## Sunday, February 22, 2015

### Reading and writing on weather maps

We need to write on and measure things on weather maps for several reasons. One is related to  evaluating a weather map so we know how much we can rely on it for weather routing decisions. A basic application is simply plot our position as carefully as possible on the latest surface analysis map at the valid time of the map and compare what it says the pressure, wind speed and direction were. We know from our own instruments what they really were, so the extent the map agrees is the extent to which we might believe the forecasts based on it.  And it is the forecasts we must use to select our route.

Then we turn to the forecast maps and do the same thing, to see what we should see when we get there, and to that extent, we know the forecast was right or not so right. This however, could well be too late!  Thus we have to know what we see now, and what is forecasted on a specific route, and then watch our actual conditions to see if they are evolving in that direction or not... and at what rate are they evolving.

In other words we do not really have to wait to see that the 48h forecast was wrong, we will see things changing from careful frequent observations to know if the rate of change seems consistent or not consistent.

Then to keep the navigator out of trouble on deck, they get to do this all over again every 6 hours to update the route selection and forecast evaluation.

The following are a few videos that address some of the issues, starting with a few of the basics of  map symbols.

We cover these processes and philosophies in the book Modern Marine Weather, but we are frequently reminded that in these modern times, videos are more popular than books. It could well be, however, that when you see our videos, you might vote for us sticking to the books. These are truly live presentations, unedited (at least for now), which has pros and cons. The cons are obvious; the pros are that doing things live you might discover interesting things or common snags that would be edited out of polished work.

Note to maybe come back to
When we are determining the direction of wind from the lay of the isobars in the videos below, it is often relative to circulation around Highs and Lows, but I want to stress that the direction of the wind does not matter where the actual Highs and Lows are located, meaning the local peaks in the distribution marked with big Hs and Ls. The direction at any specific point on the map depends only on the two adjacent isobars on either side of the point you care about.

One of these isobars will be higher pressure than the other. In the Northern Hemisphere, the wind flows toward the direction that puts the lower pressure isobar on your left-hand side facing downwind. This is the Buys Ballot law (actually discovered by the American William. Ferrel).

The rule is put your back to the surface wind, left arm out and slightly forward (20-30º) and you are pointing to the low.... which actually means pointing to the direction of the local low pressure. (The isobars could be winding all around a deep low center in some rather different direction.)

We are in effect using this rule backwards when we find wind from isobars. Place yourself on the point you care about, and then turn around so that the lower isobar is on your left and then you are facing the direction the wind flows toward.

Then with estimates of tangents to the isobars, one can fine tune that general direction with the slight shift into the lower side.. same as out of the higher side

PLEASE NOTE: the OPC has changed their website from what is in the videos. It is now ocean.weather.gov

Part 1 (3 min)

Part 2 (10 min)

Part 3 (12 min)

Part 4 (19 min)