Philip Hertzog

August 2, 2005March 18, 2021

Philip Hertzog, August 2, 2005

NOAA Teacher at Sea
Philip Hertzog
Onboard NOAA Ship Rainier
July 25 – August 13, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: August 2, 2005

Weather Data from Bridge

Visibility: 10 nm
Wind Direction: light
Wind Speed: airs
Sea Wave Height: 0 feet
Sea Water Temperature: 12.8 ˚ C
Sea Level Pressure: 1002.5 mb
Cloud Cover: 8, stratocumulus

Science and Technology Log

I got the day off from the launches so I could catch up on my paper work and study up on onboard life. I slept in an extra hour, but as I emerged from my cabin for a late breakfast, Lt. Ben Evans asked me to accompany him over to the radio transmitter site.

I grabbed my backpack and within three minutes I sped across Cushing Bay in an open Zodiac pushed by a 115 horse motor. The wind had strengthened over night and we had more waves on our side of Mitrofania Island. We bounced over the waves and quickly reached the landing site. Our coxswain carefully eased the Zodiac towards shore and as a swell moved away from the beach we jumped out into ankle deep surf. The coxswain quickly backed up the Zodiac before the next swell could push it on to the beach.

Lt. Evans and I walked up to the transmitter site located in a grassy meadow about 150 feet from the water’s edge. Lt. Evans pulled out a volt meter and soon determined that the transmitter computer failed to turn off last night and drained the batteries. The station had stopped transmitting yesterday afternoon due to a software problem. Normally a set of solar panels recharge the transmitter station’s four batteries when the computer operates correctly. The following pictures show the troublesome computer and the solar panels: Lt. Evans decided to take the computer back to the RAINIER to reload the software and figure out the system problem. A crew plans to return tonight with the computer and a fresh set of 12 volt batteries. Just as we had landed at the site, we quickly boarded the Zodiac in between swells to prevent the boat from beaching or knocking against us. We returned to the RAINIER just in time to catch her set sail for a new work location.

Around 11:30 am, the RAINIER weighed anchor and sailed 7 miles north to Fish Ranch Bay adjacent to the southwestern Alaskan peninsula. The new location will protect us from the strong north east winds that started to rock the ship yesterday. As we crossed over to the new location, the RAINIER rocked side to side as 2-foot waves blown in from the Gulf of Alaska hit our starboard side. We entered the calm waters of Fish Ranch Bay and saw a large, pyramid shaped peak to the north, green colored mountains on the east and west, and a view of Mitrofania Island 7 miles to the south across open water. The survey launches crossed earlier and passed us while mapping the harbor we’ve entered. I spent the rest of the afternoon looking at our new surroundings and finishing up my paper work. In the evening, Mike Laird and I went out with four other crew members in a skiff to try out the fishing.

Personal Log

I had a relaxed day and felt good about completing my paper work. I enjoyed seeing how the transmitter site had change since my last visit. The crew had added a battery bank, solar panels and electronics to the site that I had not seen before.

Our biggest adventure of the day started after supper with a fishing expedition. Carl Verplank and Mike Riley took Mike Laird, Matt Boles and Josh Riley (Mike R.’s cousin) out in the R8 open boat to teach us how to fish Alaskan waters. We drifted over by a small bank and soon caught fish. I caught my second “keeper” halibut just on an 8 pound test line and a cheap $ 30 pole. My line broke just as Carl netted my halibut. Both Mike R. and Carl reeled in decent halibuts and then Mike L. surprised everyone when he landed a salmon off the bottom using halibut gear.

After a couple hours, we motored over to a steep embankment below the old town site of Mitrofania. Drifting along, we suddenly had fish on three lines and started hauling in sea bass. Sea bass tend to school and go into a feeding frenzy when one fish finds food. The bass weighed from 1 to 3 pounds and measured 6 inches to one foot in length. We saw sea bass rise up from under our skiff and watched two or three fish go after one lure. We released the smaller fish and kept the larger ones.

We headed back to the RAINIER at 10:15 pm in the evening light with sunset still a half an hour away. On the back of the RAINIER we cleaned and cut up our fish with guidance from Mike Riley while Carl carefully cleaned R8 and the deck as the dusk became night. I clean and cut up my first halibut and helped Mike Laird with his fish. We finally finished the last fish and stored our catch in the reefer (boat language for refrigerator) at 1:00 am. It will be tough to get up in the morning, but I won’t forget this evening for a long time.

Question of the Day

Why do sea bass school? Is this an advantage or disadvantage to their survival as a species?

August 1, 2005March 18, 2021

Philip Hertzog, August 1, 2005

NOAA Teacher at Sea
Philip Hertzog
Onboard NOAA Ship Rainier
July 25 – August 13, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: August 1, 2005

Weather Data from Bridge

Latitude: 55˚ 53.4’N
Longitude: 158˚ 50.4’W
Visibility: 10 nm
Wind Direction: 103˚
Wind Speed: 10kts
Sea Wave Height: 0-1 feet
Sea Water Temperature: 11.7˚ C
Sea Level Pressure: 1006.0 mb
Cloud Cover: 8, cumulonimbus

Science and Technology Log

I woke up to gray skies and a 10 knot wind. The wind blew waves to around 1 foot high and rocked the RAINIER gently in Cushman Bay. We have been lucky in that no rain has fallen in the eight days since we left Kodiak and the seas have been remained calm Mitrofania Island. The deck crew lowered the launches an hour earlier at 7:00 am and I joined launch RA-4 led by Ensign Andrew Halbach with assistance from Survey Tech Dan Boles. Coxswain Carl Verplank guided the RA-4 towards the south western side of Mitrofania Island near Spitz Rock.

As we rounded the corner of the island, one to three foot swells driven by a north east wind hit us and knocked the launch around and splashed water over our bow and up onto the windows. This made for roughest conditions I have seen so far on the trip, but not rough enough to affect our sonar mapping. Carl told me that the Rainier crew has mapped ocean bottom depth in worst conditions. We stopped the launch and Ensign Halbach let me lower the SEACAT CTD (conductivity, temperature, and density) probe to the bottom 200 meters down so we can collect data to correct our sonar data. As mentioned in previous log entries; temperature, conductivity (amount of salt in the water) and pressure changes how fast sound moves in water and the CTD probe gives the computer information to correct the sonar for these factors. The CTD data changes over the day and by location so we took measurement every four hours for a total of three times. Here is a close up of SEACAT CTD probe and Dan Boles lowering it the later in the day: After the probe returned to the launch, Ensign Halbach turned on the Reson Radar which has good resolution and works the best in shallow, near-shore waters and around rocks. Our first transects took us close to the shore and Dan sat on the bow and held on tight to look for submerged rocks that could damage the launch hull and sonar probe. Dan got knocked around and splashed with water, but we quickly returned to our dry cabin as we moved further off shore:

We “mowed the lawn” following long transects that took about half an hour each to complete before turning around and moving over 100 meters to start the next transect. On transects heading into the wind, our launch traveled at 7 knots per hour and hit each wave hard with a thump and splash over the bow. On transects following the wind, the waves picked us up and we “surfed” down the backsides of two to three foot swells. The following seas pushed the launch around and Carl first turned the steering wheel hard left and then hard right to keep us on a straight line. Later in the day, I drove the launch for over an hour and learned how to set a rhythm for completing these left and right turns for each wave. At first, the launch crew remained quiet as we fought some minor motion sickness. After eating and drinking coffee and soda, most of us perked up and started talking. Carl told us about finding brown bear tracks while fishing on the main land last night near the abounded village site of Mitrofania. Dan, Carl and I told each other bear stories and eventually shifted the conversation to education. Carl and Dan both have mothers that work in public schools and told me how their parents put in long hours during the school year.

Carl, a young man in his twenties, is from Fort Wayne, Indiana and worked on the RAINIER the past four years. Carl’s Dad is an attorney and he has some younger sisters that will meet him in Homer for a visit at the end of our current leg. Carl also completed underwater dive school this past spring and can now help install tide gauge stations or inspect the RAINIER’s hull. Carl plans to stay on the RAINIER for at least another year.

Dan Boles is slightly older than Carl and has a Bachelor’s degree in geology and French. Dan grew up in Tennessee and at one point his mom raised horses on a farm. Dan has been on board for almost a year and talked his younger brother (Matt) into joining the RAINIER. Can you imagine sharing a tiny bunk bed room and working with your brother all day long? From what I saw, Dan and Matt get along well.

After 5 hours, Carl pulled the launch behind Spitz Island that provided us protection from the wind and waves, but filled the air with the foul smell of sea gull dung from the thousands of birds nesting near by. The RAINIER crew gets a half hour lunch break whether on the ship or out in a launch. Ensign Halbach, who had been up late and working on our radio transmitter site, took a nap. Dan set up his Coleman stove and cooked up some salmon fillets he brought along. The salmon tasted good after a long morning out on the water. Carl and I fished off the launch and I landed a sea bass on my first cast. I actually caught three on my first cast, but the first two fell off the hook before the third set the line. I could see several sea bass fighting for my hook. Here are some photos from lunch: After lunch we continued or mapping till around 4:30 pm. The ride back became calmer after we moved past the corner and on to the north side of Mitrofania Island which blocked the wind. We had nice views of the mountains and the RAINIER as we approached the ship. In the evening, I went out on the fan deck (very back of the ship) and fished off the side. Everyone told me the fish weren’t biting, but I tried anyway. I quickly caught a small halibut and hauled it on board with help from other crew. After carefully removing the hook, I threw it back into the water so it could grow bigger before the next fisherman comes along. I fished a little longer and caught a second halibut.

I decided to keep this one and Mike Riley, an oiler from Engineering, showed me how to bleed and fillet the fish. Halibut are more difficult to clean than other fish because they are flat, almost pancake shaped on their sides and a back bone that runs down the middle of their body. They also swim side ways with the flat side facing up and look the surface with their two eyes located on the same side of their head.

After cleaning the halibut, Mike showed me how to vacuum pack the fish and how to store it in our big freezer. Mike is in his early twenties with a shaved head and several piercings in his ears, lips and nose. Mike looks almost like a pirate or punk rocker, but the crew respects him for his fishing and filleting abilities.

The evening ended well and I retired to my bunk for a well deserved sleep.

Personal Log

I had a busy day today getting up at 5:50 am and readying myself for the launch. I really had to keep my balance on the launch today as we bounced around, but I didn’t get sea sick like some people did in the other boat. Driving the launch was the best part of my day as I skipped over waves and learned how to handle it in following seas. I learned how to focus on a point far away and to use a rhythm in steering to keep a straight course in rough seas.

Catching the sea bass and two halibuts was a real treat as many people did not catch anything today in the windy conditions. I felt a bit sorry for the fish as we cut it up, but I look forward to eating the meat upon returning from the trip.

Question of the Day

What are three factors that would make waves high out in the ocean?

July 31, 2005March 18, 2021

Philip Hertzog, July 31, 2005

NOAA Teacher at Sea
Philip Hertzog
Onboard NOAA Ship Rainier
July 25 – August 13, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: July 31, 2005

Weather Data from Bridge

Latitude: 55˚53.4’ N
Longitude: 158˚ 50.4’ W
Visibility: 10 nm
Wind Direction: 180˚
Wind Speed: 2 kts
Sea Wave Height: 0 feet
Sea Water Temperature: 12.2˚ C
Sea Level Pressure: 1009.5 mb
Cloud Cover: 1, cumulus, altocumulus

Science and Technology Log

Today I took care of tasks that come with living aboard a ship as a crew member. I’ve been on board the RAINIER for almost 9 days now and my laundry started to attract sea gulls outside my room’s porthole. Even the Sei Whales turn and swim away when they approach my side of the ship. On shore, many of my students’ moms or dads do the laundry, but on a ship this becomes your responsibility. Washing clothes at sea poses special problems because we have no sewers to dispose of waste water, only the ocean. We have to use special soap that won’t harm sea creatures and limit the amount of water used for washing. The RAINIER has a full laundry with water saving washers and energy saving dryers as you can see in this photo.

After laundry, I cleaned my room. The Captain expects everyone to keep a clean room and make up their beds everyday. He can even enter your room at any time for an inspection. I share a room with the other teacher at sea, Mike Laird. The room has two bunks, two closets, a head (known on land as a bathroom) and a desk with a computer as you can see here: Mike and I lucked out by getting an officer’s room, because many crewmembers share rooms with up to four people and only have gym lockers to store their gear.

Once I cleaned my room and vacuumed the floor, I tackled the big job of cleaning the head. Again, you can’t bring a parent along to clean the bathroom for you and this job falls on everyone on the ship. The RAINIER does its best to protect the environment by using special cleaning products that kill mold and germs, but not aquatic life that live in the water where our wastes end up. I used three different types of cleaners: one for the tub, one for the toilet, and one for everything else. I kneeled down on my hands and knees to scrub everything from top to bottom to ensure the germs die and won’t make us sick miles from the nearest doctor. My mom and students would be proud of me! The rest of the day I caught up on my paper work and read, but tomorrow I will have a big day out on the launch.

Personal Log

I enjoyed a day off the launches to get caught up on all my house work and work on my photography. My cabin had gotten messy after spending eight nights in it and I look forward to sleeping on clean sheets tonight. I met with Larry Wooten, Chief Electronics Technician, and learned how to transfer my photos over the ship’s file server. The ship’s crew is one big family and share many things. Several people have put their photos on the main server so others can enjoy and download pictures. Everyone trusts each other on the ship. We leave our doors unlocked and you can leave your wallet out on the table without a worry. I wish our society back on shore could be just as trusting. Well, I’m off to find some salmon off the back end of the ship.

Question of the Day

Why is it important to keep a ship at sea so clean? What happens if someone on a ship becomes sick?

July 30, 2005March 18, 2021

Philip Hertzog, July 30, 2005

NOAA Teacher at Sea
Philip Hertzog
Onboard NOAA Ship Rainier
July 25 – August 13, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: July 30, 2005

Reson’s sonar output that generates a map of the ocean bottom near Spitz Island.

Weather Data from Bridge

Latitude: 55˚53.4’ N
Longitude: 158˚ 50.4’ W
Visibility: 10 nm
Wind Direction: light
Wind Speed: airs
Sea Wave Height: 0 feet
Sea Water Temperature: 12.2˚ C
Sea Level Pressure: 1012.5 mb
Cloud Cover: 7, cumulus, stratocumulus, altocumulus

Science and Technology Log

I went boating into new territory today. We took launch RA-4 and headed to the western end of Mitrofania Island to map the bottom around Spitz Island and several rocks. I got to learn more about the RAINIER crew, saw a new type of sonar, met some sea lions and even drove the launch. Ensign Brianna Welton led our launch with assistance from Lorraine Roubidoux. Ensign Welton is an expert in sonar technology and I watched other crew members seek out her help when problems crop up. Ms. Roubidoux goes to school at the University of New Hampshire where she’s earning a Masters Degree. She joined the RAINIER for a month to get experience with sonar systems. Ms Roubidoux conducts research on sonar “background scatter.” Background scatter occurs when sonar signals bounce around more than once and give false readings of ocean bottom depth. Ms. Roubidoux’s research will hopefully result in better sonar for future ships.

Women play an important role on NOAA ships. They serve as officers like Ensign Welton and scientists like Ms. Robidoux. Women also play key leadership roles on the RAINIER like our ship’s XO (Executive Officer), Commander Julia Neander, who takes command of our ship when the Captain leaves. I hope my students will learn that many cool opportunities exist for women in the sciences and they should not be discouraged from taking math and science classes. Above is a photo of Ms. Robidoux running the sonar on our launch.

Coxswain (official name for a sailor who drives small boats), Corey Mussey, carefully maneuvered the launch as we approached Spitz Island. Underwater rocks make this type of mapping more dangerous and Seaman Mussey moved the launch slowly and carefully to avoid ripping off the half million dollar sonar sensor from the hull. Because we moved into shallow water, Ensign Welton turned on a different type of Sonar Sensor called the Reson SeaBat 8101. The Reson works in water depths of 4 to 150 meters and gives a sharp, clear image of the bottom. The other sonar I saw before, the Elac, operates in deeper waters ranging from 40 to 400 meters, but does not give a clear image of the bottom. Corey told me you can actually see ship wrecks in full detail with the Reson sonar.

As we mapped, I occasionally stood on the bow of the launch and looked out for rocks as we moved close into shore. We passed over underwater “forests” of bull kelp and I saw 25 to 30 feet below the surface where a long, single whip like strand moves toward the surface and attaches to a floating round bulb. Out of the bulb comes half a dozen flat fronds about 5 to 10 feet in length and four inches wide that make the bull kelp look almost like underwater palm trees. Suddenly I saw a salmon dart quickly underwater and then 40 to 50 fish appear under the launch and move just as quickly out of view to our port (left) side.

As we moved back and forth in our “mowing the lawn” mapping pattern, we saw two groups of Steller Sea Lions. Four males sat on a small rocky island while two dozen or more females beach themselves on Spitz Island three hundred yards away. Each time we passed, the Sea Lions sat up and barked at us. We may be the first humans they have seen in this remote part of the Southwestern Alaskan peninsula. As you can see, the one male challenged me with its open mouth while another sat calmly with his seagull friend.

At the end of the day, Corey let me drive the launch and run one of the transect lines for the sonar mapping. As you can see in the photos below, I looked at a computer screen that showed our boat as a red torpedo along a line on the computer screen. I had to keep the black marker on the red and green bar at the bottom of the screen exactly in between the two colors or we would miss our mapping area. This proved difficult because just as one gets lined up a wave pushes the launch off course so you constantly correct the boat’s position. I found using the computer screen to drive the launch similar to a video game except you could wreck the boat and get hurt for real if one makes a mistake. I had a great day and returned to the ship to await another adventurous day.

Personal Log

I had a fantastic day. I got to see some interesting technology and talked to professional people. Being out on the bow of the launch scared me a bit. If we had hit a rock I failed to spot, the damage to the sonar system could equal a half a million dollars. The bow also requires a lot of balance and strength. Each time a wave rocked the launch, I risked falling into the cold Alaskan water and had to really pay attention.

Though the crew of the RAINIER works hard and long hours, they do get a chance to relax and Saturday nights are special. After supper, we loaded up into the open skiff and rode about mile to a wide open, gravelly beach for a party. A few people started a large bon fire and we had soda drinks and music playing. The skiff could only carry eight people at a time, but the party grew larger and noisier each time it arrived on the beach. People talked, told jokes, found whale bones, and caught salmon all evening long. The party lasted until 11:30 pm and we rode back to the RAINIER just as the Alaskan sky started to turn dark.

After returning to the ship, I joined some of the crew in the Wardroom (ship’s lounge) and watched the video, “Napoleon Dynamite,” about a high school student. We all laughed and talked about our own high school experiences. Tomorrow we all will be tired, but ready for another two weeks of work.

Question of the Day

How large can Stellar Sea Lions get? Where do we find Stellar Sea Lions and what are their life’s history.

July 29, 2005March 18, 2021

Philip Hertzog, July 29, 2005

NOAA Teacher at Sea
Philip Hertzog
Onboard NOAA Ship Rainier
July 25 – August 13, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: July 29, 2005

Weather Data from Bridge

Latitude: 58˚ 53.36’ N
Longitude: 158˚ 50.4’ W
Visibility: 10 nm
Wind Direction: light
Wind Speed: airs
Sea Wave Height: 0 feet
Sea Water Temperature: 12.2˚ C
Sea Level Pressure: 1013.5 mb
Cloud Cover: 8, cumulus, stratocumulus, altocumulus

Science and Technology Log

Today I worked on what the hydrographic map makers call “vertical control” and “horizontal control.” When NOAA makes maps showing how deep the water is, they have a problem in that the depth changes when the tides come in and go out. If a rock exists in the water, there may be no problem at high tide, but ships can run into the same rock at low tide.

To overcome this problem, NOAA measures bottom depths on their charts starting at a constant elevation called mean lower low water. Low tide occurs twice a day, but one low tide is always lower than the other. By keeping track of all the lowest, low tides of the day and averaging their elevations over many years, scientists can come up with an elevation for mean lower low water (MLLW). You want to start measuring from your lowest tide level to ensure that ship captains can trust the chart to protect them from danger even during low tide. All of the ocean bottom charts are based on depth below MLLW. However, when you collect sonar data, your height above MLLW constantly changes with the tide in a vertical position (up and down). Hence the term “vertical control” because the chart maker needs to know how to correct the sonar data so the maps are based on MLLW, not the current tide height. In remote areas like Alaska, limited tide data exists so the RAINIER crew installed a device called a tide gauge to measure and record the rise and fall of the tide in the mapping area. The information from the tide gauge will help us to correct the sonar data so we can make the charts based on MLLW.

The RAINIER crew installed a tide gauge on Mitrofania Island 1.5 weeks ago before I got on board. Today I rode in an open boat to help the crew check the tide gauge. Ensign Andrew Halbach led our mission with assistance from Survey Technician Matt Foss and Ensign Laurel Jennings. Mike Laird, the other Teacher at Sea also joined our group. Carl Verplank, Ordinary Seaman, drove the skiff and stayed off shore after dropping us off to ensure the boat won’t get stuck when the tide goes out. Carl had the best job because he fished for salmon until we needed a pick up. I hope he shares some fish with us tonight!

Upon reaching shore, Matt Foss and I walked over to the tide gauge station to check it out. Matt carried “bear repellant” with him which is pressurized pepper put into a spray can. If a grizzly bear should approach and attack us, the pepper spray might keep the bear from eating us. On the other hand, maybe bears like to have a little pepper on their steaks. In any event, we need to stay alert in bear country.

We found the tide gauge in good working order. Matt told me that Scuba Divers helped to put the gauge in and that it sends tide information via satellite back to Washington, DC for further analysis. Now that our vertical control (up and down movement) has been taken care of, Matt and I hiked over to join Ensign Halbach and Ensign Jennings who are working on “horizontal control” or side-to-side motion.

Normally, the crew of the RAINIER knows its horizontal position through the use of global positioning satellite (GPS). As discussed in previous log entries, GPS works by using signals from several satellites to locate your horizontal position on the Earth in terms of latitude and longitude. The chart makers combine sonar data with GPS data to create accurate maps of ocean bottom depth. Atmospheric conditions can affect the satellite signals so scientists calculate correction factors. Special radio stations transmit these factors which allow the launch crews to correct the GPS data. These corrections are called “horizontal control.”

Unfortunately, the remoteness and steep mountains of the Mitrofania Island area prevent the RAINIER from receiving good radio signals. We need to set up our own radio transmission and GPS base station to get good control. This task took up the rest of our day.

Matt and I found the others busily setting up the GPS station and taking measurements to ensure good location information. Ensign Halbach carefully leveled the GPS antenna and oriented it towards north. After setting up the GPS station, Carl picked us up and drove the open boat to another location about a mile away where we repeated the process and set up a second GPS station. However, constructing the radio transmitter tower proved to be our big challenge. Nobody in our group ever set up a tower before so we worked as a team to figure it out. We returned to the RAINIER and hit the machine shop where we measured out metal, drilled boltholes and scavenged any thing to help us build the tower.

We carefully load the skiff and quickly motored back the mile across the water to the transmitter site located on a sand bar that sticks out into Mitrofania Bay. Ensign Halbach led us in constructing the tower and it went up faster than planned. Two people hold the tower straight up and balance it while the other three string guy ropes to metal stakes pounded in the ground. The tower made us proud of our team work, but no one dares to climb it. Maybe some of you students reading this log entry would like to come to Alaska and try to climb it. We returned to the RAINIER and could see our tower on the horizon where it will transmit horizontal control data to all the launches conducting sonar work over the next two weeks.

Personal Log

This was the most physical day yet on the research vessel. I actively participated in setting up the tower instead of just observing. I really enjoyed working in a team today and helping to solve problems. I also had a good physical workout by carrying heavy equipment to the GPS and radio transmitter sites. The work out really helped because the food on board the RAINIER is delicious and plentiful with three large cooked meals a day. I need to watch my weight on this trip.

The tower project showed me you need both technical training and practical construction skills when out in a remote area like Alaska. My students tend to be either hands-on or all academic, but you need a balance of both these skills to be successful upon graduation. Many of the crew on the RAINIER learned their jobs while on the boat and had to solve difficult problems without any outside help. Hopefully my students can use the RAINIER’s crew as an example on the importance of seeking balance in their lives. Speaking of balance, it’s time for me to catch a salmon. Here I am ready to go. See you tomorrow.

Question of the Day

What causes the tide to rise and fall and how does it change over the course of an entire month?