Question of the Week #4 Answer

Signs of a Cold Front Approach and Passage:

Clouds-Are our most visible clue as to what is transpiring in the atmosphere.  As a cold front approaches, the clouds at various altitudes will change alerting us to a coming change in our present weather.

Normal settle weather we will see Cumulus clouds at the lowest level of the atmosphere and possibly Cirrocumulus at the highest level of the atmosphere indicating good weather.

As a front approaches clouds at the highest level will begin to change first with Cirrostratus clouds forming followed by formation of middle layer clouds Altocumulus and Altostratus.  As the front approaches, closer Stratus and Stratocumulus will form in the lower levels of the atmosphere.  In general, the cloud cover will thicken and lower.

Frequently leading a cold front by as much as 200 miles we can see Cumulonimbus (Thunderstorms) or Nimbostratus with just rain.

Winds-In the Northern Hemisphere winds will begin to change clockwise.  This means for places like Florida and the Bahamas the normal Northeast – East winds will move into the Southeast and South and as the front gets closer and begins go pass swing into the West.  After the front passes the winds will go Northwest and likely increase in strength.

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Barometer-In the tropical regions the Barometric Pressure will rise and fall a millibar or so twice a day.  This is a normal diurnal change.  However, when you see a steady fall in Barometric Pressure it is time to take notice.  The faster and deeper the Barometric pressure falls is an indication of how fast a system is moving and how strong it may be.  Be aware though that a change in Barometric Pressure will not be your first clue the weather is about to change.  As an example, the day before a hurricane arrives the barometric pressure isn’t likely to indicate much in the way of a coming storm.  This was one of the factors in the early days of the National Weather Service’s failure to predict the Galveston Texas hurricane in the early 20^th century.

Temperature-warm and moist air in front of a Cold Front and as the front passes a rapid drop in Temperature as well as drier air.

Clouds to Look Out For-Frequently at the leading edge of a Cold Front one will find some interesting cloud formations.  In general, the darker and more menacing looking the clouds the stronger the wind will blow.  Sometimes the approaching Cold Front will roll the lower level clouds into a “Cigar” shape known as a Roll Cloud.  When you see this cloud formation approaching-REEF NOW AND REEF DEEP-you are about to experience a very rapid and large increase in wind strength!  Usually the water directly under the cloud will be boiling, if you can see this happening with the naked eye, the wind increase is very close!  I use to tell my students whose first inclination upon seeing interesting cloud formations was to grab a camera and start taking pictures “that if a cloud formation is suddenly worthy of a picture, forget the camera and reef because you are about to get slammed”.

Another sky/cloud scenario to beware of is a very dark, pea green and purple sky, not only is there a lot of wind in this sky, but there are likely very severe thunderstorms in the area and likely a tornado.  This color sky in the mid-west is referred to as Tornado weather.

Your first clue a front is approaching will likely be a change the high-altitude clouds, followed by a shift in wind direction.  As the weather system approaches the sky will become very active with changes in cloud formations at all levels of the atmosphere and a continued clocking of the wind.  It is time to make sure the anchorage you are in is secure for the expected wind shifts and if you are at sea to prepare for heavy weather.  While you may not be able to predict how strong the coming wind will be or for how long it will rain or how much rain you will get there will still be clues to the weathers severity that you must heed.

While modern communications can give us several days advanced notice of approaching bad weather keep in mind the meteorologists sometimes get it wrong or communications break down for any number of reasons.  It is still your job as a Seaman to keep an eye on the sky, the winds, the barometer and the thermometer to keep your crew and vessel safe.