Atmospheric Pressure and Winds

Geog 096: Weather, Climate & Landscapes

Factors influencing air pressure

Arbogast, Fig. 6.2

• Air density (gravity)

• Air temperature

• Lifting of air (local heating & convection, fronts, topography)

differences in atmospheric pressure cause air to move …

weak pressure gradient strong pressure gradient

820

830

840

850

860

870

880

890

820

830

840

850

860

Measuring atmospheric pressure

At sea level:•76 cm Hg (30 in Hg) •1013.2 millibars •101,320 Pascals (101

kPa)Arbogast, Fig 6.3

Atmospheric Pressure with altitude

Arbogast, Fig. 6.4

Atmospheric Pressure:

Force exerted by the atmospheric per unit area

Meteorology 101

Air Pressure

On average, air weighs about 14.7 lb/in2

14.7 lb/in2 =29.92 “inches of mercury”

Air Pressure varies over the globe

1” 1”

“Top”

Sea level) و فشار سطح دریا(Station pressure)فشار ایستگاهیpressure)

Meteorology 101

Low or lowering pressure = “Lousy” weather

Meteorology 101

Changing Pressure - Winds

Take more out than put in – decrease pressurePut more in than take out – increase pressure

Meteorology 101

Changing Pressure - Temperature

Cold Warm

Coldest column = highest pressure **Warmest column = lowest pressure **

Meteorology 101

Pressure Differences Create Wind

Air moves from higher toward lower pressure

Vertical motions also occur

Meteorology 101

Air “converges” at lows, and rises.

Air “diverges” at highs, and sinks.

High) و پرفشار(Low pressure)ایجاد مراکز کم فشارpressure)

Airflow Patterns, Surface and Aloft

(Isobarخطوط هم فشار )

Isobars

Mapping atmospheric pressure

نیروهای موثر بر (Pressure gradient force, PGF)- نیروی گرادیان فشار 1حرکت باد

Meteorology 101

Actual winds around highs and lows

Reality is more complicated

Factors affecting wind movement

1. Pressure gradient force Air moves perpendicular to isobars

Arbogast, Fig. 6.10

Factors affecting wind movement

2. Coriolis effect

Air is deflected; moves parallel to

isobars

deflection:right in no. hemisphereleft in so. hemisphereArbogast, Fig. 6.11

نیروهای موثر بر (Coriolis force, CF)- نیروی کوریولیس 1حرکت باد

The Coriolis Effect

- عوامل موثر بر نیروی کوریولیس: سرعت جسم، عرض جغرافیایی1

(Anticyclone) و واچرخند (Cyclone)چرخند

(Anticyclone) و واچرخند (Cyclone)چرخند

Factors affecting wind movement

3. FrictionDrag on air as it moves across the earth’s surface

(reduced with altitude)

Arbogast, Fig. 6.13

Combined effects Combined effects of pressure of pressure

gradient, Coriolis gradient, Coriolis effect and effect and frictionfriction

Image: Christopherson 2001, Elemental Geosystems

Which of the following statements is NOT true

A. Air tends to move from High to Low pressure

B. Low pressure systems circulate counterclockwise inward in the northern hemisphere

C. Low pressure systems are called “anticyclones”

باد زمینگرد:

Geostrophic Winds

Operate in the upper atmosphere

Not affected by friction

Image: Christopherson 2001, Elemental Geosystems

The Jet Stream ( a geostrophic wind)

Image: Christopherson 2001, Elemental Geosystems

: یک پارامتر برداری است که دارای سرعت و (Wind) بادجهت می باشد.

CodeSpeed

Kilometers per Hour

Description

0 < 1 calm

1 1 - 5 light air

2 6 - 11 light breeze

3 12 - 19 gentle breeze

4 20 - 29 moderate breeze

5 30 - 38 fresh breeze

6 39 - 51 strong breeze

7 51 - 61 near gale

8 62 - 74 gale

9 75 - 86 strong gale

10 87 - 101 whole gale

11 102 - 120 storm

12 > 120 hurricane

Cyclonic and Anticyclonic Winds

Circulation on a Non-Rotating Earth

Meteorology 101

General Circulation

Warm

Cold

Circulation on a Rotating Earth

Surface Pressure

Sea and Land Breezes

Valley and Mountain Breezes

Meteorology 101

Cold Front

Cold air advancesWarmer

Colder

Warm Front

Warm air advances*

Colder

Warmer

Stationary FrontWarmer

Colder

Meteorology 101

What happens when air masses meet at fronts?

Cold air lifts the warmer air.

Clouds and precipitation form.

WarmCold