1. In a mountain wave situation, the worst of the turbulence is most likely to be found when flying:
a. at about mid-height between the lenticular and roll cloud b. in or just below the roll cloud
c. in the cap cloud d. in the lenticular cloud
2. Complete the following sentence: Low level windshear:
a. is rare where there is a strong inversion level close to the surface b. is found only under the core of the anvil of a thunderstorm c. is only ever found on the fringes of a micro-burst
d. may be experienced 15 to 20 miles ahead of a fast-moving thunderstorm
3. A light aircraft flying at low level near a mountain range across which a strong wind is blowing may expect:
1. severe turbulence below or within the rotor zone
2. downdraughts on the leeward side of the mountain, below ridge level, which may exceed the climb rate of the aircraft
3. strong smooth upcurrents on the windward side of the mountain 4. wavelenticular cloud
a. Only 1 and 3 are correct b. Only 1 and 2 are correct c. 1, 2, 3 and 4 are correct d. Only 1, 2 and 4 are correct
4. The pilot of an aircraft which is approaching a mountain from the downwind, or leeward, side, a few hundred feet above ridge level, observes lenticular cloud above. Which of the following conditions would the pilot expect to encounter as the flight continues?
a. Strong thermal upcurrents at the top of the ridge
b. Strong downdraughts and turbulence after passing over the ridge c. Strong downdraughts immediately before the ridge of the mountain
is reached, with strong updraughts after passing the ridge to the windward side
d. Strong updraughts before the ridge is passed and strong downdraughts after the ridge is passed
CHAPTER 9: TURBULENCE QUESTIONS
5. An aircraft is flying in the vicinity of a range of hills, lying North/South, across which a wind is blowing from the West to the East. Which of the following situations might cause the aircraft to encounter dangerous downdraughts?
a. When flying East towards the hills from the West b. When flying West towards the hills from the East c. When flying North towards the hill from the South d. When flying South towards the hill from the North 6. A low level temperature inversion may produce:
a. windshear at the inversion boundary b. an on-shore breeze
c. a decrease in stability
d. good visibility by day because of the steep temperature lapse rate
Question 1 2 3 4 5 6
Answer
The answers to these questions can be found at the end of the book.
CHAPTER 10
CLOUDS AND
PRECIPITATION
CHAPTER 10: CLOUDS AND PRECIPITATION
INTRODUCTION.
This chapter deals with clouds, their formation and characteristics, their relevance to aviation, and the type of precipitation which may be associated with them. A discussion of precipitation and how it is produced will then follow.
CLOUDS.
Cloud Amounts.
Meteorologists often measure cloud amounts in units called OKTAS, literally meaning eighths of the sky. So, if half of the sky were covered with cloud, the cloud amount would be reported as 4 oktas. Figure 10.1 represents a typical sky for which the cloud would be reported as 4 oktas.
In aviation weather reports, the extent of cloud cover is normally expressed using the following words.
FEW = 1-2 oktas
SCATTERED = 3-4 oktas
BROKEN = 5-7 oktas
OVERCAST = 8 oktas
Figure 10.1 Four oktas or scattered cloud cover.
Cloud Base.
Cloud base may be defined in several ways. One definition is: “the lowest zone in which the type of obscuration perceptibly changes, from that corresponding to clear air haze, to that corresponding to water droplets or ice crystals”. A further definition of cloud base is given as: “for a given cloud layer, the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.”
In Terminal Area Forecasts (TAFs) and Meteorological Aerodrome Reports (METARs), cloud base is given as height above aerodrome level. In area forecasts, cloud base is shown as altitude above mean sea-level.
There are several ways of determining the cloud base.
• Cloud base can be reported by aircraft in flight, (see Figure 10.3).
• Cloud base may be measured by timing a balloon with a known rate of ascent, in order to calculate the vertical distance between the ground and the cloud, (see Figure 10.4).
• The most common modern tool used by aviation meteorologists to measure cloud base is the laser cloud base recorder.
In TAFs and METARs, cloud bases are given as heights above aerodrome level;
in area forecasts, they are given above mean sea level (AMSL).
Figure 10.3 Cloud base can be observed in flight.
Figure 10.4 Cloud base may be
determined by timing a balloon’s ascent. Figure 10.5 Cloud base may be measured using a laser device.
CHAPTER 10: CLOUDS AND PRECIPITATION
Cloud Tops.
The heights of the cloud tops are much more variable than the level of cloud base.
Cloud growth ceases when the air within the cloud is no longer rising or when there is no longer sufficient water vapour in the air for saturation to occur. The vertical growth of cloud comes to a halt when the temperature of the air within the cloud reaches the same temperature as that of the environmental air. We may see from a graph showing both Environmental Lapse Rate (ELR) and Saturated Adiabatic Lapse Rate (SALR) (see Figure 10.6) that the vertical growth of cloud will cease where the ELR and SALR lines intersect. This point quite often lies on a temperature inversion, which is also shown in Figure 10.6.