Aviation Weather CH 4 - 6

Q&A from Jeppesen Aviation Weather Chapters 4 - 6

Moving Air

Carries heat, moisture and pollutans from one location to another

Winds

Move atmospheric mass and therefore affect changes in atmospheric pressure

vertically

When air moves from one location to another it can move both horizontally and…

Thunderstorms and Mountain lee waves

Horizontal air motions are much stronger than vertical motions except in…

vector quantity (has magnitude and direction)

Wind velocity is a…

Temperature and pressure

Examples of Scalar quantities - Only have magnitude

The wind speed, expressed in knots (NM/hr), MPH (SM/hr), KPH or m/s

The magnitude of the wind velocity is…

Wind direction

is the direction from which the wind is blowing, measured in degrees, or to eight or sixteen points of the compass, clockwise from true north (360°)

Meteorologist

Always state their wind direction relative to true north

Air traffic controllers

Always state their wind direction in terms of magnetic north

Annemometer and wind vanes

Common techniques to measure wind velocity (vector = magnitude + direction) at the surface

15 knots or more

When a standard wind sock becomes fully inflated, the windspeed will equal how many knots?

Aerovane

An annemometer + a wind vane with the shape of a little airplane. The rotation of the propeller determines the wind speed while the “airplane” shape will fly “into the wind”

30 ft ( 9 m )

Regular height for wind measurements

Sustained wind speed

Reported wind spees and directions are usually one or two minute averages. This average wind speed is also refered to as…

Gust (racha)

Is reported when there is a 10-knot variation between instantaneous peaks and lulls (calma) during the last 10 minutes

Squall (Ráfaga, turbonada)

Reported when there is a sudden increase of wind speed by at least 16 knots to a sustained speed of 22 knots or more for a one-minute period.

Peak wind

Maximus instantaneous wind speed greater than 25 knots since the last hourly observation.

The wind direction in reference to true north

What does the first three digits of the wind group on the METAR represent?

VRB03KT (wind direction is variable, wind speed is 3 knots)

How is wind reported on the METAR if the wind direction is variable.

020V090 = Wind direction variable between 020° and 090° (Goes after the wind group)

If the wind direction varies 60° or more and the speed is above 6 knots, how is wind reported on the METAR?

08032G45 → with a G followed by the highest gust over the las ten minutes.

How are gusts reported on the METAR?

PK WND 35035/1915 → three digits for direction, 2 or 3 digits for speed and the time in hours and minutes

How is peak wind reported on the METAR remarks?

The conservation of momentum.

“If an object of mass, M, is subjected to an unbalanced force (F_total), it will undergo an acceleration, A, that is:

F_{total = MA}

Newton’s second law

What makes the wind blow?

Acceleration

Change of the speed and/or direction of a masss or air as it moves along its path. F_total is the sum of all the horizontally directed forces which act on a partidular mass of air.

1. Pressure gradient force

2. Coriolis force

3. Frictional force

The most important forces that affect air motions

Pressure gradient force

The difference in pressure between two points divided by the distance between the points. The greater the__________the greater the acceleration.

Horizontal pressure gradient force

The atmosphere causes air parcels to be accelerated across the surface of the earth toward low pressure, that is when a____________exist.

The difference in temperaure and thus changes in pressure in a coastline between the land and the sea

Example of the creation of a horizontal temperature gradient by differential heating.

in Cold air

Where does pressure decreases more rapidily with height? in cold or warm air?

Thermal circulation

The movement of air which results from differential heating. They have two horizontal branches: an upper branch or “return flow” and a lower branch (sea breeze, land breeze, mounain breeze and valley breeze). They occur over distances of ten to a hundred miles.

Perpendicular to the isobars toward the lower pressure

In reference to the isobars the pressure gradient force is always directed….

the wind

In a thermal circulation, the stronger the temperarure gradient, the stronger the pressure gradient and the stronger…

Coriolis force

Affects all objects moving across the face of the earth (ocean currents, and the paths of airplanes). As soon as air begins to move, it is incluenced by it. Its effect is only significant when an object moves over large distances (several hundred miles or more) allowing the weak force of time to act.

to the right

In which direction is the deflection of objects in the North Pole (rotation of the earh is ccw) due to the Coriolis force?

to the left

In which direction is the deflection of objects in the South Pole (rotation of the earth is cw) due to the Coriolis force?

in the Northern Hemisphere

Where does Coriolis force always acts 90° to the right of the wind?

In the Southern Hemisphere

Where does Coriolis force always acts 90° to the left of the wind?

It doesn’t affect wind speed only wind direction, however, is the wind speed is zero there’s no Coriolis effect. If the wind speed is zero, Coriolis forces is zero. The greater the wind speed, the greater the coriolis force.

How is Coriolis force related to wind speed?

It varies from zero at the equator to a maximum at the poles.

How does latitude affect Coriolis force?

Its effect is minimal for smale-scale circulations and very important for large-scale wind systems

How does Coriolis force affect small vs. large air movements?

to the Left in the Norther hemisphere and to the Right in the souther Hemisphere
Correct left - North
Correct Right - South

How must aircraft tracks be corrected in each hemisphere due to Coriolis force?

Geostrophic Balance / Geostrophic wind

Pressure gradient force and Coriolis force tend to balance each other when the scales of atmospheric circulations are large enough. When air travels over distances of hundreds of miles or more (the farther the better), Coriolis and Pressure gradient forces tend to be equal in magnitude, but opposite in direction. This condition is known as…
and its wind is….

on the left

In the Northern Hemisphere, the geostrophic wind in parallel to the isobars with the lowest pressure on…

“With your back to the wind, the low pressure is on your…

Cyclonic flow / anti-cyclonic flow

In the Northern Hemisphere winds tend to blow ccw around low pressure centers (cyclones) and cw around high pressure centers (anti-cyclones) therefore ccw and cw motions are often described as…

Sea breeze, thunderstorms, tornadoes and dust devils (remolinos de polvo)

Due to the much greater influence of pressure gradient force over Coriolis force in small scale circulations, Geostrophic Balance does not occur in…

Coriolis Force

Winds do not blow directly from large scale high pressure areas to low-pressure areas because of…