I hear people around here talking about “minus tide”, especially as it relates to clamming and oystering. What’s a “minus tide”? It seems to be an especially low tide, but what is it “minus” in relation to? Average low tide?
Minus tides refer to low tides that drop below the “Mean Low Water” level. This generally occurs around the full and new moons when the tidal changes are greatest. It’s important for mariners because nav charts generally list depths in relation to Mean Low water depths. It’s probably important to clammers because it means that seabed that is usually underwater is exposed during these very low tides.
Doh… slight correction. Tide levels (including minus tides) and nautical charts are based on a reference point of “Mean Lower Low Water,” which is a bit lower than the “Mean Low Water” point. Here’s the definition of the two terms:
Mean Low Water (MLW)
The average height of all low water at a reference station over a 19 year cycle.
Mean Lower Low Water (MLLW)
The average height of the low water spring waters over a 19 year cycle. [“Spring waters” refers to spring tides which occur during the full and new moons. Neap tides occur during the first and third quarters of the moon]
Here’s a pretty good article explaining tides in general:
http://www.sailnet.com/collections/articles/index.cfm?articleid=sexton056