Weather – Page 5 – NOAA Teacher at Sea Blog

NOAA Teacher at Sea
Karen Meyers & Alexa Carey
Onboard NOAA Ship Albatross IV
August 15 – September 1, 2006

Mission: Ecosystem Monitoring
Geographical Area: Northeast U.S.
Date: August 22, 2006

Weather Data from Bridge
Visibility: 8 nautical miles
Wind direction 270^o
Wind speed: 5.5 kts
Sea wave height 1-2’
Swell wave height 2’
Seawater temperature 19 C
Sea Level Pressure: 1017.4 mb
Cloud cover: 6/8, Cumulus, Cirrus

Science and Technology Log

We’ve done 4 stations on our watch and that’s it for today because we’re heading back into port to exchange personnel. We expect to dock around 4 p.m. and then leave Wednesday morning around 11.

I went up to the bridge to get weather data today and came away again with a wealth of information from Captain Steve Wagner. He explained the difference between sea waves and swell waves. Swell waves are generated by distant weather systems and tend to have longer wavelengths. Sea waves are created by local winds – they’re more like chop. There can be swells coming from different directions and this is the source, he said, of the belief among surfers that every third wave is a bigger wave. If there are swells approaching a beach from two different directions, sometimes they’ll come together in constructive interference, resulting in a wave that’s larger than either and other times they’ll cancel each other out in destructive interference. It may be every third wave that they come together or it may be every fifth wave or whatever. They estimate the heights of the waves and the swells visually. Seawater temperature is measured by a hull sensor. Cloud cover is also measured visually by dividing the sky into 8^th’s and estimating how many 8^th’s are made up of clouds. Visibility is measured visually as well but confirmed, if possible, by radar or land sightings. For instance, right now Martha’s Vineyard is visible and they know the distance to the island so that can help them come up with a visibility number. If they’re out at sea and there’s nothing to use as a marker and the horizon appears crisp, they post a 10-mile visibility. They send all their weather data to the National Weather Service every 3 hours. They have a book–the same one with the Beaufort Scale ratings–that has pictures of cloud formations, each with a number and letter to identify it so they can use that for their reports.

He also explained that when they’re estimating visibility, they have to take into account “height of eye” which is how far above the water they are when they’re looking out. For Steve Wagner on this ship, it’s about 26 feet because the bridge is about 20 feet above the water and Steve himself is 6 feet tall. That affects the visibility distance and there’s a formula they can use which takes the square root of height of eye and multiplies by 1.17 to correct the visibility figure.

We also discussed the fact that US offshore charts use fathoms (1 fathom = 6 feet) while the charts of harbors, which have shallower water and so require greater resolution, use feet. Canadian charts use meters. So a mariner has to be aware of what measurement the chart he’s looking at uses. He said the Spanish have their own fathom which is less than 6 feet.

I find it fascinating that there’s such a combination of information from high-tech sources like GPS and low-tech sources like the human eye used in piloting, navigation, and weather prediction.

Personal Log – Karen Meyers

I got very said news via email yesterday. A woman who worked in the business office at my school and was an experienced horsewoman was killed in a riding accident. The service was today. I’ll look for a sympathy card and send it to her family while we’re in port.

Alexa, Tamara, and I are going on a shopping trip to Falmouth. I have a list of things to buy including a deck chair, if I can find one. No one here seems to object to the concept of deck chairs but there are only 3 on the whole ship and they’re in much demand. If I can find a cheap, lightweight one in Falmouth, I’ll buy it and then just donate it to the ship when I leave, along with the book Cod by Mark Kurlansky which I finished and passed on to Jerry Prezioso and my cache of granola bars if there are any left (which there almost certainly will be).

NOAA Teacher at Sea
Barney Peterson
Onboard NOAA Ship Rainier
August 12 – September 1, 2006

Mission: Hydrographic Survey
Geographical Area: Shumagin Islands, Alaska
Date: August 18, 2006

wet and dry bulb thermometer — Wet and dry bulb thermometer

Weather Data from Bridge
Visibility: 10 nm
Wind direction: 220˚
Wind speed: light 0 – 2 knots
Sea wave height: 0 – 1’
Seawater temperature: 9.4 ˚C
Sea level pressure: 1017 mb
Cloud cover: cloudy (8/8)

Science and Technology Log

Wednesday I spent time on the bridge, observing what happens when the ship is traveling at sea. My classes at James Monroe Elementary have participated in the GLOBE program, acquiring and sending weather data daily to be used to form a picture of conditions around the world. It was particularly interesting to me to learn that the crew of NOAA ships take much the same readings hourly and report them every 4 – 6 hours to the National Weather Service to help develop the predictions that help us all guide our day to day lives. I was especially impressed that the readings I saw were made using traditional instruments, not an automated electronic weather device.

One of the people in the pilot house logs weather every hour on the hour. There is an outside station on the starboard wall of the pilot house. This gives a temperature reading and allows them to calculate relative humidity. That is the difference between how much moisture is in the air, and how much total moisture the air is capable of holding. It may be expressed as a percentage, or decimal number. For hourly reporting, the relative humidity is not recorded and it is calculated automatically by when the “Big Weather” is submitted to National Weather Service. Both temperature of the air and sea water are read in ˚Fahrenheit and converted to ˚Celsius for reporting.

Wind speed is read from an anemometer mounted on the ship’s mast. This reading is a bit trickier if we are under way. When the ship is moving, the ship’s speed is subtracted from the anemometer reading to give a corrected wind speed. (Otherwise, the reading is like what you would get running while holding a pinwheel in front of you…much faster air movement than what is actually happening.) There is a wind vane mounted on the front of the ship and also an electronic gauge for reading wind direction.

The barometer (at left) is used for reading air pressure. It is located on the back wall of the pilot house and always gets a gentle tap before a reading is taken. This measurement is important because trends up or down in air pressure give clues to developing weather systems. The pressure is recorded in milibars. The ship’s barometer is shown at left. Some measurements involve using experience and personal judgment as well as instruments. These are the ones for wave height, swell height, cloud cover amount, cloud height, and visibility. The accuracy of these readings depends upon the experience and care of the person making them. The sea wave and swell can be estimated by careful observation, which seems to become second nature to the crew because they are exposed to them all the time. They are recorded in feet. The direction of the swell is always shown as the direction in which the swell is going. It can be measured using a device mounted on the deck outside the pilot house.

Cloud cover is measured in eighths. The observer divides the sky, calculates by observation how many eighths of the sky are covered by clouds, and reports that fraction. Likewise, a person must be a careful observer to note the kind of clouds they are seeing and where they mostly appear in the sky. There is a cloud chart available that shows pictures of cloud types and tells the altitudes at which they are commonly formed. This is a great help. (The cloud chart is shown at the right.) When there are low clouds, and there is land nearby, the observer can check the elevation of a point of land and judge the elevation of the lowest clouds as they appear against that point. Another measurement that may sometimes have to be an experienced estimate is visibility. Again, if land is visible, the observer tells how far away she/he can clearly see according to landmarks and the distances on charts or the ship’s radar screens. It is a lot harder to make this judgment when the ship is at sea, with no landmarks to help. That is when experience is especially important. One aid in this case is that the known distance to the horizon, due to the curvature of the earth, is eight nautical miles. That means that if the observer can see clear to the horizon, visibility is at least 8nm.

This day I watched Able Bodied Seaman (AB) Jodi Edmond take weather readings and report “Big Weather” to the National Weather Service using the internet.

A cloud chart on the NOAA’s National Weather Service Web site.

Personal Log

I am running about a day behind writing and submitting my logs. There is so much to do and see that I forget to spend enough time writing. I am using the personal journals that my students gave me at the end of the school year to record my impressions and thoughts every evening. Those act as memory-joggers when I sit down at the computer to do my formal writing.

Everyone aboard the RAINIER is very friendly and helpful. I am still making a few wrongs turns or selecting the wrong stairs to get to where I need to go. The officers and crew are great about pointing me in the right direction and giving me clues to help me remember how to find where I need to be when.

Every afternoon the orders for the next day are posted in several spots throughout the ship. These list the survey boats that will be going out, and their crews and assignments. The list also tells about responsibilities on board ship…both for the officers and the crew. These are called the Plan of the Day (POD) and are important for everyone to read when they are posted.

Question of the Day

How is wind direction normally reported: do we tell the direction from which the wind comes, or the direction toward which it is blowing?

Tag: Weather

Karen Meyers & Alexa Carey, August 22, 2006

Barney Peterson, August 18, 2006