Basic Weather

8/3/2019 Basic Weather

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BASIC WEATHERErin Moore

S/V Romany Star

Be aware of your surroundings and how they can affect wind and waves You can experience local or “loco” conditions that are specific to your area and won’t be part of anyforecast.

Land and Sea Breezes

These occur in coastal areas and are important to consider when anchoring. As the land heats up duringthe day, the warm air above it rises, and the cooler air over the water moves in to replace it. At night,the relatively warm air over the water rises and is replaced by the cooler air over land, thus the “landbreeze”.

Land features

Winds coming down canyons and off promontories and headlands often blow as much as twice thespeeds forecast for the area. Watch for this in moutainous areas (especially the north of Baja), andwhen rounding a point (the “punta effect”). The Bahia L.A. area is known for the “elefante” winds thatfunnel down from canyons between high mountains to the west.

Keep an eye out at all times

Keep your weather eyes peeled. You don’t want to be caught by surprise by a squall you would havenoticed had you looked behind you to see an approaching bank of roiling black clouds.

As a boater, you’re most interested in wind strength and direction and wave height. The longer thewind has been blowing and the larger the area of water it’s been blowing over (called “fetch”), thehigher the wave height will be. It can take a full 24 hours for big, sloppy seas to die down after ahowling norther in the Sea of Cortez. So how are you going to know what’s coming when you’rethinking of crossing the Sea or just heading up to Partida Cove?

A good place to start is to listen to weather forecasts over your HF radio.

A few SSB and Ham radio nets with weather for Mexico

Amigo Net with Don: SSB Net. 8.122. 1400 warm-up/1415 weather (7:15 a.m. MST)Sonrisa Net* with Geary: 3.968. 1430/1445 (7:45 a.m. MST)Southbound Net with Don: 6.516. 0145/0205. (7:05 p.m. MST)

All times are Zulu (UTC or Greenwich Mean Time). Mountain Standard=UTC-7Some nets observe Daylight Savings Time; some don’t.* HAM Net (license required to transmit)

You can read Geary’s Amigo net forecast on line at www.sonrisanet.org. Click on “Today’s Weather

Recap”.

If you want to familiarize yourself with HF radio forecasts, it’s good practice to listen to the weatherfor a week straight. Have a chart handy for reference until you’re familiar with the geography of thearea, especially the towns and points (Abreojos, Cabo Falso, Pt Concepcion, Cabo Corrientes). It getseasier once you learn the names and the pattern of the forecast (e.g., Geary does today and tomorrowfor many areas. Outside first, then N to S on the inside of the peninsula, then N to S mainland side.Then he does a 3-day forecast.) Northern and Southern crossings:N: San Carlos to Santa Rosalia



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S: Mazatlan to Muertos

But what are they talking about on these nets?

High and Low Pressure Systems

Warmer air rises, and cooler air sinks. At the poles, cool air sinks toward the earth, creating vast areas

of high pressure. Think of the air “pressing” down on the earth. At the equator, warm air rises from theearth’s surface, creating areas (also called “systems”) of low pressure. If the earth didn’t rotate andthere were no land masses, movement of high and low pressure areas would be simple—southwardfrom the poles at low altitude, and northward from the equator at high altitude. But the rotation of theearth and the friction caused by land masses make things much more complicated. Just remember thatmasses of air of different temperatures are associated with all pressure differences.

What do Areas of High and Low Pressure Mean to You on the Water?

High pressure usually leads to clear skies, lighter winds, and a reduced chance of precipitation.

Low pressure leads to unsettled weather with increased cloudiness, increased winds, increasedtemperatures, and an increased chance of precipitation—increased everything! Hurricanes and winterstorms are intense varieties of low pressure systems. To figure out where a low is in relation to yourposition, stand with your back to the wind and point to the left. You’re pointing toward the low. (In thesouthern hemisphere, point to the right.)

High pressure=Usually good weather (think of getting high)Low pressure=Usually not so good (you feel low)

Air flows from areas of high pressure to areas of low pressure. That’s why a high pressure area overthe Four Corners in the U.S.—something you’ll often hear referred to on the nets--often brings a“norther” (strong winds, sometimes gale force, blowing out of the north) to the Sea of Cortez. The airis being “sucked” from the Four Corners region to an area of lower pressure to the southwest of us herein La Paz, resulting in high winds for us and Santa Ana winds in southern California.



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Reading Surface Analysis Charts or Weather Faxes

What in the heck are all those squiggly lines?

“H” is for high pressure area, and you can guess what “L” is for.

1013.23 millibars is average sea level pressure (29.92 inches). The lowest pressure ever recorded was870 mb at the center of a hurricane. In the diagram above, “12” means 1012 mb; “20” means 1020 mb.“98” would mean 998 mb.

Isobars: a contour line of equal pressure. If they are tightly spaced and look “squished” together, windspeeds are probably high.Pressure gradient: Change in pressure measured across a given distance. Just think “pressure change”.

The higher the gradient, the higher the winds.Ridge: An elongated area of high pressure.Trough/trof: An elongated area of low pressure, often associated with a front. We’ll get to fronts in aminute.



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The straight arrows in figure “b” show the direction of movement of the low pressure system towardthe northeast. A circled “X” in the direction of movement would predict the position of the system 24hours from the time of the forecast.

High pressure systems generally move more slowly than low pressure systems. High pressure systems

rotate clockwise in the northern hemisphere; low pressure systems rotate counter-clockwise. Think “Low – Left, High – Right”. Just to keep things confusing, rotation is the opposite in the southernhemisphere.

You can tell what direction the winds are likely to come from depending on where you are in relationto the center of the system. Someone to the east of the center of high pressure in “d” will probablyhave northerly winds, while someone on a boat to the west of the system center will experiencesoutherly winds. The opposite applies to low pressure system “a”. If the center of a storm passes overyou, you’ll experience wind from every direction of the compass as the system moves through.

It’s important to know the prevailing wind pattern, as that will tell you where the storm is probably

headed (in La Paz, the prevailing winds are westerly, so a system would probably move to the east).Watch for areas between high and low pressure systems. These may have dangerously high andconfused seas that are not shown on the charts.

What are fronts?

Fronts are the boundaries between air masses of different temperature and density (cold air is denser—weighs more—than warm air). They often bring significant changes in the weather. If warmer air ismoving toward colder air, it is a “warm front”. These are shown on weather maps as a red line with



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scallops on it. If colder air is moving toward warmer air, then it is a “cold front”. Remember “warmand cold” are relative. A cold front won’t necessarily be cold, just colder than the air surrounding it.

Cold fronts are always shown as a blue line with arrow points on it (think of certain body parts gettingpointy when you’re cold). If neither air mass is moving much, it is called a “stationary front”, shown asan alternating red and blue line.

An “occluded front” occurs when a faster moving cold front rides up over a warm front. There is oftenrain in front of and behind these fronts, and sometimes thunderstorms. Occluded fronts are indicatedon a weather map by a line with alternating half-circles and triangles pointing in direction of travel.

Map Symbols of Fronts

1. cold front2. warm front3. stationary front4. occluded front5. surface trough6. squall line7. dry line8. tropical wave



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A stationary front is an airmass that is not moving.

A cold front is an area of cooler air, not cold air. It ispossible to have cold fronts inthe summer. A cold front oftentriggers storms.

A warm front is a leading edge

of warmer air.

An occluded front occurswhen a faster moving coldfront passes under a warmfront, and catches up toanother cold front. There isoften rain in front of andbehind these fronts.

Now let’s go back to that confusing weather graphic on page 3 and see if things make more sense...

What will you see from your boat as a front approaches?

Usually high, thin cirrus clouds are the first sign of an approaching warm front (this could be 24 hoursin advance). The clouds keep getting lower and lower in the sky, and pressure drops. There may besome wind gusts, and temperatures will rise, then level off. When the front arrives, there will be steadyand heavy rainfall. As the front passes, the wind will shift in a clockwise direction in the northernhemisphere (e.g., northeast to southeast).

Gathering cumulus clouds often indicate an approaching cold front. It will probably arrive in 6-12hours. As it comes closer, you will have increasing winds and possibly some squalls and thunder—maybe even hail. Pressure will drop dramatically and rise again rapidly after passage. Cold frontsmove faster than warm fronts, so changes occur more quickly and violently. In the northernhemisphere, the wind will shift in a clockwise direction as the front passes over, with strong gusts afterfrontal passage. Fair skies with scattered showers and cooler temperatures usually follow the passageof a cold front.



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Cloud Formations

Cirrus are thin clouds very high in the sky, often indicate the approach of a warm front.Nimbostratus are your basic rain cloud: heavy, gray clouds, often with a jagged lower edge, with theirbase at about 3,000 feet.Cumulus are the “cotton ball” clouds that you see on sunny days. If they start clumping together into

bigger and bigger clumps, be ready for a change in the weather.Cumulonimbus are the result of many cumulus clouds accumulating. They’re the classic thundercloud,reaching great heights, often with a top edge flat like an anvil. The cloud may mostly be white, but thebase is usually gray. These can bring heavy rain and sometimes hail. Steer clear!Fog: Learn how to use your radar before you need it. Check the manual and see what tricky things youcan accomplish--a great way to stay awake during a boring night watch.

If possible, keep clear of cumulonimbus clouds—or at least reduce sail!



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Wind direction and speed indicators on weather maps

The tips of the tip-less arrows on a GRIB file point in the direction the wind will blow. The tailindicates the direction the wind will blow from. The hatches on the arrow’s tail indicate wind speed. Ahalf line is 5 knots, and a full line is 10 knots. A pennant or triangle is 50 knots. You don’t want to seethose!

Wind is always discussed in terms of what direction it will blow from, not to. A northerly wind blowsfrom the north.

The information for wind speed is always given in knots (1 knot = 1.15 miles per hour).

On weather faxes and grib files and in radio forecasts, wind speeds are averages. Gusts may be up to40% stronger than average speeds. If you see 25 knots on the GRIB file, expect some 35 knot gusts.The same applies to wave height: wave and swell heights represent the average of the highest one thirdof waves. The maximum wave height can be up to twice as high!

Some sources other than HF radio nets for weather conditions:

GRIB Files (via Winlink or Sailmail)GRIB (stands for “gridded binary”). Five-day forecasts with wind vectors and/or sea state. These areespecially useful when you can’t hear HF radio nets due to bad propagation, engine noise, or just beingbusy dealing with something on deck, like ironing your bikini.

In order to receive GRIB files, you must be able to receive e-mail via either WinLink orSailMail: http://www.sailmail.com/ and http://winlink.org/

Sailmail charges annual fee of $250. Winlink is free, but requires a General HAM license.Software is free for both.



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Send the following:To: [email protected] grib:10N,35N,090W,160W|1,1|12,24,48,72,96,120,144,168,192|or to receive files daily at about 1200Z send:sub grib:10N,35N,090W,160W|1,1|12,24,48,72,96,120,144,168,192 days=30

You can also receive text documents with the forecast for a specific area by sending a differentquery to [email protected]. Ask Susan on Two Can Play for more information.

Weather fax

WORLDWIDE MARINE WEATHERFAX BROADCAST SCHEDULEShttp://www.nws.noaa.gov/om/marine/rfax.pdf

In order to determine optimum propagation times, use one of the readily available propagationsoftware packages.Don’s favorite is available at http://elbert.its.bldrdoc.gov/pc_hf/hfwin32.html

Buoy weather

From www.buoyweather.com for $10 you can get 100 messages, triggered by your own email, eachgiving a voyage forecast.

Commanders’ Weather: www.commandersweather.com

Not the cheapest option, but one of the simplest. They’ll essentially e-mail you a personalized forecastand recommend a departure date for your passage.



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The Coriolis Effect

Warm air rises and cold air sinks. A very basic description of worldwide air movement: warm air risesat the equator, creating low pressure. Cool air sinks at the poles, creating high pressure. If the Earthweren’t rotating, air masses would move south to north. To an Earth-based observer, the Earth’s

rotation makes the flow of air seem to curve to the right in the Northern Hemisphere, and to the left inthe Southern Hemisphere. But a space-based observer would see the air masses moving in a straightline north to south and south to north as the earth rotates below...This is called the Coriolis Effect.

Coriolis Effect

Basic Weather

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