Orographic Cloud

Hills deflect winds upwards, so that the air cools, possibly to dewpoint. In that case, clouds form at the Lifting Condensation Level (LCL) (Note 8.B), shown in Figure 8.1. For example, the west coast of the South Island of New Zealand is more cloudy and wet than the east coast (Chapter 10) because of the orographic uplift of the mainly westerly winds.

A cap cloud, shaped like a contact lens, forms at the crest of an isolated mountain, if winds rise over it sufficiently high to attain the Lifting Condensation Level (LCL). Wind blows through the clouds, condensing on the upwind side and evaporating in the lee. The first reported unidentified flying objects (UFOs) were actually cap clouds, seen above an isolated 4 km-high volcano, Mt. Rainier in Washington State, USA. The likelihood of cap clouds depends on the atmosphere's stability and the wind speed: a stable weak wind tends to flow around rather than over a mountain.

The height of the LCL ranges from below 300 m in winter in wet regions of high latitude, to more than 3 km over northern Chile or inland Australia, which are dry. It is governed by the difference between dry-bulb and dewpoint temperatures at the surface (Note 8.B), so cloud base is usually lower at night than during the day, in winter than in summer, and at the coast than inland. A city's relative dryness (Section 6.4) raises the LCL there by a few hundred metres.

Apart from cap clouds on the mountain-tops, a range of mountains can also create wave clouds, which are long and lens-shaped

Orographic Lifting Diagram

temperature

Figure 8.1 Derivation of the Lifting Condensation Level (LCL) and the Convection Condensation Level (CCL) for a measured temperature profile and surface dewpoint Td (at J) plotted on a skew T—log p diagram.

Figure 8.1 Derivation of the Lifting Condensation Level (LCL) and the Convection Condensation Level (CCL) for a measured temperature profile and surface dewpoint Td (at J) plotted on a skew T—log p diagram.

Condensation Diagram

Plate 8.1 Orographic clouds on the peaks at the south end of Lord Howe Island at 32°S.

Figure 8.2 The formation of mountain waves. Wind crossing a mountain range experiences a wave motion, which creates the possibility of lenticular clouds in the crests of the waves.

(lenticular), parallel to the range and downwind of it (Figure 8.2). They are due to waves in the atmosphere of several kilometres amplitude, caused by the range when there is a stable layer near the ridge-top level, against which the air bounces down again. The wind has to be at least 7-15 m/s across the ridge of the range, and faster at higher levels. Uplift towards the wave crests creates the cloud, if the wind is close to saturation.

Wave clouds are stationary; the air moves through them, condensing on the uphill side of the crest and evaporating on the downward side. The space between adjacent rows, called the foehn gap, can be used to estimate the wavelength of the waves, typically 10-20 km. Five or more rows may sometimes be seen in satellite pictures of eastern New Zealand, when westerlies blow over the north/south ranges. Also they often occur over Sydney when strong westerly winds blow over the Blue Mountains just inland. The waves have their highest amplitude well above the height of the mountain ridge, e.g. the effect of a range 1 km high can be felt at 10 km. But the uplift in wave clouds is insufficient to create rain.

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Renewable Energy 101

Renewable Energy 101

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