Orals - Meteorology
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millibar? Q. What is atmospheric pressure? What is hPa and how is it related to millibar? A. Atmospheric pressure is the pressure exerted by a column of earth’s atmosphere. It is the force exerted by the column (given by: mg) per unit area. Atmospheric Atmospheric pressure is also known as barometric pressure. 2 On average, this column weighs about 1.02 kg over an area of 1cm . This is known as 1 bar. Millibar (mb) is used for smaller values. However as per SI units, mb is now being replaced by hPa. hPa stands for hecto Pascal an d is 2 equal to 100 Pascals. (hecto means 100). Pascal is SI unit of pressure and is measure d in N/m . Numerically, 1 hPa = 1 mb How 1 hPa equals 1 mb? 2
1 hPa = 100 Pa = 100 N/m .
2
1 bar corresponds to 1.02 kg column per cm 2 2 Or, 1 bar = 1.02 kg f/ cm = 1.02 x 9, 81 N/cm 2 2 = approximately 10N/cm = 100,000 N/m 2
Thus, 1 mb = 100 N/m = 100 Pa = 1hPa Q. What do you understand by pressure gradient? A. Pressure gradient means rate of change of pressure with distance. Thus it is a measure of how strongly or mildly the wind will blow across isobars. Winds always blow from high pressure area to low pressure area and the force driving them is the pressure gradient. Greater the pressure gradient, stronger is the wind and vice versa. Graphically pressure gradient is represented by spacing of isobars on weather maps. Closer the isobars, greater is the pressure gradient and vice versa. Q. The low pressures at the centers of a TRS and a frontal depression are comparable to each other; yet the winds associated with a TRS are much stronger. Why? A. It is because the isobars associated with a TRS are closely packed (pressure gradient is high) and isobars in frontal depression are spaced wider (pressure gradient is low). Or, the average diameter diameter of a TRS is about 500 miles and that th at of a depression is about 1500 miles.
temperature and what is its significance to a mariner? Q. What is dew point temperature A. Dew point temperature temperature is the temperature at which dew forms or water vapour present in the air condenses. At this temperature, dry bulb and wet bulb thermometers have the same reading. At this moment relative humidity is 100 % or is saturated. It is significant on two counts: 1. It helps a mariner in deciding when and whether or not to ventilate ventilate cargo holds for preventing possible damage to cargo due to cargo or ship sweat. 2. It is useful in knowing if vessel is likely to encounter sea (advection) fog. The approximate time time of meeting the fog can also be assessed. Q. So, how will you decide weather to or not to ventilate cargo hold?
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Orals - Meteorology
A. Decision is made after comparing dew point temperature of air with cargo hold temperature. If dew point temperature is greater than cargo hold temperature: Do NOT ventilate hold If dew point temperature is less than cargo hold temperature: temperature: DO ventilate hold Q. How is fog predicted at sea? s ea? A. Fog is predicted by plotting sea water temperature and dew point temperature of air against time on a common scale. The readings are plotted at intervals of at least 10 minutes. If the two curves are converging, they can be extended to meet at point, the time reading of which will give the predicted time of fog. This procedure is recommended especially when air temperature is same as or mor e than sea water temperature. Q. When, in a year, is fog likely to be encountered the most? temperate and higher latitudes) during spring and an d A. Fog is most likely to be encountered ( in temperate early summer, because during these times the sea temperatures are quite low. Q. Why is it important to post extra look out at high place (like Monkey Island) in restricted visibility? (advection) fog is usually shallow and affects visibility at low levels only. At higher A. Sea (advection) heights the visibility is usually better and masts etc, of a vessel can be seen clearly and reported to bridge team. Q. How should sea swell be recorded in log book? A. Sea swell should be recorded in terms of its height and length. Length usually precedes height. Height: Low: 0 – 2 m Moderate: 2 – 4 m Heavy: > 4 m Length: Short: 0 – 100 m Average: 100 – 200 m Long: > 200 m Examples: Short moderate swell, long heavy swell, average moderate swell etc. Q. What do you understand by doldrums? A. Doldrums is often used by mariners to describe a broad belt of low pressure area encircling the earth around equator. High temperatures around this area result in low pressure, causing air to rise and travel towards N and S latitudes. On reaching higher latitudes, the air descends and comes towards doldrums in the form of NE and SE trades. Pressure gradient being low, winds are usually mild or calm in this belt. This belt moves N and S seasonally in synchronization with the sun’s declination, though with a time lag of about 1.5 to 2 months. This movement is more pronounced in the vicinity of large landmasses. Doldrums is also known as Equatorial Trough (Interestingly, the dictionary meaning meaning of doldrums is a state of inactivity, slump or stagnation).
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Orals - Meteorology
Within the doldrums, lies ITCZ – a zone where NE and SE trades converge. It is characterized characterized by a line/band of dense cumulonimbus clouds associated with heavy rains, thunderstorms and squalls. Thus weather in doldrums doldrums is calm/light variable winds interspersed with squalls and thunderous showers. It is the doldrums; where the disturbances originate which later develop develop into tropical storms. Q. What do you understand by isopleths? A. Isopleths are lines or curves representing equal values. Thus isotherms are isopleths of temperature temperature and isobars are isopleths of pressure. Q. Define isallobars? A. Isallobar is a line/curve joining places having equal change in atmospheric pressure in a given time interval. Thus, they are lines showing equal pressure tendencies. tendencies. It is of two types: Anallobar or Positive Isallobar: line/curve joining increasing pressure changes or positi ve pressure tendencies. Katallobar or Negative Isallobar: line/curve joining decreasing pressure changes or negative pressure tendencies. They are used to forecast the movement movement of low and high pressure systems. While lows move in the direction of maximum pressure falls, highs move the direction of maximum pressure rises. The speed of movement of the pressure systems is proportional to the isallobaric gradient.
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