List the assumed conditions on which the International Standard Atmosphere (ISA) is based.
The International Standard Atmosphere (ISA) is a set of atmospheric conditions which are used as a ‘yard stick’ to calculate aircraft performance given these conditions. The conditions are rarely the same as ISA on any given day, but calculations can be made for changes.
ISA consists of the following conditions:
Sea level Pressure = 1013.2 hPa
Air Temperature = +15c at sea level
Temperature lapse rate = 1.98c/1,000ft
Temperature above 36,090 = Constant -56.5c
Pressure lapse rate = 1 hPa per 30 feet
State the unit of pressure commonly used in meteorology.
The unit used in aviation used to measure pressure is the pascal, commonly referred to as hectopascal (hundreds of pascals).
Hectopascals are written as hPa
State the concept of convergence and divergence and describe how the associated subsidence or ascent of air influences the type of weather commonly associated with pressure systems.
Horizontal inflow is the convergence of air; and divergence of air is outflow of air.
The convergence of horizontal winds causes air to rise, whereas the divergence of horizontal winds causes downward motion of the air (subsidence).
Explain the relationship between pressure gradient, isobars and wind velocity (airflow around pressure systems).
On weather charts the pressure gradient is indicated by the closeness of the isobars.
When they are close together they imply a greater difference in pressure over a small distance
The pressure gradient is strong and winds will be strong
When isobars are far apart they imply a small difference in pressure over a larger distance, the pressure gradient is slack and the winds will be light
Wind strength is directly proportional to the pressure gradient
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)
Describe the source of atmospheric pressure.
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.
Explain how temperature influences the ability of air to hold water vapour.
Temperature influences how much water vapour air can hold;
Warmer air can hold more water vapour than cold air.
State the average pressure lapse rate in the lower atmosphere.
The average Pressure Lapse Rate in the lower atmosphere is a;
Reduction of 1 hPa per 30 feet gained