Wind Flashcards
Define: geostrophic wind.
Air that is in motion due to:
» pressure gradient force
» geostrophic force
which are in equilibrium.
Define: geostrophic force.
Force due to the angular velocity of the Earth.
It acts at right angles to the pressure gradient force: to the right in the northern hemisphere and to the left in the southern hemishere.
Define: Buys Ballot’s Law.
If an observer stands with their back to the wind, the lower pressure is:
» on their left in the northern hemisphere
» on their right in the southern hemisphere
Define: pressure gradient force.
The force that moves the air from high pressure to low pressure.
Define: gradient wind.
The resultant wind combined from:
» pressure gradient force
» geostrophic force
» centripetal force
Describe how gradient wind changes as it approaches the surface.
Entering the friction layer means that:
» wind is slowed
» coriolis effect is reduced
» coriolis deflection reduces
» the wind backs in the northern hemisphere, veers in the southern hemisphere
Define: isobars.
Lines joining points of equal atmospheric pressure.
Principally, what is a deciding factor on whether a wind will be geostrophic wind or gradient wind?
» straight isobars: geostrophic wind
» curved isobars: gradient wind
Define: hurricane.
A wind with a mean surface velocity of 64 kt or more.
What does the geostrophic wind velocity formula signify?
That the magnitude of the geostrophic wind velocity increases with decreasing latitude.
Define: wind veering.
A clockwise change in wind direction.
What overall effect does friction layer have on wind?
It slows it down.
What is the nature of friction layer turbulence?
Mechanical and thermal.
Define: gust.
A short variation from the mean wind speed in which the wind speed varies by 10 kt or more.
What is the primary cause of wind?
Pressure differences, which are usually caused by temperature differences.
Is there a difference in depth of the friction layer between night and day?
Yes, at night the friction layer depth reduces to around 1,000 ft due to the absence of thermal turbulence.
Define: gale.
A wind with a mean surface velocity of 34 kt to 47 kt.
What are the requirements for pure geostrophic wind?
» steady pressure gradient force
» completely horizontal motion of air
» straight and parallel isobars
Define: isotach.
A line of equal wind speed.
Define the formula for:
(a) geostrophic force
(b) pressure gradient force
and identify the elements.
The formula is the same for both (a) and (b), because they are assumed to be in equilibrum in geostrophic wind.
GF/PFG = 2 · Ω · ρ · VW · sin Φ
Ω = Earth’s angular velocity
ρ = air density
VW = geostrophic wind velocity
sin Φ = latitude sine
Define: wind backing.
An anti-clockwise change in wind direction.
In which direction does the pressure gradient force act?
Towards the low pressure, perpendicular to the isobars.
What is an alternative name for the geostrophic force?
Coriolis force.
What does the GF formula signify?
That the magnitude of the geostrophic force increases with increasing latitude.
Define: friction layer.
Turbulent layer of air that extends (generally) up to about 3,000 ft.
Define the formula for the geostrophic wind velocity.
VW = (PGF) / (2 · Ω · ρ · sin Φ)
What does isobar spacing indicate?
The strength of the pressure gradient force.
Define: wind.
Horizontal motion of air.
How does wind speed affect TAS?
TAS increases with increasing headwind or decreasing tailwind.
TAS decreases with decreasing headwind or increasing tailwind.
What is the relationship between isobar spacing and wind strength?
The closer the isobars, the stronger the wind.
