Lesson 4 Weather

Question 1

1. What is a Synoptic Chart?

Answer 1

A map of a geographic region that represents atmospheric conditions that have been analysed or forecast for a given period of time. Aka weather charts, pressure charts or mean sea level pressure charts.

Question 2

2. What is an isobar?

Answer 2

A line on a weather chart that joins places that have equal atmospheric pressure.

Question 3

3. What do isobars on a chart do?

Answer 3

Isobars define the areas of high pressure ( anticyclones or ridges) and low pressure ( cyclones or troughs) . They indicate where there is a pressure gradient and therefore stronger winds , I.e. where isobars are closer together, and therefore stronger opposite where they are further apart.

Question 4

4. What are the 2 basic rules that govern wind direction in the southern hemisphere?

Answer 4

1) winds tend to flow outwards from and anticlockwise around a high pressure system. ( isobars close together)
2) winds tend to flow into and clockwise around a low pressure system.

Question 5

5. What is a ridge?

Answer 5

An elongation of isobars in a high pressure system.

Question 6

6. How are troughs shown on a synoptic chart?

Answer 6

Dashed lines across the isobars

Question 7

7. When is Australia’s tropical cyclone season ?

Answer 7

Between November and April

Question 8

8. What action should mariners take to prepare for a cyclone?

Answer 8

• find a safe haven
• secure hatches, clear decks, stow loose equipment, board or tape large windows
• centralise weight distribution & keep heavy objects low in the vessel.
• check bilge pump
• remove engine from tender , may fill tender with water to prevent it blowing around.

Question 9

9. What is a barometer?

Answer 9

An instrument that measures the weight of a column of air above it.

Question 10

10. In general, what does a rise in barometric pressure indicate?

Answer 10

Improving conditions

Question 11

11. In general, what does a sharp drop in barometric pressure indicate?

Answer 11

Worsening weather

Question 12

12. In Queensland, what is the average barometric pressure?

Answer 12

1013 HPa

Question 13

13. What is a flood tide

Answer 13

An incoming tide

Question 14

14. What is an ebb tide?

Answer 14

An out going tide

Question 15

15. What is the ‘range of tide’?

Answer 15

The difference between a high and a low tide. ( or vice versa)

Question 16

16. What is a spring tide?

Answer 16

The tide during a full or new moon, when there is the greatest range of movement.

Question 17

What is a ‘Neap tide’?

Answer 17

The tide when the moon is in the quarter phase , when there is the lowest range of movement.

Question 18

What is the ‘rule of twelfths’?

Answer 18

A tough method of calculating the height of the tide for a certain time based on the tide movement over a 6 hour period.
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