Understanding Boating Weather
Weather is not magic, but it is a mystery some times. While meteorologists spend years learning weather patterns and details of weather phenomena to provide us with great information; we can learn to make well-educated plans for our boating adventures by learning some basic principles of weather systems and how they interrelate.
On this site, we discuss weather in the northern hemisphere, where low pressure gradients rotate in a counter-clockwise manner and high pressure gradients rotate in a clockwise manner. We also take some liberties to explain basic weather. Weather is not an exact science and there are many factors that influence our day to day weather systems. We will keep our discussion at a high level, so if you are a meteorologist, please forgive me for minor inconsistencies; my goal is not teaching all about weather, merely helping non-expert weather forecasting boaters to look up and go, "Ah ha!, I know what's going on today and why, and have a good idea about tomorrow now!"
HIGH PRESSURE GRADIENTS ("HIGH PRESSURE SYSTEMS")
High pressure is caused by descending air; the center of an air mass where air is coming down from the upper atmosphere and spreading out over the ground, like pouring liquid on a cookie sheet. In the northern hemisphere, this air (or "liquid" in the example) is pushed to the right as it hits the ground, causing a clockwise spin around the center of the air mass (the Coriolis Effect). High pressure systems tend to be colder and drier air than low pressure systems.
LOW PRESSURE GRADIENTS ("LOW PRESSURE SYSTEMS")
Low pressure will form between two high pressure systems (or air masses) as air rises back away from the surface. Think of it as if the liquid being poured onto the cookie sheet (the "high" pressure systems) in two locations collides with enough force on the cookie sheet to push a wall of liquid back upwards. The center of the rising air mass (or the "liquid wall" in the example) going upwards is the low pressure point. In the northern hemisphere, because the air is spinning clockwise around both of the high pressure systems beside it, the lower pressure air will spin counter-clockwise as it rises back up to the atmosphere. Low pressure systems tend to wetter and warmer than high pressure systems.
[One way to remember this is to think of a clock sitting "high" on a wall, so "highs" always spin clockwise; therefore a "low" will always spin counter-clockwise]
HOW DOES WEATHER "HAPPEN"?
Four basic ideas to understand.
- Sufficient moisture combined with a lifting mechanism (or on occasion surface cooling) produces clouds and precipitation.
- The intensity of the lifting system and amount of moisture present determines the strength of any associated storm and precipitation.
- Winds are generated by the location of different atmospheric pressure gradients (Highs and lows) or local geographical causes (onshore and off shore breezes, mountain breezes, etc).
- The intensity of the wind is caused by the speed of weather fronts or instability in the air related to the degree of temperature, atmospheric pressure and moisture differences in adjacent weather systems.
There are Six Basic Weather Principles to Remember
Principle 1: Cold air is more dense than warm air
Principle 2: Dry air is more dense than moist air
Principle 3: Warm air can hold MUCH more moisture than cold air
Principle 4: Moist air cools less upon expansion than dry air
Principle 5: When air is cooled below its saturation point (dewpoint) it must expel liquid water
Principle 6: When water vapor is converted back to liquid water (clouds and precipitation), heat is released, warming the air surrounding it
Weather Associated With Different Types Of Weather Fronts
Cold air is more dense and hugs the ground, pushing in under warmer air. Because of this, cold fronts are typically strong lifting fronts. The colder air acts like a plow and pushes hard against the warm air in front of it, causing it to lift quickly. If moisture is present the cold air will lift this warm moist air to create large cumulonimbus clouds; bringing heavy rain, gusty winds and thunderstorms.
|Before Passing||While Passing||After Passing|
|Temperature||warm||sudden drop||steadily dropping|
|Pressure||falling steadily||minimum, then sharp rise||rising steadily|
|Clouds||increasing: Ci, Cs and Cb||Cb||Cu|
|Precipitation||short period of showers||heavy rains, sometimes with hail, thunder and lightning||showers then clearing|
|Visibility||fair to poor in haze||poor, followed by improving||good, except in showers|
|Dew Point||high; remains steady||sharp drop||lowering|
Warm fronts are more gentle, riding in high over colder air with much less intensity than a cold front and therefore do not bring as much "lifting force" to the front. This means that one of the mechanisms of forming big storms is missing and the weather is not as powerful as when a strong cold front moves in. When lots of moisture is present, warm fronts typically bring steady rain with less likelihood of damaging winds associated. When the cold air under an approaching warm front is very cold and warm front brings enough moisture or moisture is pulled into the area by a pressure gradient; snow and ice are possible.
|Before Passing||While Passing||After Passing|
|Temperature||cool-cold, slow warming||steady rise||warmer, then steady|
|Pressure||usually falling||leveling off||slight rise, followed by fall|
|Clouds||in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer||stratus-type||clearing with scattered Sc; occasionally Cb in summer|
|Precipitation||light-to-moderate rain, snow, sleet, or drizzle||drizzle or none||usually none, sometimes light rain or showers|
|Visibility||poor||poor, but improving||fair in haze|
|Dew Point||steady rise||steady||rise, then steady|
Stationary Fronts and Occluded Fronts
Cold Fronts tend to move faster than warm fronts, so if they happen to catch up with a warm front, a stationary front forms, and the frontal boundary stops moving as fast-- basically a warm front/cold front combination. These tend to bring long, rainy periods that do not move much. If this Cold Front pushes the warm front upwards and runs into the back of another colder air mass, an Occluded Front may form. Depending on the amount of moisture present and the amount of lift presents, an Occluded Front can have all the weather associated with Warm Fronts and Cold Fronts together along the various associated air boundaries or Fronts. Wind direction tends to become variable as the Occluded Front sits over head, and reverse with the passing of an Occluded Front.
One of the most important things to remember is "Winds Blow, They Don't Suck" so when you hear about a "South Wind" or wind is "Southerly" you know that the wind is BLOWING OUT of the SOUTH in a NORTH DIRECTION. Likewise, a "North Wind" or a "Northerly" is coming FROM the NORTH; it NOT blowing towards the north as it may sound.
WIND BLOWS FROM THE DIRECTION IT ORIGINATES OR IS DESCRIBED.
I know this is common sense to many; but for some folks just learning weather principles, this may be difficult to grasp initially; so we'll give them a break here, ok?
Remember the direction that air travels around each of the High and Low air pressure systems, and you can have a better understanding of the type and direction of wind to expect.
Also keep in mind that wind speed is determined by a number of factors, with the pressure differential between a high and low pressure system and the distance between the two causing the greatest variance. A high pressure system very close to a low pressure system will have strong, gusting and steady winds between them. A high pressure system farther away from a low pressure system will have lower wind speeds with fewer strong gusts expected.
In the northern hemisphere middle latitudes (30-60 N) , the high altitude winds (Westerlies or "anti-trades") that move air masses (Jet Stream Winds) travel generally from west/northwest to east, so weather patterns will follow this same general direction. The jet stream winds will dip south and rise towards the north based on high and low pressure gradients. The high altitude winds are the steering winds for large storm systems, so knowing the current jet stream pattern, you can better forecast the direction of storm travel and forecast wind changes as the pressure system rides the path of the jet stream.
In the tropics, at the lower north latitudes (0-30 N) the higher altitude winds (Trade Winds) travel generally travel from east/southeast to west, opposite the Westerlies. Surface winds will vary considerably however. In addition, the Coriolis effect due the earths rotation will cause wind traveling from high pressure to low pressure to curve towards the right.
On a larger scale the type of pressure system being moved by high altitude winds, the temperature and moisture content of the air associates with that pressure system, and the temperature and moisture of the air it is running into will all influence wind direction.
Remember that a high pressure system forms when air is pushed down onto the surface and wind is associated with that air moving out and away from the center of the high pressure area, and curved to the right. It then moves toward the oppositely rotating low pressure system and winds are steered in a counter clockwise direction as they rise upward near the low pressure area.
On a smaller scale, winds associated with strong storm fronts, wind associated with temperature differences in land and water and wind associated with topography like mountains will all impact wind speed and direction.
On Shore and Off Shore Breezes
During the day, heating of the land happens quicker from sunshine than the temperature of the water, so depending on location; warmer air over the ground is less dense, rises upwards and creates a localized low pressure system. The air over the water is colder and more dense, creating a localized high pressure system. The air (wind) will blow onshore from high pressure to low pressure (an "Off Shore" breeze; remember that wind is describes by where is comes from).
At night, the opposite happens. The ground cools faster than the water and sets up a small high pressure area over the ground, and air over the warmer water sets up a local low pressure system so the direction of the air (wind) changes to blowing from water to shore, and blows from shore to water (an "onshore" breeze). These nighttime breezes are typically much lighter than daytime due to the less rapid changes in air temperature over water and land at night.
Fog happens when the air temperature is cooling to a point that equals its dewpoint, but there is not sufficient lifting force to move it away from the surface. Remember, anytime air temperature reaches the air masses dewpoint, the air must release moisture in the form of visible water. This is a cloud initially (fog on the surface), and if the dewpoint and temperature meet with enough moisture present trapped in the air mass, it become rain.
Radiation Fog: Ground cools the air to saturation point (clear skies at night, moist air present = fog in the early morning)
Advection Fog: This typical SEA FOG. Warm moist air on land travels into an area of cold surface water temperatures and is cooled to saturation point, creating a fog. Most common in spring time when the land temps are rising and ocean or water temp is doing so at a much slower pace.
With sunshine present, the air temperature begins to rise and the lifting from warmer air around/above the fog layer will start to create a lifting force and increase air temperature, so the fog will lift or dissipate.
LAKE EFFECT WEATHER
Primarily a concern over very large lakes. Air flowing over the lake picks up moisture and becomes unstable; or it then cools over the land leading to clouds and associated precipitation. Depending on the temperature difference in the air over the lake and the lake water, wind may also be created by warm water causing rising air, which pulls cooler air over the lake; created off shore wind. The opposite is what causes an on-shore wind.
MOUNTAIN EFFECT WEATHER
Although many boats may never deal with close by mountains, they can affect some of us, so I wanted to throw this in for information if needed. This illustration does a good job of demonstrating the basic ideas of what to expect when boating near mountains. What side of the mountain you are on makes the difference in the weather you may have. Note that large lakes next to mountains may have the same effects as the ocean shown in the picture.
Putting It All Together To "Forecast" Your Weather
Search for a weather site that works well for you and provides the best information.
A great weather related site for boaters is WWW.PASSAGEWEATHER.COM
- This site is self-supported and uses some adverting, but needs donations to keep running, so if you use the site often, please consider also providing a small donation on the site to help with management expenses. I have no affiliation with this site, but feel strongly about supporting these great webmasters!
Another good site is the NOAA Marine Weather Site Here
Many professional forecast maps use Station Barbs to indicate weather observation.It's important to learn to read wind barb station forecasts to understand current weather:
- You can download a complete Weather Station Reading Legend HERE from me for FREE. (This legend is provided by the NOAA and National Weather Service)
Be certain to also evaluate wave and tide information for your location
- Passageweather.com provides wave and tide information on their site as well
A FINAL POINT TO REMEMBER:
When considering any weather forecast (yours or one provided to you) remember if the current weather you might be experiencing is different than the weather originally forecast for that time period, any future forecast associated with your location will also have a strong chance of being incorrect; so get an updated forecast!