AOS101Lecture 10

• A severe thunderstorm is defined as a thunderstorm that produces- Hail of 1 inch diameter (in central US) or larger and/or

wind gusts 58 mph or greater and/or a tornado

• Occurs most frequently during the spring and summer when there are the following atmospheric conditions:

– Conditionally unstable atmosphere – Moisture – Upward vertical motion (“Lifting”)– Wind shear

A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACT with the ground.” –NWS

•Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.•An area of rotation, 2-6 miles wide, now extends through much of the storm. •Most strong and violent tornadoes form within this area of strong rotation

• Several favorable environmental conditions must be in place before a tropical cyclone can form:– Some initial disturbance such as a thunderstorm

complex, which may slowly develop– Warm ocean waters (at least 80°F)– Potentially unstable atmosphere favorable to

convection– Moist air near the middle of the troposphere– Low values of vertical wind shear between the

surface and upper troposphere• If these conditions persist for several days,

a tropical cyclone may form

• When these disturbances first appear, they are called tropical depressions– Not named yet

• Once the disturbance has developed with surface wind speeds stronger than 39 mph, the storm is classified as a tropical storm

• After further strengthening and surface wind speeds greater than 74 mph, the system is upgraded to a hurricane

• Atlantic Hurricane season is June 1 – November 30

EYE

EYEWALL

RAINBAND

HURRICANE KATRINA

What makes the wind blow? We need to think about Newton's Laws 1st Law

◦ An object at rest will remain at rest; an object in motion will remain in motion as long as no force is exerted on the object.

2nd Law◦ The total force exerted on an object is equal

to the acceleration of the object times its mass.

Pressure Gradient Force (PGF)Coriolis force (CF)Centripetal forceFrictional Force

Our atmosphere is full of forces that become balanced

As a result, we can say something about it’s motion

Balanced forces tell us many things.◦For example, the wind direction is a balance between the Coriolis force, PGF, and frictional force

The pressure gradient is a change in pressure over a given distance.

Pressure gradient force compels fluids to

move from high pressure to lower

pressure. The PGF acts to increase

lower pressure and decrease higher pressure

Direction of PGF – always points from HIGH pressure toward LOW pressure, directly perpendicular to an isobar

Magnitude of PGF- strength is directly related to the strength of the pressure gradient

The PGF is the force that causes the wind to blow!

Pressure gradient = 4 mb per 100 km

◦ When isobars are very close together, the numerator in the PGF equation is large (a very large change in pressure)

◦ So the pressure gradient is large, and thus, the PGF is very strong.

The Coriolis force is an apparent force that results from the constant rotation of the Earth.

In N. Hemisphere, acts at a 90° angle to the right of the object in motion (such as the wind)

This means that a wind from the south would have a CF acting toward the east

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s.

The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX

Imagine Dallas, TX fires a nuclear missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s.

The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX

Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of 37.28 N, rotates with the Earth at a speed of 465.11 m/s.

Missile travels toward Winnepeg which, at a latitude of 52.00 N, rotates with the Earth at a speed of 286.35 m/s.

The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX

Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target

Missile lands north of Ottawa.

We cannot see the planet rotating, so when something is moving, we perceive it as being deflected to the right of its intended path in the N. Hemisphere

Deflection is dependent on latitude◦ 0 at equator and maximum at the poles

Deflection intensity is directly related to wind speeds◦ Acts only as the wind starts to blow

http://www.youtube.com/watch?v=mcPs_OdQOYU

A balance between the ◦ Pressure gradient force◦ Coriolis force

Balance allows PGF to be equal and opposite the CF. This balance will tell use the magnitude of the geostrophic wind

The geostrophic wind blows parallel to lines of constant pressure, with low pressure on the left

Movie: What happens if its not in balance

Geostrophic Balance

996 mb

1000 mb

1004 mb

L

H

x

Geostrophic Balance

996 mb

1000 mb

1004 mb

L

H

Coriolis Force

Pressure Gradient Force

Where the pressure gradient is small, the PGF is also small, resulting in a weak wind.

996 mb

1000 mb

1004 mb

L

H

Coriolis Force

Pressure Gradient Force

Geostrophic Wind

Where the pressure gradient is large, the PGF is also large, resulting in a strong wind.

The wind can be approximated as nearly geostrophic in the upper levels of the troposphere.

PGF

CF

How are there different heights at 500 mb?

PGF/ CF/ Centripetal

Friction affects geostrophic balance by putting a drag-force on the air: friction always acts in the direction opposite the direction of the wind

windFR

This throws the wind out of geostrophic balance – there is now a net force acting on the wind in the direction opposite its motion

windFR

PGF

CF

The Frictional Force

• Since friction acts in the opposite direction of the wind, it slows the wind

• Change in speed change in magnitude of the Coriolis force

• Friction + Coriolis force ~ PGF no longer geostrophic balance and winds can cross the isobars

How does friction affect geostrophic balance?

Friction

996 mb

1000 mb

1004 mb

L

H

Upper Level WindBalance: PGF/ CF

Lower Level WindBalance: PGF/ CF/ Friction

Causes wind to cross isobars at ~30° angle at surface

• Height above the surface– The further away from the surface, the less friction– For instance, the winds at 300 mb experience less

friction than the winds at the surface

• Wind speed– The stronger the wind, the more friction will oppose

the motion– Therefore, slower winds experience less friction

than fast winds

• Surface Type– The rougher the surface, the greater the friction– For example, the friction over an open body of

water is weaker than that over a mountainous terrain

More factors that affect the frictional force:

In regions (upper levels) where friction is negligible, the winds are approximately geostrophic