The Alpine föhn ( German : Alpenföhn ) is the name given to the föhn wind in the Alpine region . The name föhn was originally used to refer to the south wind which blows during the winter months and brings thaw conditions to the northern side of the Alps. Because föhn later became a generic term that was extended to other mountain ranges around the world that experience similar phenomena, the name "Alpine föhn" ( Alpenföhn ) was coined for the föhns of the Alpine region. The wind can cause heavy storms with winds of hurricane strength and top speeds of up to 150 km/h (93 mph). The south wind on the northern side of the Alps is also called the south föhn ( Südföhn ), its opposite number on the south side of the Alps is also called the north föhn ( Nordföhn ).
37-555: Föhn conditions are known for their warm air and unusual cloud and atmospheric appearance. A considerable proportion of föhn days are not accompanied by any precipitation south of the Alpine chain, so that the thermodynamic föhn theory does not explain the warm air of the Alpine föhn. The föhn phenomenon on the northern side of the Alps can, however, be explained by the fact that the air which
74-403: A fore-and-aft vessel going dead downwind. In light winds, certain square-rigged vessels may set studding sails , sails that extend outwards from the yardarms , to create a larger sail area for points of sail, ranging from downwind to a close reach. Sails for a fore-and-aft rig and a square rig in use downwind True wind ( V T ) combines with the sailing craft's velocity ( V B ) to be
111-452: A higher velocity made good downwind, by sailing on whatever broad reach is most efficient on that particular craft, and jibing as needed. The longer course is offset by the faster speed. For instance, if a vessel sails alternately in the directions 45° from the downwind direction, it will sail √ 2 (≈1.4) times farther than it would if it sailed dead downwind. However, as long as it can sail faster than 1.4 times its dead downwind speed,
148-407: A sailboat experiences a wider range of apparent wind angles than does an ice boat, whose speed is typically great enough to have the apparent wind coming from a few degrees to one side of its course, necessitating sailing with the sail sheeted in for most points of sail. On conventional sail boats, the sails are set to create lift for those points of sail where it's possible to align the leading edge of
185-409: A sailboat, which is resisted by the shape and configuration of the hull (or hulls, in the case of catamarans) and the weight of ballast, and can be further resisted by the weight of the crew. As the boat points off the wind, lateral force and the forces required to resist it become reduced. On ice boats and sand yachts , lateral forces are countered by the lateral resistance of the blades on ice or of
222-399: A stationary observer. The motive power , and thus appropriate position of the sails, is determined by the apparent wind : the wind relative to an observer on the sailing craft. The apparent wind is the combined effect of the velocities of the true wind and of the sailing craft. A sail with the airflow parallel to its surface, while angled into the apparent wind, acts substantially like
259-414: A vessel that can point into the wind, the shorter its "course made good" to an upwind destination. Beating upwind, a vessel alternates between having the wind come on the port and starboard sides (the port and starboard tack). Changing from one tack to the other, by steering through the wind direction, is called tacking , or going about . A craft sailing with the true wind on its side (within limits)
296-408: A wing with lift as a force acting perpendicular to its surface. A sail with the apparent wind perpendicular to its surface, acts substantially like a parachute with the drag on the sail as the dominant force. As a sailing craft transitions from close-hauled to running downwind, the lifting force decreases and the drag force increases. At the same time, the resistance to sidewards motion needed to keep
333-403: Is reaching . Wind is flowing over the surface of the sail, creating lift (like a wing) to propel the craft. Because lift is more powerful than drag on this point of sail, sailing craft achieve their highest speeds on a reach. A variety of high-performance sailing craft sail fastest on a broad reach with the sails close-hauled at speeds several times the true windspeed. Depending on the angle of
370-457: Is close-hauled. Sailing craft B is on a beam reach. Sailing craft C is on a broad reach. Boat velocity (in black) generates an equal and opposite apparent wind component (not shown), which adds to the true wind to become apparent wind. The speed of sailboats through the water is limited by the resistance that results from hull drag in the water. Ice boats typically have the least resistance to forward motion of any sailing craft; consequently,
407-410: Is detectable as a föhn in the northern Alpine valleys does not come from the southern foot of the Alps, but from higher up; the windward air beneath it forms a layer of stable air and is prevented from crossing the barrier. Through the deeply incised Alpine passes, some of this relatively cool, trapped windward air reaches the north as a moderate föhn. In Switzerland, on the other hand, the term "föhn"
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#1732793170843444-473: Is the side distant from or physically in the lee of the prevailing wind, and typically the drier. In an archipelago windward islands are upwind and leeward islands are downwind of the prevailing winds, such as the trade winds of the Atlantic and Pacific oceans. Points of sail A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over
481-493: Is too close to the direction from which the wind is blowing, because the sails cannot generate lift in this no-go zone. A craft passing through the no-go zone to change tacks from one side to the other, must maintain momentum until its sails can draw power on the other side. If it remains in the no-go zone, it will slow to a stop and be in irons . This is called missing stays . To recover, that craft typically must return to its original tack and pick up sufficient speed to complete
518-418: Is used only if a clearly warm downslope wind is meant, which is caused by the additional heat of condensation (thermal energy) during periods of rainfall on the southern side of the Alps (northern side of the Alps when there is a north föhn). The föhn-like high altitude winds are mostly large-scale, central Atlantic or African air masses from the southwest to south, which cause unusually warm conditions north of
555-459: The Age of Sail , a vessel always sought to use the wind to its advantage, maneuvering if possible to attack from windward. This was particularly important for less maneuverable square-rigged warships, which had limited ability to sail upwind, and sought to "hold the weather gage " entering battle. This was particularly important once artillery was introduced to naval warfare. Ships heel away from
592-424: The Age of Sail , the term weather was used as a synonym for windward in some contexts, as in the weather gage . Since it captures rainfall , the windward side of a mountain tends to be wetter than the leeward side it blocks. The drier leeward area is said to be in a rain shadow . The term "windward" has roots in both Low German and Old English. The word "lee", which means a place without wind, comes from
629-441: The apparent wind velocity ( V A ); the air velocity experienced by instrumentation or crew on a moving sailing craft. Apparent wind velocity provides the motive power for the sails on any given point of sail. The apparent wind is equal to the true wind velocity for a stopped craft; it may be faster than the true wind speed on some points of sail, or it may be slower e.g. when a sailing craft sails dead downwind. Sailing craft A
666-406: The wind . Windward is upwind from the point of reference, i.e., towards the direction from which the wind is coming; leeward is downwind from the point of reference, i.e., along the direction towards which the wind is going. The side of a ship that is towards the leeward is its "lee side". If the vessel is heeling under the pressure of crosswind , the lee side will be the "lower side". During
703-587: The Alps even at low wind speeds. Triggers are usually slow-moving or blocked Atlantic depressions in the area of the British Isles and North Sea, which move air radially at their outside edge, i.e. the cold front . In particularly extreme conditions, however, this also results in a valley föhn. They are föhn-like inasmuch as the moisture carried along has often been precipitated in the Pyrenees or French Alps . These southerly winds can bring Saharan dust to
740-407: The Alps, for example. In real föhn conditions, the clear temperature differences of well over 10 °C (18 °F) between the föhn area and its immediate neighbourhood, especially in radiation fog , are only explainable by föhn processes. When there is a föhn the temperatures may rise or fall by up to 25 °C (45 °F). Föhns are also responsible for quite a few winter temperature highs. On
777-553: The Old Norse "hle" for "cover" and has been used in marine navigation in Germany since medieval times. The word "wind," meaning "air in motion," comes from Proto-Germanic *winda- and has evolved over time, with pronunciation changes influenced by similar words like "windy." The word "wind" has been associated with emptiness and vanity since the late 13th century. Additionally, "wind" has been used figuratively in phrases like "which way
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#1732793170843814-403: The craft on course also decreases, along with the sideways tipping force. There is a zone of approximately 45° on either side of the true wind, where a sail cannot generate lift, called the "no-go zone". The angle encompassed by the no-go zone depends on the airfoil efficiency of the craft's sails and the craft's lateral resistance on the surface (from hydrofoils , outriggers , or a keel in
851-436: The indirect route will allow it to arrive at a chosen point sooner. Craft running downwind increase power from the sails by increasing total area presented to the following wind, sometimes by putting out sails that adapt well to the purpose, such as a spinnaker on a fore-and-aft rigged vessel. Another technique is to place the jib to windward (opposite to the main sail)—called "wing on wing" or one of several other terms—for
888-410: The maneuver. The span of the no-go zone depends on the efficiency of a sailing craft's sails and its resistance to sideways motion in the water (using a keel or foils) on ice or on land, typically at an angle between 30 and 50 degrees from the wind. A craft stopped in the no-go zone is said to be in irons. A square-rigged vessel in irons by accident is taken aback with the sails blown against
925-413: The mast or laid aback if deliberate. In either case, the stopped vessel will be blown backwards, which with proper positioning of the rudder allows the vessel to point outside the no-go zone and resume forward motion, once the sails can draw power. Iceboats are often parked in irons with a brake applied to the ice to prevent motion. To commence sailing, the craft is guided to one side and boarded, once
962-506: The north, and föhn windows with possibly elevated temperatures in the south. In contrast to the föhn north of the Alps, however, the north föhn will often appear as a relatively cold storm, as this wind situation usually occurs after the passage of a cold front from the west. Windward and leeward In geography and seamanship , windward ( / ˈ w ɪ n d w ər d , ˈ w ɪ n ər d / ) and leeward ( / ˈ l iː w ər d , ˈ lj uː ər d / ) are directions relative to
999-618: The northern side of the Alps, the föhn is associated with very good visibility due to the low humidity . In winter and spring the dry air and the high temperature encourages rapid snow melt. During inverse pressure conditions, on the southern side of the Alps, a north föhn arises, known in Italian as the tramontana or tedesco ("the German"). The effects are not exactly symmetrical, as northern air has different characteristics from southern air. North föhn winds result in clouds with rain in
1036-410: The sail can draw power. A sailing craft is said to be sailing close-hauled when its sails are trimmed in tightly and are acting substantially like a wing , relying on lift to propel the craft forward on a course as close to the wind as the sail can provide lift. This point of sail lets the sailing craft travel upwind, diagonally to the wind direction. The smaller the angle between the direction of
1073-478: The sail with the apparent wind. For a sailboat, point of sail significantly affects the lateral force to which the boat is subjected. The higher the boat points into the wind, the stronger the lateral force, which results in both increased leeway and heeling. Leeway, the effect of the boat moving sideways through the water, can be counteracted by a keel or other underwater foils, including daggerboard, centerboard, skeg and rudder. Lateral force also induces heeling in
1110-517: The same meaning are widely used, particularly upwind and downwind . Among sailing craft, the windward vessel is normally the more maneuverable. For this reason, rule 12 of the International Regulations for Preventing Collisions at Sea , applying to sailing vessels, stipulates that where two are sailing in similar directions in relation to the wind, the windward vessel gives way to the leeward vessel. In naval warfare during
1147-415: The surface. The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. For many sailing craft 45° on either side of the wind is a no-go zone, where a sail is unable to mobilize power from the wind. Sailing on a course as close to the wind as possible—approximately 45°—is termed beating , a point of sail when the sails are close-hauled . At 90° off
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1184-419: The true wind and the course of the sailing craft, the higher the craft is said to point . A craft that can point higher or sail faster upwind is said to be more weatherly . Pinching occurs as a craft's point of sail approaches the no-go zone and its speed falls off sharply. In order to sail upwind, sailing craft must zig-zag across the direction of the oncoming wind, called beating to windward . The higher
1221-444: The true wind with respect to the course sailed, a reach may be close , beam , or broad , as follows: Sailing with the wind or running before the wind , the sails generate power primarily through drag (like a parachute) with the true wind directly from behind the sailing craft. A sailing craft running more downwind than a broad reach cannot attain a speed faster than the true wind. However, higher-performance sailing craft achieve
1258-407: The water, runners on ice , or wheels on land ). A craft remaining in its no-go zone will slow to a stop—it will be "in irons". The recognized points of sail are judged relative to the true wind direction. They include: The range of directions into the wind, where a sailing craft cannot sail is called the no-go zone . A sailing craft cannot sail directly into the wind, nor on a course that
1295-657: The wind blows" to indicate the current state of affairs. The suffix "-ward," meaning "toward," is an adverbial suffix in Old English derived from Proto-Germanic *werda-, which itself comes from the PIE root *wer- meaning "to turn, bend." The original notion of "-ward" is "turned toward." Windward and leeward directions (and the points of sail they create) are important factors to consider in such wind-powered or wind-impacted activities as sailing , wind-surfing , gliding , hang-gliding , and parachuting . Other terms with broadly
1332-423: The wind, a craft is on a beam reach . The point of sail between beating and a beam reach is called a close reach . At 135° off the wind, a craft is on a broad reach . At 180° off the wind (sailing in the same direction as the wind), a craft is running downwind . A given point of sail (beating, close reach, beam reach, broad reach, and running downwind) is defined in reference to the true wind —the wind felt by
1369-419: The wind, so the leeward vessel would expose more of her topsides to shot, in extreme cases even part of her bottom. The terms windward and leeward are used in reference both to sides (and climates ) of individual islands and relative island locations in an archipelago . The windward side of an island is subject to the prevailing wind , and is thus the wetter (see orographic precipitation ). The leeward side
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