2.3 Ingeniería de Requisitos
2.3.2 Definición de requisitos
The rest of this chapter deals with more localised winds. The first of these are land and sea breezes. Land and sea breezes are local winds that can be found along coastlines, particularly, but not exclusively, in the tropics. Land and sea breezes only occur when the general wind conditions are slack and there are no major pressure systems dominating the coastal regions.
Sea Breezes.
Figure 12.13 shows a cross section of the land and sea during the day. The land will heat up faster than the sea and will therefore be warmer. As a result there will be warmer, less dense air above the land surface than above the surface of the sea.
As thermal mixing increases during the
day the surface wind speed becomes closer to the geostrophic wind speed and so does the Coriolis force affecting it. As these both increase the surface wind will veer towards the direction of the geostrophic wind. As the thermal mixing decreases over night the surface wind speed is reduced by friction; this reduces its Coriolis force and the wind backs and decreases.
Figure 12.12a Diurnal Variation of the Surface Wind.
Figure 12.13 The Sea Breeze.
This warm air over the land will start to rise and the cooler air over the sea will tend to sink. This will gradually set up a small scale cycle of air up to about 1 000 ft as shown. Notice that at the surface the wind will blow from the sea to the land, and hence be called a “sea breeze”. This wind will reach its maximum strength when the lands is hottest which will be at about 15:00 local time.
Land Breezes.
The process is reversed at night. Figure 12.14 shows a cross section of the land and sea during the night.
The land will cool down faster than the sea. As a result there will be warmer, less dense air above the sea surface than above the land. This air will be heated and start to rise. This situation will cause a localised reduction in pressure and air will start to be drawn in from over the land towards the sea.
The airflow from the land to the sea is known as a land breeze, and its strength will be maximum just after sunrise where we have the minimum land surface temperatures.
Again this flow of air creates a circulation pattern up to approximately 1 000 ft.
Figure 12.14 The Land Breeze.
Figure 12.15 The sea breeze and the land breeze will produce a cross wind day and night at this coastal airfield.
During the day the land surface becomes warmer than the sea and the surface wind blows form sea to land, a sea breeze. At night the situation is reversed and the breeze blows from land to sea, a land breeze.
CHAPTER 12: LOW LEVEL WINDS
Land and sea breezes are particularly important to coastal airfields. Shown in Figure 12.15 is an airfield near the sea with a runway parallel to the coastline. During the night and day, the sea and land breeze effect will always produce a wind across the runway, but remember that sea and land breezes will only develop when there is a weak pressure gradient, but are still subject to Coriolis force, and will, if persistent, end up blowing parallel to the coast.
Anabatic Winds.
There are another two other types of localised winds worth a mention. Again, these only occur when the general wind conditions are slack and there are no major pressure systems dominating the region. They are anabatic and katabatic winds.
On the sloping terrain in Figure 12.16, during the day, the land will warm and in turn warm the air adjacent to the surface.
This air will be less dense and start to flow up the slope, to be replaced by cold air flowing in from elsewhere. These upward flowing winds are called Anabatic winds.
Sometimes clouds on the top of the ridges are signs that anabatic winds are present.
Always treat mountain ridges with extra caution regardless of the presence or lack of clouds.
Katabatic Winds.
During the night the process is reversed.
The surface of the slope will cool faster than the air in the valley and the air overlying it will become heavier and denser. As a result it will sink down the sloping terrain and collect in the valley bottom below. This wind is called a Katabatic wind as illustrated in Figure 12.17. Throughout the night, cold air will continue to collect in the valley. If the dew point is reached this can lead to the formation of valley fog.
Figure 12.16 Anabatic Wind Formation
Figure 12.17 Katabatic Wind Formation
The Föhn Effect.
A Föhn wind is caused when stable air flows over a mountain and descends on the other side. As air is forced over a mountain it will cool at the dry adiabatic lapse rate of 3ºC per 1 000 feet. If the dew point is reached then cloud will form. From then on, if forced ascent continues it cools at the Saturated adiabatic lapse rate of 1.5ºC per 1 000 feet. There may be precipitation produced on the windward slope which reduces the water content of the air mass. However, because the air is stable, it will flow back down the leeward side of the mountain and as it descends it will warm adiabatically.
At first it warms at the saturated adiabatic lapse rate until the cloud disperses and then at the dry adiabatic lapse rate as it descends further.
Notice that the air on the leeward side is mostly warming at the DALR rate of 3°C per 1 000 ft. This creates some unique effects. If the air on the windward side cooled mainly at 1.5°C per 1 000 ft but when it descended on the leeward side it warmed at 3°C per 1 000 ft then the air at the base of the leeward side will have become much
warmer than when it started on the windward side. (See Figure 12.18).
The Föhn wind is a fairly common occurrence in the Southern Alps especially so in winter as the air from mainland Europe gets drawn up and over the mountains into the Mediterranean. These winds can cause major problems in certain areas of the world causing large scale rapidly moving forest fires and this is famously apparent in California around Los Angeles where this wind is called the “Santa Ana”.
This completes the chapter on low level winds. Surface winds are always best understood when comparing them to the geostrophic wind and so a good comprehension of how the geostrophic wind occurs and of the modifications it undergoes in the friction layer are essential. When examining a pressure chart to find the winds to help in flight planning, do not forget also to bear in mind the local effects of hills, valleys and sloping terrain. These may significantly alter the wind from what is suggested by the surface pressure charts.
Figure 12.18 The Föhn Wind Effect.