Chapter 34: Weather Elements
Pressure patterns drive wind; air masses and fronts drive weather changes. Reading a synoptic chart lets the navigator anticipate rather than react.
Pressure, wind and the wind laws
Isobars connect points of equal pressure; the closer they are, the stronger the pressure gradient and the wind. Above the friction layer wind blows parallel to the isobars; at the surface friction backs it 10°–20° over sea and more over land. Buys Ballot's Law: in the northern hemisphere, stand with your back to the wind and low pressure is on your left. Circulation is counter-clockwise and inward around a northern-hemisphere low, clockwise and outward around a high; reversed in the southern hemisphere.
- Tight isobars = strong wind
- Buys Ballot (NH): back to the wind, low on the left
- Surface wind is backed from the geostrophic direction by friction
Air masses and fronts
Air masses are classified by source: maritime or continental, tropical, polar or arctic. A warm front brings a long slope of cloud — cirrus, then altostratus, stratus, steady rain — with slow veering and rising temperature. A cold front is steeper and faster: cumulonimbus, squalls, a sharp veer, a pressure rise and clearing. An occluded front forms when the cold front overtakes the warm and mixes the two signatures. A stationary front lingers with persistent cloud.
Fog and visibility
Advection fog forms when warm moist air moves over a colder sea — the Grand Banks and California coast — and can persist in strong wind. Radiation fog forms over land on clear calm nights and drifts seaward at dawn. Sea smoke (arctic smoke) forms when very cold air passes over warmer water. Frontal fog accompanies warm-front precipitation. Dew point approaching air temperature is the warning sign; a psychrometer and a rising sea-air temperature difference give useful lead time.
Key takeaways
- • Isobar spacing is a wind-speed gauge; use Buys Ballot to locate the low.
- • Cold fronts bring squalls and a sharp veer; warm fronts bring long steady rain.
- • Advection fog is the fog that persists in wind — plan for it, do not wait for it.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 34 quiz
0 of 3 answered
1.In the northern hemisphere, standing with your back to the surface wind, low pressure lies:
2.The passage of a cold front is typically marked by:
3.Advection fog forms when: