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The air UHI in the UCL (below the level of the buildings' roofs, Figure 5.1)
occurs in 95% of the nights and 85% of the days, with median intensities between
1.7 and 2ÂșC. It is more intense in summer and in situations with moderate W and
NW winds, in opposition to the general rule of stronger UHI in windless situations.
This is because Tagus and Ocean breezes frequently occur in Lisbon on windless
situations cooling the southern districts. Daytime UHI in summer was less frequent
at 2 pm than at 5 pm (Figure 5.6c) because at 2 pm the city is often affected by
Tagus and Ocean breezes that reach Restauradores (R in Figures 5.3b and 5.5) and
occasionally Saldanha (S, Figures 5.3b and 5.5); at 5 pm the breeze has often been
replaced by the prevailing N or NW winds [ALC 87] (see section 5.7.3.2). The UHI
intensity remains high all night long during spring and summer (Figure 5.6b), while
in autumn and winter the UHI is at its highest intensity after sunset according to the
general model [OKE 87].
Figure 5.6. UHI in Lisbon [ALC 07]. a) Frequency of nocturnal UHI;
b) mean intensity of nocturnal UHI; c) frequency of diurnal UHI
5.7.3. Measuring and modeling wind over Lisbon
5.7.3.1. Prevailing N wind in the summer
Wind is another weather parameter modified by urban areas. As shown in Figure
5.7, the prevailing winds at Lisbon Airport (located windward of the city, Figure
5.4b) are N and NW, particularly during the summer months. Urban roughness
decreases air flux and, therefore, the urban logarithmic wind profile is different from
that of an open area [MOR 93].
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