The frost night

At ten o'clock the thermometer in the garden said four degrees Celsius. I had just planted a row of young lettuces, and I knew that frost, which starts below zero, would kill them.

At $22:00$ the thermometer said $4{}^{\circ }\mathrm{C}$. My lettuces die in frost, that is, below $0{}^{\circ }\mathrm{C}$.

$22:00$, temperature $T=4{}^{\circ }\mathrm{C}$; frost $F\iff T<0$.

The air was cooling by one degree every hour until four in the morning. Four hours of cooling would bring it to zero at two o'clock, and after that there would be frost.

The air cools by $1{}^{\circ }\mathrm{C}$ every hour until $04:00$: after $t$ hours, $T=4-t$. So $T=0$ at $t=4$, that is $02:00$.

$T=4-t$ for $0\le t\le 6$; $T=0\iff t=4$ ($02:00$).

By four it would be minus two. I had an old sheet, and I knew a sheet keeps plants about three degrees warmer than the open air. So I spread it over the row.

At $04:00$, $t=6$ and $T=4-6=-2$. A sheet keeps plants about $3{}^{\circ }\mathrm{C}$ warmer, so I spread one over the row.

$t=6\Rightarrow T=-2$; sheet: $+3$.

Under the sheet, the coldest moment was minus two plus three, which is one degree above zero. In the morning the grass was white and the lettuces were green.

Under the sheet the coldest moment is $-2+3=1>0$. In the morning the grass was white and the lettuces were green.

$-2+3=1>0$ ✓.