You are not heating the outdoors, Mr. Grower. You are heating a room, and tonight the ceiling of that room is about sixty feet over your head. Everything you are going to spend before sunrise depends on how tall that room is — and on nothing else you have been told.
THERE are two different nights and the trade calls both of them a freeze. One of them you can fight. The other one you cannot, and I sold pots for nine years before I would print that sentence — I farmed twenty acres of Bosc myself until 1938 and I have stood in both kinds.
An ADVECTION FREEZE is a cold air mass arriving with wind behind it and no top to it at all. It is thousands of feet deep. Nothing you can burn will hold it. Any man who sells you a heater on the strength of an advection night is selling you a lottery ticket, and I will not do it.
A RADIATION FROST is the other night, and it is the one you have tonight. Clear sky, no wind, low dew point. The ground radiates its heat straight up to space, the air touching the ground cools by contact, and the cold layer stays SHALLOW. That shallowness is the whole economics of this business.
On such a night the temperature increases with height. Fifty feet up it is genuinely warmer than it is at your knees. That is the INVERSION, and it is the lid on the box. Your heat rises until it reaches air of its own temperature and then it spreads and stays. You are heating a room.
So the first number you need is not the forecast. It is the strength of your own inversion, read off two thermometers — one at five feet in the rows, one at fifty feet on the packing-shed roof. EIGHT DEGREES of spread and protection is cheap. TWO DEGREES and the lid is high and leaky and you are heating the sky at eleven and a half cents a gallon.
Demand climbs with the degrees of rise you want AND climbs sharply as the inversion weakens, together. Four degrees under a strong inversion may run you FORTY GALLONS an acre-hour. The same four degrees under a weak one will run past a hundred and twenty and still not arrive. There is a circular going around Medford quoting a heating rate flat, with no inversion figure beside it anywhere on the sheet. That is a figure with the bottom cut off it.
Big fires. A few hot stacks run wide open build a buoyant column that punches clean through the inversion, vents your orchard's heat to altitude, and pulls cold air in underneath to replace it. The man who lights half his pots and opens every damper has spent his oil building a chimney. MANY SMALL FIRES, burning cool and low and evenly spaced, is the whole method, and it is why you light from the outside in and give the upwind and downhill borders a double row.
Cold air is dense and it behaves like water. It drains downhill, it ponds against a hedgerow or a fill or a road embankment, and the coldest spot in your orchard is the low corner with something solid across the bottom of it. Cutting a gap through that hedgerow is worth more than twenty heaters and costs you a Saturday.
When the falling air temperature reaches the dew point, water condenses out and gives up its latent heat, and the curve flattens hard. A night with a dew point of 30 will very likely not go below 30. A night with a dew point of 22 has nothing under it and will keep going down until sunrise. The dew point taken at four in the afternoon tells you more about your minimum than the thermometer does at four in the afternoon.
Light at your critical temperature PLUS your lag. A cold orchard full of cold wood runs fifteen to twenty minutes behind the pots. And the critical temperature is a property of the BUD STAGE, not the crop: pear at swollen bud takes 24°F for half an hour, at first white 27°F, at full bloom 28°F, at small fruit 29°F. The same night is nothing in February and ruin in April.
It does not make wind. A wind machine is not a heater — it reaches up and pulls your warm ceiling air down and mixes it, and it is worth its price in exact proportion to your inversion strength and worth precisely NOTHING on a night without one. A machine plus a light rate of heating beats either alone, because the heat gets mixed instead of climbing.
It does not sprinkle. Sprinkling works on latent heat of fusion — freezing water releases about 144 BTU to the pound and holds the wet surface at 32°F while ice is forming — but you must not shut it off until the ice is melting off on its own, or evaporative cooling will take the bud lower than it would have gone dry. Rate goes up sharply with wind and with dryness.
It does not smoke, and that is not a courtesy. A smoke cloud holds essentially no heat in; the particle size is wrong to block the outgoing long-wave radiation, which is the whole of what you are losing. Black smoke costs you oil, costs you goodwill on the highway, and buys you nothing.
New this season. The collar is a rolled sleeve seated on three lugs above the stack throat rather than pinned through it, so the sleeve expands on its own and never binds at temperature. One quarter turn carries it from a 3/8-inch crack to full. It holds a setting through a nine-hour burn without creeping open, which the pinned collar did not, and that is the end of what I have to say about it.
Oil delivers about 140,000 BTU to the gallon. A stack pot at half a gallon an hour is putting out about 70,000 BTU an hour. Forty heaters to the acre is not a sales figure, Mr. Grower. It is a division problem, and you can do it at your own kitchen table.
Ran 38 of your No. 7 to the acre on the Applegate place the night of March 3rd, all of them at the low setting you specify, borders doubled. Held 4 degrees on 5 degrees of inversion and burned 19 gallons the acre over five hours and forty minutes. I had figured on 26. The collar did not creep on me one time.
I ran the recommended rate on the 12th of last April and lost the low forty anyway. Buds were at first white and the shed thermometer never showed under 26. I do not know whose fault that was and I am not saying it was yours. I am saying I paid for the oil and I have no crop off that block, and I would want the arithmetic explained to me before I bought again.
Cut the hedgerow at the bottom of my east block after reading your Bulletin 14 and the low corner has come up two full degrees on every clear night since. That is two degrees I am no longer buying. I still put in 60 heaters and I am not sorry, but the hedgerow was the cheaper half of it.
Crated 24 to the lot, F.O.B. Porterville. Rogue Valley deliveries by rail to Medford and truck out. Order before the 20th of March for April placement. Repair collars and stack sleeves stocked separately, catalog page 11.
Every figure on this page is computed at 140,000 BTU to the gallon of orchard-grade distillate and at this model's rated output of 70,000 BTU per hour at half damper, which is one-half gallon per hour. The co-op's plain stack pot is rated the same at half a gallon and it will do that honestly; what it will not do is hold the setting through a nine-hour burn, and a pot that creeps open at two in the morning is a pot burning three-quarters of a gallon at four in the morning and you will find that out from the fuel wagon and not from the thermometer.
Required heat is not proportional to degrees alone. It rises with degrees of protection wanted and it rises sharply as the inversion weakens, and the two multiply. Below about 2°F of inversion there is no practical lid on the box and no rate of oil we sell will buy you four degrees; the wheel on this page prints a bar of ink instead of a number in that case, and the bar of ink is the correct answer. Wind at four feet adds a real penalty because it drags your heated layer downhill and out from under you; at 4 mph you may add better than half again.
Smoke is not insulation. The particle size in oil smoke is wrong by more than an order to intercept the outgoing long-wave radiation from the orchard floor, which is the entire loss you are fighting. Any density of smoke that shades the orchard at sunrise costs you more in delayed warming than it ever held overnight. Burn clean.
Tonight, in this valley: dew point 22, and there is nothing at 22 to flatten the curve before daylight. Present shed reading 34, falling about a degree and a half the hour. Critical 24°F at swollen bud. Inversion measured at the shed roof, five feet against fifty, 5°F. 42 acres. Oil at 11½ cents. Set the wheel and tell the wagon before dark.
Fill in and mail. No salesman is going to drive out to your place unless you write that you want one.