Rule of Thumb on overnight temperatures

The TV weather man happened to mention this rule of thumb the other day so I looked it up.

This is a good post.

when forecasting the overnight low temperature, it should never be more than a degree or two cooler than the dewpoint temperature.

Latent heat of course.

From the post.

This is why warm, moist air is so important when looking at the formation of thunderstorms. The heat capacity of water is pretty incredible, and it provides that extra fuel that makes storms so violent.

Speaking of rules of thumb, some of us have heard; hot to cold ventilate bold, cold to hot ventilate not.

One of my long-time chief mates had never heard this, he lived in Florida so I told him if we were gong from Florida to Maine we needed to get rid of the bad Florida air and replace with good Maine air and if we had good Maine air in the holds we needed to keep it.

Condensation on a car ship is bad news, the water doesn’t hurt the cars but the various debris carried does, soot from diesel exhaust, dirt, paint chips, rust etc.

We learned that our assumptions on what the humidity of the air in the holds was not reliable so when there was a possibility of moisture in the hold being a problem we sent the cadet around in the morning with sling psychrometer. Having the wet/dry temps from each deck was a big assist. We had been at times ventilating and making things worse. To measure is to know.


I always thought that was just for bulkers - never occurred to me that car carriers would follow as well.

Cold soaked is a term we used if the temp control wasn’t cranked up high prior to a rapid ascent in a Lear. It would take a while to warm up the cabin after the skin had absorbed the cold air at high altitude.

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It was strictly followed in general cargo ships and psychrometer readings were recorded daily on voyages from Europe in the past. On the voyage North the holds were full of frozen cargo and it didn’t matter.

When I first started on the car ship the routine was to ventilate the holds/decks every other day which required running a second generator. Later when fuel went up in price it was “as needed”.

After about four days or so the holds will get hazy from the gases thrown off by the plastics from the new cars, bad enough that the smoke detectors will start going off.

It’s not too hard to get the ventilation right but in general they get real fussy about new cars.

For a cargo of cars condensation is a second order problem. For example if the holds are clean the run-off doesn’t cause any damage.

The first time we had problems was from these deck vents:

Not the big blowers next to the rails but the so-called "skylight just to port of the fire main running down the center line. Sticking up above the deck are they cool down faster then the rest of the holds.

Over the ;years, a few dust storms etc this area got overlooked during hold cleaning. This was our first time getting dinged in this way.

Excess water was never a problem, the water led to other problems.

We used the same term in the AF and would crank the temperature controller all the way down to “cold soak” our KC-135 during training flights prior to descent and 2 hours+ in the traffic pattern shooting touch and go’s at Castle AFB in central California. The air-packs on the old A-models just couldn’t keep up with the summer heat.

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