Define and describe:
a) isobar;
b) wind velocity;
c) anticyclone (high);
d) depression (low);
e) ridge of high pressure;
f) trough of low pressure;
g) col;
h) pressure gradient.
a) An isobar is a line of equal pressure; as displayed below
b) Wind velocity is strength and direction of the wind for a given height or area.
Such as 270/15 this means the wind is coming from 270 degrees, at 15 knots
c) Anticyclone (high) System with greatest pressure in the the centre. Wind rotates Anticlockwise (Southern Hemisphere). Air subsides(goes down) compresses and warms.
d) Depression (low)
System with lowest pressure in the the centre. Wind rotates Clockwise (Southern Hemisphere). Air rises causing expansion and cooling thus invisible water vapour may condense into visible water, clouds.
e) A ridge is an extension off a High and has the same characteristics as the High
f) A trough is an extension off a Low and has the same characteristics as a Low
g) A col is an area between 2 highs and 2 Lows. Wind is very light or calm.
h) Pressure gradient is the atmospheric pressure change per horizontal distance.
Higher – Stronger Wind Lower – Winds light
The Pressure Gradient is the difference in pressure per horizontal distance – it is a force – always acting at right angles to the isobars and always from High to Low
Explain the effect of temperature on Pressure Lapse Rate.
The Pressure Lapse Rate in warm air is less than that in cold air
When going up in a column of warm air, the air particles are further apart, thus a greater height is involved and the fall off in pressure, otherwise known as the Pressure Lapse Rate is less compared to cold air.
Explain the basic principles of operation of the barometer.
A barometer is a weather instrument that measures atmospheric pressure, or the weight of the air at that given datum.
A mercury barometer, which is becoming obsolete, works on the principle that the air’s pressure is able to support a 0.76 metre column of mercury at sea level. As height is gained this value reduces, meaning that the pressure reduces.
Another more commonly used type of barometer is the aneroid capsule. The ambient atmospheric pressure attempts to collapse the capsule, which is attached to a leaf spring. With an increase in ambient pressure, the capsule is in effect crushed inwards; this moves the leaf spring down, which indicates the change in the atmospheric pressure.
Modern aircraft use electrical barometers. These pressure instruments use either a aneroid cell or ceramic capsule, which works on the principle of capacitance to indicate the atmospheric pressure.
The unit of pressure indicated by barometers is the hectopascal (hPa)
Atmospheric pressure is the result of the weight of a column of air above the selected datum (place).
This is the resulting weight of a column of air above a datum. e.g. More air thus more pressure at sea level than there would be at the top of Mount Cook.
By understanding this we can understand why atmospheric pressure changes depending on where we are located.
Pressure Lapse Rate is the reduction in ambient pressure with the increase in height.
This is due to there being less weight of air above the measuring point; as elevation increases there is less overlying atmospheric mass, and so less ambient pressure.
State the composition and structure of the atmosphere.
The atmosphere is an envelope of air between the Earth’s surface and space. These gasses dictate how the Earths atmosphere behaves. This envelope of air is made up of:
78% Nitrogen
21% Oxygen
1% comprising of Argon, Hydrogen, Ozone and Carbon Dioxide.
The atmosphere is divided into layers extending from the Earth’s surface. The first layer is the Troposphere – in which most of the air is concentrated, and and generally contains most of the weather which affects us as pilots. The Troposphere extends 30,000ft over the poles and 60,000ft over the equator. This is capped off by the Tropopause.
The second layer is the Stratosphere, extending from the Tropopause to the Stratopause; which generally extends to 150,000ft from the Earth’s surface.
This layer generally contains little to no air or moisture, meaning it is not common for cloud to form in the stratosphere.
Describe the presence and importance of the following in the atmosphere:
a) carbon dioxide
b) ozone
c) water vapour.
All three gases are present in our atmosphere, and all three affect and change the atmospheric conditions and how our aircraft performs.
Carbon dioxide and other similar ‘waste gasses’ have the ability to trap heat; with excess amounts of these waste gases the ‘greenhouse effect’ takes place and the atmosphere is warmed.
Ozone is an important gas which filters out and blocks much of the harmful ultraviolet radiation from the sun reaching the Earths surface.
Water vapour is present in variable amounts depending on the temperature of the air; the warmer the air, the more water vapour it can hold. Generally, when air cools some of the vapour turns to liquid water or ice and clouds.
