ppl_04_e2

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daily and seasonally. Figure 4.21 shows that, while the temperature of the world’s oceans generally varies by only five degrees in a 24-hour period, the temperature of the land masses varies by up to three times that figure.

The diurnal

variation of the sea

temperature

is less than that of land temperature. This is the underlying cause of sea breezes, by day, and land breezes, by night.

Figure 4.21 Diurnal temperature variation of the land and sea.

During the day, and in the summer months, the land will be at a higher temperature than the sea, but, during the night and in the winter months, this situation is reversed, with the sea generally being warmer than the land.

The differences in temperature between the land and the sea are the cause of sea breezes and land breezes We will examine sea and land breezes in Chapter 12.

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C H A P T ER 4 : T EM P ER A T U R E Q U ES T IO NS

R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n k n o w l e d g e o f T e m p e r a t u r e .

1.If a temperature inversion is present in the lower atmosphere:

a.there is no horizontal gradient of temperature

b.there is no change in temperature with height

c.there is an increase in temperature as height increases

d.there is a decrease in temperature as height increases

2.The surface of the Earth is heated by:

a.convection

b.conduction

c.long wave solar radiation

d.short wave solar radiation

3.Which of the following words defines temperature remaining constant with an increase in altitude?

a.isotherm

b.isogonal

c.isobar

d.inversion

4.The method by which heat energy is transferred from one body to another, with which it is in contact, is called:

a.radiation

b.convection

c.conduction

d.latent heat

5.The diurnal variation of temperature is:

a.greater over the sea than the land

b.less over desert areas than over temperate grassland

c.increased by convection currents

d.greater when the wind is strongest

6.Replace the missing words:

The sun radiates ______ amounts of heat energy with ______ wavelengths. The Earth radiates ______ amounts of heat energy with ______

wavelengths.

a.great, short, smaller, long

b.small, short, greater, long

c.great, long, greater, long

d.great, long, smaller, short

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C H A P T ER 4 : T EM P ER A T U R

7.Cloud cover will reduce surface diurnal variation of temperature because:

a.incoming solar radiation is reflected back to space and outgoing terrestrial radiation is reflected back to earth

b.incoming solar radiation is re-radiated back to space and atmospheric heating by convection will stop at the level of the cloud layer

c.the cloud stops the sun’s rays getting through to the earth and also reduces outgoing conduction

d.incoming solar radiation is reflected back to space and terrestrial radiation is absorbed by the cloud and re-radiated back to the surface.

8.Diurnal variation of the surface temperature will:

a.be unaffected by a change in wind speed

b.decrease as the wind speed increases

c.increase as the wind speed increases

d.be at a minimum in calm conditions

9.The primary source of atmospheric heating is:

a.long-wave solar radiation

b.long-wave terrestrial radiation

c.short-wave solar radiation

d.latent-heat of evaporation

Question	1	2	3	4	5	6	7	8	9
Answer

T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o u n d

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CHAPTER 5

PRESSURE SYSTEMS

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C H A P T ER 5 : P R ES S U R E S Y S T EM S

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INTRODUCTION.

The different pressure systems found across the surface of the Earth play a primary role in determining the Earth’s weather. Understanding pressure systems is central to the understanding of weather itself.

It is important to note, from the outset, that in low pressure areas, air is rising, while in high pressure areas, air is descending. This general vertical movement of air constitutes the primary distinction between high and low pressure systems. Pressure systems are not defined by the numerical value of the prevailing atmospheric pressure within the systems themselves, but by the relative pressures.

Figure 5.1 Lows, Highs and Cols.

The two principal types of pressure system, are low pressure systems (also called depressions or cyclones) and high pressure systems (also called anticyclones). There are also a number of subsidiary pressure systems called cols, ridges and troughs.

LOW PRESSURE SYSTEMS.

There are two forms of low pressure system: small scale low pressure areas and large scale low pressure areas.

SMALL SCALE LOWS.

Small scale lows, or depressions, can be found almost anywhere on the Earth’s surface. They are created when there is unequal heating of the Earth’s surface.

As you have learnt, an increase in temperature leads to a decrease in air density. Air lying above a warm surface will be heated by that surface through conduction. (See Chapter 4.) So, the heating process and the associated reduction in air density will cause air to rise. The rising air travels up through the atmosphere and eventually, when it reaches high altitudes and has cooled again to the temperature of the surrounding air, diverges, or spreads out. The total weight of the column of air above the warm surface of the Earth reduces as the air diverges, causing the atmospheric pressure to fall at the surface.

High pressure

areas are also referred to as “anticyclones”.

Low pressure areas are often called “depressions”.

The difference

between a low pressure area

and a high

pressure area is defined by the fact that in a ‘low’ the air is rising, and in a ‘high’ the air is descending.

Variations in

pressure, both horizontally

and vertically,

are a basic cause of weather.

Surface

atmospheric pressure

falls in low pressure systems.

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C H A P T ER

Small scale lows are

created by unequal

heating of the Earth’s surface which gives rise to convection. Condensation in the rising air leads to the development of cumulus cloud.

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As air rises, the air surrounding the low pressure area will be drawn inwards in an attempt to fill the low, and return the air pressure to equilibrium. However, the inward-moving air will experience friction as it moves over the Earth’s surface, slowing it down. Consequently, more air leaves the divergence area in the upper atmosphere than can be replaced at the surface. Therefore, low pressure above the warm surface is maintained, and air pressure may continue to fall over time as this process evolves. The development of a small scale low is depicted in

Figure 5.2.

Figure 5.2 Rising air in a small scale low.

As the air in the centre of the depression rises, the volume of air will expand, because pressure decreases with increasing altitude. This expansion will cause the rising air to cool as it ascends by a process called adiabatic cooling. Adiabatic cooling will be covered in detail in Chapter 8. Condensation will take place when the air temperature has fallen to its dew point. The dew point is the temperature to which the air must be cooled, at constant barometric pressure, for the water vapour contained in the air to condense. As water vapour changes state to become liquid water droplets, cloud is formed. The clouds which are created within a small scale low develop vertically, and are called cumuliform clouds, or, more simply, cumulus clouds. (See Figure 5.3.)

Figure 5.3 Cumuliform clouds formed by adiabatic cooling, as air rises in a low pressure area.

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HAZARDS TO AVIATION FROM SMALL SCALE LOWS.

Small scale low pressure areas can set in motion large amounts of energy, and may present some serious hazards to the general aviation pilot. Hazards associated with small-scale low pressure areas include:

•Turbulence.

•Precipitation.

•Icing.

•Poor visibility.

T u r b u l e n c e a n d P r e c i p i t a t i o n .

The velocity of the rising air can be very significant inside a vigorously developing cumuliform cloud. In some cases, hail stones up to 2 lbs (1 kg) in weight may be suspended inside the cloud by strong upcurrents. When the weight of the water droplets or hail stones suspended within the cloud exceeds the force of the rising air, they will fall to the Earth as precipitation. The onset of precipitation associated with small scale lows is generally both rapid and intense, resulting in air being forced downwards toward the Earth’s surface, creating very active down draughts. The strong updraughts and downdraughts within small scale lows generate moderate to severe turbulence. (See Figure 5.4.)

Figure 5.4 Turbulence and precipitation hazards in small scale lows. Strong updraughts are generated inside the clouds, while strong downdraughts are associated with precipitation.

Ic i n g .

Another phenomenon associated with small scale lows and of significance to aircraft is icing. Icing occurs when sub-zero liquid or supercooled water, held within the cloud, freezes onto an aircraft’s surfaces. The formation of ice dramatically affects the aircraft’s performance and ability to remain airborne. (See Figure 5.5.)

Aircraft Icing will be covered in detail in a later chapter; for the moment, it is important that you should note that the icing risk can be moderate to severe in weather phenomena associated with intense small-scale low pressure areas.

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C H A P T ER

The hazards of small scale

lows are turbulence

and icing. Visibility, outside cloud and showers, is good, but visibility in precipitation may be very poor, reducing to almost zero in heavy precipitation.

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Figure 5.5 Icing may be moderate to severe in cumuliform clouds.

V i s i b i l i t y .

One other concern for pilots, connected with small scale low pressure areas, is the horizontal visibility near the Earth’s surface. When air is converging and rising, any impurities near the Earth’s surface will be drawn up into the upper atmosphere leaving much clearer air at the surface. Nevertheless, although surface visibility may be good, in precipitation, visibility can be very poor, reducing to almost zero in heavy precipitation.

THE EQUATORIAL LOW PRESSURE BELT.

Small scale lows commonly form over land masses in the summer months, especially in Asia, Central Europe and the USA. However, the most frequent occurrence of small scale low pressure areas is around the Equator. Figure 5.6, depicts bands or belts of low pressure systems centred on the Equator, created by warm rising air. The central belt is the Equatorial Low Pressure Belt where very extensive cumuliform cloud developments occur.

Figure 5.6 A global pressure distribution model.

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