Meteorology

Meteorology
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Tides Currents World Wind Patterns NOAA Online School For Weather

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Tides

Tides
Because Being Stuck With Your Ass In The Mud Is Kinda Bad
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The Basics
Tides - basically changes in the mean sea level. They're caused primarily by the gravitational pulls of the moon and sun, but also because the moon's pull causes the Earth to wobble slightly in it's rotation. As a result, water bulges form on opposite sides of the Earth.
As the Earth revolves beneath this bulge, the tidal effects are felt, and usually result in 2 similar high and 2 similar low tides per day, otherwise known as a Semi-Diurnal Tide Cycle. Some places only experience one high and one low tide per day; this is known as a Diurnal Tide Cycle. Other regions experience Mixed Tidal Cycles characterized by 2 highs and 2 lows per day, but the heights of the highs varies (one will be higher than the other), and the heights of the lows varies (one will be lower than the other). When the moon, sun and Earth are all in a straight line, the tidal range is it's greatest. We call this a Spring tide. The tidal range is at it's greatest because the pull's from the moon and sun add together to pull in the same direction. When the moon and sun are at right angles to each other, the tidal range is it's least. We call this a Neap tide. Because the moon and sun's pulls are at right angles, they are not additive, and the high tides will be lower and the low tides higher. Perigee - the point at which the moon is closest to the Earth in it's orbit around it. Apogee - the point at which the moon is farthest from the Earth in it's orbit around it. When Perigee occurs during a Spring tide, the tidal range will be at it's absolute greatest. When Apogee occurs during a Spring tide, the tidal range will not be as extreme as it would be during a normal Spring tide. When Apogee occurs during a Neap tide, the tidal range will be very minimal. When Perigee occurs during a Neap tide, the tidal range will be greater than it would be during a normal Neap Tide.

Solving for Tides/Currents

Finding Highs and Lows Finding Height At Any Time Finding Time at any Height

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Tides

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Time of Highs and Lows

Time of Highs and Lows

Because Surf's Up At High Tide, Man
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To find the tides at a given location,

you'll need to use the Tide Tables for the appropriate region. The tide tables provide tidal information in the Daily Pages for a certain number of sites within a designated region. These sites are referred to as Reference Stations, and tidal information for every day of the year is listed in the Daily Pages. The tide tables also allow you to find the tidal information at hundreds of additional stations, known as Subordinate Stations because to find their tidal information, you must apply a correction to the reference station data. Before you begin solving for tides, first set up a table like the one at right, with the time and height of high and low tides across the top, and Reference, Corrections, and Subordinate station along the side. Next, in the back pages of the Tide Tables you'll find the Index to Stations. Look up the Index Number of the station you are seeking. If the station has an * and a page number in parenthesis next to the name, lucky you because this is a reference station and you won't need to correct, you can just go straight to the Daily Pages and find your info. Using your Index Number, go into Table 2 Tidal Differences and Other Constants, and under the "Differences" column, note the corrections from your subordinate station to the reference station. These should fall right into the table you set up before, in the "Corrections" row. If you find that under "Differences" your correction reads "Daily Predictions," your station is a reference station and you can go straight to the Daily Pages. Still within the "Differences" column of Table 2, look up the column until you come across a line that reads "On ..... page..." The station referenced in this line will be your Reference station. For example, if your subordinate station is Port Wentworth, you would look up the column and find that your reference station would be Savannah on page 104. Next, turn to the appropriate daily page for your reference station, find the date in question and find the times and heights of tides as desired, noting them in the Reference Station row of your table.

Time Time Height of High of Low of High Data at Reference Station Correction to Subordinate Station Actual Tide at Subordinate Station

Height of Low

The last step is to simply add or subtract the corrections to or from the reference station data to find the data for the subordinate station. If

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Time of Highs and Lows

your height correction has an * next to it instead of a + or -, you will multiply the reference station data by your correction rather than adding or subtracting. If you need to correct for daylight savings (MarOct), simply add an hour to the reference station times before adding in the corrections.

Back to Tides

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Height At Any Time

Height At Any Time

Because You Can't Always Be High
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So, finding the time of the high and the time of the low is useful, but it'd be nice to be able to find the height of the tide at a given time. For example, if you know that you'll be coming into port at 0830, but the high is at 1000 and the low is at 0745, you need to know what the height will be at 0830 so you know your overhead and under keel clearances.
Step 1: Find the high and low tide that bracket (are on either side of) the time that you want for your subordinate station. (If you haven't already corrected the height from the reference station to your subordinate station, do that first.) For example, if you need the height at 0830, and the tide tables list a high at 0200, a low at 0815, a high at 1414, and a low at 2022, select the low at 0815 and the high at 1414 and continue on with the problem. Step 2: Set up an interpolation table like the one at right to make your calculations easier. This table should consist of 2 columns and 3 rows, with enough space between each to allow for some writing. Step 3: In the first column in the top row, place the time of the high or low tide occuring before the desired time. In the 2nd column, place the height of that tide. Repeat this step, placing the time and height of the high or low after the desired time in the bottom row. In the 2nd row of the 1st column, place the desired time. Step 4: As per the directions in the interpolation table (table 3) of the tide table, find the difference (range) of heights between the high and low, difference in time between the high and low (duration of rise or fall), and the difference in time between the nearest tide (either high or low) and the desired time. These should each be noted along the margins of your interpolation table just so you can keep track of them. Step 5: Using the interpolation table (table 3) found within the tide tables and the values you found in step 4 above, follow the table until you find your Height Correction. -First find the row for the nearest value of the duration of rise or fall -Move across the row until you find the time from the nearest high or low -Move down that column until you come across the row that best describes your range of tide -Where that row and column intersect, that is your height correction

Time of high or low prior to desired time Desired time Time of high or low after desired time

Height of high or low prior to desired time Height at desired time Height of high or low after desired time

Finding Highs and Lows Finding Time for any Height Back to Main Tides Page

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Height At Any Time

Step 6: Apply that Height Correction to the height of the nearest tide (whichever you used to find the difference in time). As it says at the bottom of the table, if the nearest tide is a high water, subtract the correction. If the nearest tide is a low water, add the correction. Step 7: That's it man, you're done. To find the clearance under your keel, add the tidal correction you just found to the charted depth in the area you are passing through. From this, subtract your draft, whatever's left is your keel's clearance. Finding the vertical clearance is a little more complicated, and look for it in a different lesson

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Finding Time for Given Height

Finding Time for Given Height

Because Blindly Assuming You'll Clear The Bridge Is A Bad Way To Sail
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So lets say that you know you'll be

coming into port on a given day, but need to know what time to arrive so that you'll have enough water beneath your keel to clear an obstruction (a sandbar for example). What do you do? Like solving for the height at any time, this problem is all about interpolation, and it's fairly simple. Step 1: Find the high and low tide that bracket (are on either side of) the time that you want for your subordinate station. (If you haven't already corrected the height from the reference station to your subordinate station, do that first.) For example, if you need the height to be 3.2 ft, and the tide tables list a high of 3.1 at 0200, a low of -0.5 at 0815, a high of 5.1 at 1414, and a low of -0.6 at 2022, select the low at 0815 and the high at 1414 and continue on with the problem. Step 2: Set up an interpolation table like the one at right to make your calculations easier. This table should consist of 2 columns and 3 rows, with enough space between each to allow for some writing. Step 3: In the first column in the top row, place the time of the high or low tide occuring before the desired time. In the 2nd column, place the height of that tide. Repeat this step, placing the time and height of the high or low after the desired time in the bottom row. In the 2nd row of the 2nd column, place the desired height. Step 4: As per the directions in the interpolation table (table 3) of the tide table, find the difference (range) of heights between the high and low, difference in time between the high and low (duration of rise or fall), and the difference in height between the nearest tide (either high or low) and the desired height (listed as height correction in table). These should each be noted along the margins of your interpolation table just so you can keep track of them. Step 5: Using the interpolation table (table 3) found within the tide tables and the values you found in step 4 above, work backward through the table until you find your "Time from nearest high or low water" -Using your known Height Correction and your Range of Tide, find the column where they meet and follow it upward into the next table -Move upward through the column until you come across the row that describes your Duration of Rise or Fall -Where these two values meet, that is your

Time of high or low prior to desired time Desired time Time of high or low after desired time

Height of high or low prior to desired time Height at desired time Height of high or low after desired time

Finding Highs and Lows Finding Height At Any Time Back to Tides

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Finding Time for Given Height

Time from nearest high or low water Step 6: Apply that Time from nearest high or low water correction to the time of the nearest tide (whichever you used to find the difference in height). If the desired tide is after the nearest, add the correction to the time. If the desired tide is before the nearest tide, subtract the correction. Step 7: That's it man, you're done. It might also be useful to know the amount of time that the water will be at that level so that you don't enter the channel safely only to be stuck 10 minutes later.

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Currents

Currents
Because That Message In A Bottle Wont Float Itself To Africa
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Currents are essentially just the movement of water. They can be caused by tides (tidal
currents), gravity, or just the Coriolis effect (open ocean currents). Tidal currents occur when the tidal bulge moves across the earth. This bulge draws water with it and as that water flows into a bay or past a reference point, the current can be noted. For example, when a high tide is coming into San Francisco Bay, the current under the Golden Gate Bridge can be very strong leading into the bay. Tidal currents in most areas are described as Reversing because the flow of the tide comes in, stops, then flows out. When the tide is coming in, or transitioning from a low to a high tide, it is said to be Flooding. When the current is going out, or transitioning from a high to a low tide, it is said to be Ebbing. The strength of the current is highest halfway between the high and low tides, and it 0 or close to 0 exactly at the time of high or low tide. The strength of the current depends largely on the height of the tide. Unusually high tides cause unusually strong currents because a larger volume of water must flow into an area in a set amount of time. Rotary Currents are also tide driven, but rather than being reversing as described above where the flow of water stops and reverses, water is always flowing past, but it is constantly changing direction.

Time of Slack/Max Current Speed at Any Time Time at Any Speed Duration of Slack

Open Ocean Currents are caused by the differing densities of ocean waters. Water at the equator is hot and water at the Poles is cold. As a result, the warm water moves toward the colder denser water, forcing the cold water to move as well. Because the Earth is spinning, the Coriolis Effect deflects the water movements, causing currents that skirt the edge of most ocean basins. Click the link below to see a diagram of Open Ocean Currents.

Ocean Current Patterns

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Currents

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Time of Slack/Max Current

Time of Slack/Max Current

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Solving for currents is fairly similar to solving for tides in that you have a subordinate and reference station, corrections, and interpolations. Only this time you're using slightly different numbers. Step 1: Build A Table, like the one at right. -The headings will line up with the corrections from the Tidal Current Tables (TCT). Step 2: Find your station's index number in the back of the TCT Step 3: In the corrections section of the TCT, use your index number to find the corrections for your station. Note these corrections under the correct headings in your table Step 4: Note which station the corrections are to be applied toward Step 5: Turn to the Daily Pages for the correct reference station and pull of the values that you need. -For each day, the times in the left half of the column are the times of slack water, the times on the right are for either Max Flood or Ebb, and the farthest right numbers are the applicable speeds of the current, noted as either F (flood) or E (ebb) Step 6: Add or subtract the corrections to your reference station number to get the times of max currents at your subordinate station.

MBF = Minimum Before Flood (ie. slack water before flood)

F = Flood (maximum)

MBE = Minimum before Ebb (ie. Slack before Ebb) F (direction)

E = Ebb (maximum) E (direction)

MBF F MBE E F (speed) (time) (time) (time) (time) Reference Station Speed corr. is a multiplying factor (ie. multiply this by speed in daily pages to get correct speed)

E (speed)

Corrections

Speed corr. is a multiplying factor (ie. multiply this by speed in daily pages to get correct speed)

Direction found on the Corrections page needs no adjustment

Direction found on the Corrections page needs no adjustment

Subordinate Station

Back to Currents

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Time of Slack/Max Current

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Current At Any Time

Current At Any Time

Because if you can avoid being pushed around, do it
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First things first, if you haven't already found the times and speeds of the nearest flood and ebbs to the time you need, do that. See the page on Times of Slack/Max Current for help. All you need to do to find the speed at any time is interpolate those values, so without them, you're fucked. Step 1: Find the times of the slack and max current that surround your desired time, (see Time of Slack/Max Current for additional help) Step 2: Build a Table like the one at right and fill in all of the info you already know Step 3: Find the time difference between Slack and Max Current, the time difference between Slack and Desired Time, and note the speed of nearest max current. Step 4: In the Tidal Current Tables, you'll find a Table for interpolating to find Speed of Current at Any Time near the back. Open up to this page and read the titles on the sides of the table. Step 5: Using the values that you know, interpolate using the table and find your Speed Correction Factor. Step 6: Multiply your Speed Correction Factor by the speed of nearest Max Current to find the current at the Desired Time.

Time of Slack or Max before desired time Desired Time

Speed of max or slack (0.0) Speed at desired time

Time of Slack or Max after Speed of Max or desired time Slack (0.0)

Back to Currents

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Time at any speed

Time at any speed

Because it never hurts to plan ahead
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So if you know that you'll be coming into an area of high current and you don't want to have to fight the current the whole way in, what do you do? You ride in on a slack or flood. How do you know when those occur? Guess. No, solve like you did for Speed at Any Time, but in reverse. Step 1: Solve for the times of the nearest slack and max currents surrounding your desired date. You should at least know what date you'll be coming in. Step 2: If you know the maximum current you want, pick the times of slack and max current that would provide you with the desired current nearest to the desired time. Step 3: Build a table like the one at right to simplify your addition and subtraction. Step 4: Divide your desired current by your max current to find your Speed Correction factor. Step 5: Find the Time Between Slack and Max Current, and note your speed of max current. Step 6: In the interpolation table in the back of the Tidal Current Tables, interpolate to find the Time From Slack To Desired Time. Step 7: Add or subtract this value to the time of Slack to find the nearest time at your desired speed. To find how long the current will remain that speed, see Duration of Slack.

Time of Slack or Max before desired time Desired Time

Speed of max or slack (0.0) Speed at desired time

Time of Slack or Max after Speed of Max or desired time Slack (0.0)

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Duration of Slack

Duration of Slack
Because we're all a little late sometimes
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This one's really simple. All you gotta do is go into the Tidal Current Tables, find the table that tells you Duration of Slack, and solve. You'll need to know the speed of max current

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Ocean Current Patterns

Ocean Current Patterns

Because Everybody Likes To Go With The Flow
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North Atlantic:

-Gulf Stream: carries warm water from the Caribbean north along the east coast of the US -Labrador: carries cold water from the Arctic south along the west coast of Greenland, meets up with the Gulf Stream off of Canada. -North Atlantic Drift: transitional current that runs East from Greenland to Britain -Spitzbergen: transitional current that carries moderately warm water from the Gulf Stream north along the west coast of Britain and up off of Norway -Canary: carries cold water from Northern Europe down along the west coast of Europe and the Canary islands -North Equatorial: transitional current that carries water westward just north of the equator

South Atlantic:

-South Equatorial: transitional current that carries water westward just south of the equator -Brazil: carries warm equatorial water south along the east coast of South America -Faulkland: carries cold antarctic waters north along the east coast of Argentina, meets up with the Brazil current -West Wind Drift: circumpolar transitional current that runs all the way around the earth, carrying cool water eastward, as it is not restricted by land -Benguela: carries cold water north along the west coast of Africa

North Pacific:
-California: carries cold Alaskan waters south along the west coast of the US -Davidson: fed by the Equatorial Circumpolar, carries warm Central American waters north along the west coast of Mexico until it meets the California current near LA -North Equatorial: transitional current that runs westward just north of the equator -Equatorial Circumpolar: transitional current that runs eastward along the equator -Kuroshio: carries warm equatorial waters north along the east coast of Japan -North Pacific Drift: transitional current carries waters eastward from Kamchatka south of the Aleutians toward Canada -Bering: breaks off from the North Pacific Drift and carries transitional waters north of the Aleutians into the Bering Sea

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Ocean Current Patterns

-West Wind Drift: circumpolar transitional current that runs all the way around the earth, carrying cool water eastward, as it is not restricted by land -Humboldt: carries cool arctic water northward along the west coast of South America -Peru: fed by the Equatorial Circumpolar, this carries warm waters south along the west coast of Peru until it meets the Humboldt -South Equatorial: transitional current that carries water westward just south of the equator -East Australian: carries warm equatorial waters south along the east coast of Australia

South Pacific:

Indian Ocean:

-South Equatorial: transitional current that carries water westward just south of the equator -Agulhas: carries warm waters from the Persian Gulf south along the west coast of Africa -West Wind Drift: circumpolar transitional current that runs all the way around the earth, carrying cool water eastward, as it is not restricted by land -West Australia Current: carries cool antarctic water north along the west coast of Australia

Back To Ocean Currents

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World Wind Patterns

World Wind Patterns

Because its not just the dress you're wearing, it is a little breezy in here
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Global wind patterns develop as the result of subtropic and subpolar highs and lows and the rotation of the earth. The Coriolis effect deflects winds to the right in the Northern Hemisphere, and to the left in the Southern.
At the equator: Warm air rising at the equator causes a low pressure region here, which draws winds in. This region is also known as the InterTropical Convergence Zone because winds and currents approach from either side of the equator, causing fluctuating conditions at the equator itself. Yet another term for this region is the Doldroms, as there is little wind because most of the air movement here is vertical rather than horizontal. From 0-30: The prevailing winds in this region are from the East and are known as the Trade Winds (the NE Trades in the Northern Hemisphere and the SE Trades in the Southern). Air from the Sub Tropic High blows toward the Equatorial Low and is deflected to the East by the Coriolis effect. At 30: Also known as the Horse Latitudes because years ago when sailors were trapped in this region of little wind, they threw their heavier cargo (ie. horses) overboard to try and lighten their load. This Sub Tropical High Pressure Zone sees little wind as most air movement hear is vertically downward. From 30-60: The prevailing winds in this region are known as the Westerlies. Winds blow from the Sub Tropic High toward the Sub Polar Low and are deflected to the West by the Coriolis effect. In the Southern Hemisphere, the region between 40S and 60S is known as the Roaring Forties. Prevaling winds in this region are unusually strong because this belt continues all the way around the earth without being interrupted by land. At 60: This belt is known as the Sub Polar Low sees little wind as a result of warmer upwellings of air traveling vertically rather than horizontally. From 60-90: Prevailing winds in this region blow almost directly from the East because as winds blow from the Polar High to the Sub Polar Low, they are greatly redirected by the Coriolis effect, which is very pronounced at this latitude. At the Poles: The Polar High Pressure Region is the result of a vertical downdraft of air as it cools above the colder Polar regions.

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World Wind Patterns

Winds here are primarily local and are the result of land formations and local irregularities.

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