Chapter 4 summery Flashcards
Tropical storm:
An organized center of low pressure that originates over warm tropical oceans
The maximum sustained surface winds of tropical storms
Range from 63 to 118 kmph
Tropical storms generally develop
During the warmer months (late summer and early autumn)
The main parts of a tropical cyclone:
- Rain bands
- The eye
- Air spiral in toward the center in a counter-clock wise pattern in NH and out the top in the opposite direction in the center of the storm air sinks forming an eye
- Mostly cloud free
- The eyewall
- The strong rotation of air around the cyclone balances inflow to the center causing air to ascend about 16-32 km from the center forming the eyewall
- Consist of tall thunderstorms that produce
- Heavy rains
- The strongest winds
The destructive high winds accompanying tropical cyclones produce:
Phenomenal seas (wave height larger than 14 m) which are dangerous both for vessels out at sea and those moored in harbors.
How are swells created?
The strong winds associated with offshore storms blow on the surface of the sea
Swells:
Waves that have been created sometime before by the winds blowing in an area some considerable distance away
The size of the swell depends on:
- The intensity of the wind within a storm
- Fetch
- The amount of distance the wind blows across the water
- The amount of time the wind blows at a sustained speed
The long swell ahead of a tropical storm is sometimes experienced at a distance
Of more than 1000 miles from the storm’s center
The long swell ahead of a tropical storm travels
- Much faster than the storm itself
- Cause storm tide
- Result in disastrous flooding on the coast
- Cause storm tide
A storm generates
Winds that impact a region over open water
Fetch:
The “length” (distance) wind blows over open water
Sea:
Area where wind waves are generated, mixed period and wavelengths
Ocean swell:
Refers to series of ocean surface waves that were not generated by the local wind
Tropical cyclone is a major hazardous weather system for:
Ships travelling in the oceans
Important prediction for tropical cyclones approaching:
- Cyclone intensity
- Movement
Essential information to understand for mariners for safe voyage
- Wind
- Wave distribution in the vicinity of the tropical cyclone
Tropical cyclone route and air flow in the NH:
- Travel from east to west
- Air flow towards the center of a tropical cyclone in a counter-clockwise direction
Dangerous semicircle:
- High winds and waves are normally most prominent on the right hand side of the moving cyclone
- The speed of forward movement of the cyclone itself is added on top of the wind circulating around the cyclone
During the tropical storm season the navigator must
Exercise constant vigilance to ensure that he is not caught unaware in the path of the storm
The mariner’s aim must be to
Avoid getting near the center of the storm
Certain common features of these storms have enabled:
The evolvement of guidance ‘rules’ (warning signs) to help in keeping the ship out a serious trouble
Warning signs include:
- Barometric pressure
- Appearance of the sky
- Swell
- Visibility
- Winds
Note: except for the variation of the barometric pressure, any one of the above warning signs, if taken alone is only an uncertain indication of the approach of a TS. All these warning signs must be considered together.
Barometric Pressure:
If there is a marked departure in the barometric pressure (3 hPa or more below the mean pressure) from the diurnal variation there is a risk of a tropical storm forming or developing.
- The barometric pressure is as low as 5 hPa below normal, it should be taken to mean that a TS is almost certainly somewhere in the area and probably within 200 miles of the observer.
Appearance of sky:
Vivid (bright) coloring at sunrise and sunset are often a warning feature, accompanied or followed by high cirrus clouds of the eye.
- Cirrus may first appear when the storm is from 300 to 600 miles away and is often the first warning of a TS.