In this case, broken clouds were reported.

Sea Level Pressure:

The last three digits of the sea level pressure reading in millibars (mb).

Wind Barb:

The wind barb. The wind barb indicates wind direction and wind speed.

Page 29: If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases

Surface Maps

Use available surface data

Shows:TemperatureWeatherDew point temperatureCloud coverPressureWind direction and speed (wind barb)


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