Anyone who has whiled away a hot summer day by a lake or sea is bound to have noticed that there is always a cool breeze blowing in from the water. But why does this cool breeze from the lake or sea blow towards the banks? This is because of the sea breeze, which is part of the land and sea breeze cycle. In this article, we attempt to provide a physical explanation of the land and sea breeze system.
What is the land-sea breeze system?
The land and sea breeze cycle is a thermally driven local wind system that is prominent on seacoasts but also occurs to a lesser extent on the banks of larger lakes. The prerequisites for the emergence of a diurnal wind system include minor but large-scale differences in atmospheric pressure, as well as insolation and terrestrial radiation that are impeded as little as possible by cloud. Provided there are no strong winds – due to the general weather situation – that prevent the formation of a land-sea breeze system, the wind direction can be seen to switch depending on the time of day along seacoasts and the banks of larger lakes. The origin of the land and sea breeze cycle is thermal in nature and is primarily due to the differing thermal behaviour of land and water.
How does the land and sea breeze cycle form?
Prerequisites for the formation of a land-sea breeze system include weather situations that are characterised by balanced pressure conditions. This is illustrated by the horizontal isobars (lines of equal atmospheric pressure) in Figure 1a). In the late mornings, after the sun has risen, the lower thermal capacity of the land surface relative to the water surface means that the land heats up many times faster. Air over the land therefore becomes warmer, while the air over the water barely changes. The warmer air begins to rise over the land, raising the altitude of the isobars (see Figure 1b)). Consequently, the atmospheric pressure is higher over the land than over the water at the same altitude (dashed line). A small-scale upper-level high (H) therefore forms over the land, and an upper-level low (L) forms over the water. The atmosphere wants to equalise these pressure differences, and an upper-level compensatory flow forms from land to water (see Figure 1b)). Due to this flow, "mass" accumulates over the water, leading to a pressure increase at the surface and thus to a localised surface high (H). Conversely, the surface pressure falls over the land (air flows away upwards) and a surface low (L) forms (see Figure 1c)). At the surface, a compensatory flow then becomes established, flowing from the surface high over the water to the surface low over the land. Due to its origin, this wind is known as a "sea breeze" (or "onshore wind"), and it often transports cooler sea or lake air towards the land. These pressure distributions are aligned in exact opposition to one another at the upper level and the surface, ultimately producing a closed cycle.
In the morning there is no wind; late in the morning warm air rises over the land; in the afternoon a sea breeze develops from the water toward the land; at night the pattern reverses and a land breeze blows from the land toward the water.
At night, the flow conditions reverse. After the sun sets, the land surface and the air above it cool down faster than over the water, producing a thermal high over the land. A thermal low forms over the water. Due to these pressure conditions, which are the opposite of those seen during the day, the circulation reverses over the course of the night, forming the "land breeze" ("offshore wind"), which blows from the land towards the lake (see 1d)).
In Switzerland, land and sea breezes can be found at some larger lakes, such as Lake Constance, Lake Geneva or Lake Zurich, as well as at Lake Lucerne (see "Oberwind" below). Particularly in the vicinity of the Alps and their foothills, however, the land-sea breeze system is often overlaid with an upslope or valley wind.
The "Oberwind": a local sea breeze in the Lucerne basin
The "Oberwind" (a fair-weather wind) is a thermal sea breeze blowing from a southeasterly to easterly direction in the Lucerne basin, at the lower end of Lake Lucerne. It reaches a wind speed of 5 to 10 km/h and sometimes slightly more. The Oberwind extends from the entrance of the Lucerne basin, along the line between the Meggenhorn and Haslihorn mountains, to the city of Lucerne. In sunny weather, it blows in the second half of the morning, until around midday, and subsides again towards the evening. It tends to occur mainly in the warmer months. In winter, it is rarer and only develops under certain temperature conditions.
Formation of the Oberwind
Insolation warms not only the built-up areas of the city of Lucerne, at the lower end of the Bay of Lucerne, but also the south-facing slopes of the basin, such as those of the Dietschiberg. This contrasts with the relatively cool water surface of the Bay of Lucerne, as well as the Kreuztrichter basin (where the four arms of the lake meet). An air flow therefore emerges, moving from the relatively high pressure over the cooler water surfaces to the relatively low pressure over the warmed city and the adjacent sunny slopes. The development of the Oberwind is favoured if the overlaid wind field is generally coming from a southerly direction. On the other hand, the Oberwind is inhibited or prevented by the presence of overlaid northerly winds or an increase in pressure.
The Oberwind is generally driven by insolation. However, there is also a variant that occurs in bad weather. In this case, it is cloudy over Lake Lucerne and there is still occasionally some precipitation falling in the Alpine foothills. In the Swiss plateau, on the other hand, conditions are already partly sunny. This causes cooler air to flow outwards from Lake Lucerne towards the northwest. This constellation can occur following the passage of a cold front, for example, if there is residual cloud clinging to the Alpine foothills.
Another special case of the Oberwind occurs in conjunction with showers or thunderstorms. This version is significantly stronger and gustier, reaching wind speeds of over 20 km/h. The prerequisites for this type of Oberwind generally include multiple showers or thunderstorm cells over Obwalden and Nidwalden. These conditions cause a flow of cooler air northwards towards the Swiss plateau. A strong southeasterly wind temporarily blows over the Lucerne basin.