Wind Direction

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Winds drawn in towards the centre of a tropical cyclone do not trace the circular or elliptical shape of the isobars shown on synoptic weather charts, but instead follow spiral streamlines (Fig. 5.11). The angle of deviation of wind

Tropical Cyclones
Fig. 5.11. Surface streamlines for an intense tropical cyclone, drawn with respect to an arbitrary southward direction of motion. Adapted from Simpson and Riehl (1981).

bearing from the isobar can be measured, and varies from 20° to 40°, depending on the distance from the storm centre. Localised influences interfere with the general pattern of streamlines. There is much disruption of air flow over and around obstructions, such as irregular topography or buildings in urban areas, giving gusty winds with erratic changes in direction. Squally cells of heavy rain embedded within the storm also tend to show frequent fluctuations in wind pattern.

The combined effect of the inward-spiralling wind pattern and the general advance of a tropical cyclone along its track means that at a particular location the bearing of the wind changes as a storm approaches, passes nearby and then moves away. Such shifts in wind direction are illustrated in Fig. 5.12, which shows the track of Tropical Cyclone Eric in January 1985 across the southern tip of Espiritu Santo island in Vanuatu. At Luganville town on the southeast coast of Santo, the wind first blew from the northeast while the cyclone approached from the west. As TC Eric came nearer, the winds veered round to the northern quadrant, at the same time intensifying from gale to

Wind Direction Cyclone

Fig. 5.12. Changing wind direction and intensity for Espiritu Santo island in Vanuatu, during the traverse of Tropical Cyclone Eric from 15 to 16 January 1985. At Luganville town in the path of the eye, the wind bearing swung around a full 180°, veering from the northeast as the cyclone approached to the southwest as it migrated away. Dates and times (dd hh:mm) are given in GMT.

Fig. 5.12. Changing wind direction and intensity for Espiritu Santo island in Vanuatu, during the traverse of Tropical Cyclone Eric from 15 to 16 January 1985. At Luganville town in the path of the eye, the wind bearing swung around a full 180°, veering from the northeast as the cyclone approached to the southwest as it migrated away. Dates and times (dd hh:mm) are given in GMT.

storm to hurricane force. Conditions then dropped to calm as the eye passed over Luganville shortly before midday on 15 January (GMT).

On the other side of the eye, hurricane-force winds returned again, but now blowing from the south. Thereafter the wind intensity waned to storm and then to gale force. The wind bearing veered to the southwest as the track took TC Eric away to the east of Santo island. Overall the wind bearing at Luganville swung round a full 180° from northwest to southeast during the direct overhead traverse of TC Eric. Elsewhere in Vanuatu fluctuations in wind direction were also experienced, but the swing in degrees as the storm passed was less for islands farther away from the line of the track.

Hourly Incident Winds direction (° true)

Maximum Wind Gusts 20 (m s-1) 10 0

Hourly Incident Winds direction (° true)

Maximum Wind Gusts 20 (m s-1) 10 0

Wind Bearing Directions

Fig. 5.13. Relationship between shifting wind direction and fluctuating maximum wind speed at Apia in Samoa, during the passage of Tropical Cyclone Heta in early January 2004. Extract from Australian Bureau of Meteorology (2004).

Fig. 5.13. Relationship between shifting wind direction and fluctuating maximum wind speed at Apia in Samoa, during the passage of Tropical Cyclone Heta in early January 2004. Extract from Australian Bureau of Meteorology (2004).

Figure 5.13 provides another example, showing the relationship between fluctuating wind direction and maximum wind speed measured at Apia, the capital of Samoa, during the passage of Tropical Cyclone Heta in early January 2004. Shifting wind directions with respect to the orientation of a coastline, especially whether winds are offshore or onshore, are important for determining the extent of sea flooding by storm surge. This is examined in Section 5.3.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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Responses

  • fannie
    Why wind changes directions with cyclones?
    8 years ago
  • esko
    What causes wind to shift direction in cyclone?
    8 years ago
  • wilibald
    Why are there changes in wind speed for cyclones?
    6 months ago
  • pandora
    Why is there changes in wind direction in cyclones?
    6 months ago

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