What is the temperature in the weather forecast based on?

The meteorological department stipulates that 20 o'clock every day is the limit of one day, and the highest (low) temperature is generally observed at this time. Therefore, the instrument used to measure the maximum (low) temperature is required to be different from the general thermometer in structure, so as to ensure that the indicator does not decrease (increase) with the decrease (increase) of temperature when increasing (decreasing) to the maximum (low) value. The thermometer used to measure the highest temperature has a narrow mouth at the upper end of its ball. When the temperature rises, the mercury in the ball expands and the indicator increases. When the temperature drops, the mercury shrinks, but due to the narrow mouth at the upper end and the contraction pressure caused by the temperature drop, the mercury cannot shrink back into the target, and the indicator remains unchanged. Generally speaking, when the temperature of this thermometer changes, the mercury column can only rise, not fall. Therefore, after observing at 20 o'clock every day, the observer must hold the watch body with the ball facing down, stretch his arm for about 30 degrees, and use the big arm to drive the small arm to shake the thermometer back and forth several times, so as to reduce the mercury column in the thin tube to the temperature of 20 o'clock for reuse the next day. The thermometer used to measure the lowest temperature takes advantage of the low freezing point of alcohol, replaces mercury with alcohol, and puts an I-cursor in the thin tube of the casing thermometer. When the temperature drops, the alcohol column shrinks and the cursor is brought to the ball by the tension on the alcohol surface. When the temperature rises, alcohol rises from the gap between the cursor and the pipe wall, while the cursor remains in place. Therefore, the thermometer can also be said to be a thermometer with a cursor that only drops but does not rise when the temperature changes. After the observation at 20 o'clock every day, the observer must lift the bulb of the thermometer by hand to move the cursor forward (raise) to the temperature value at that time, and then use it again the next day. Choose the highest (lowest) value from the highest and lowest temperatures in ten days, January or a year as the highest (lowest) temperature extreme value in ten days, January or a year, and the meteorological department calls it the extreme highest (lowest) temperature in ten days, months or a year.