Question

Under calm, clear conditions during the summer in a temperate climate similar to Boston’s, meteorologists can predict the minimum night time temperature quite accurately by making measurements of the relative humidity during the day. Explain why estimates made the same way are much less accurate in the winter, when maximum daytime temperatures are close to 0 °C.

 

 

Answer 1

Apart from usual gases like oxygen, nitrogen, carbon dioxide, atmosphere also contains water vapor which it gets due to process of evaporation from waterbodies like lakes, ponds, oceans etc. The fraction of water vapor present in atmosphere changes from place to place. It also changes due to height. It changes due to season. During rainy season, it increases while during summer it decreases

 

The amount of vapor in atmosphere is measured in relative terms. It is known as relative humidity. A

relative humidity implies that amount of humidity present in the atmosphere is half the humidity which can saturate air with vapor. Hence, it can be expressed in mathematical form as follows.

 

When air containing some amount of vapor is cooled gradually, the air tends to become saturated with vapor. At some temperature, air becomes fully saturated with vapor and further beyond this temperature causes vapor present in air to become visible in the form of dew.

 

The temperature at which it occurs, is known as Dew point. Dew point of air depends on its moisture content at that instant. Higher the moisture content, lower will be the Dew point. Temperature of a place can not be cooled below its Dew point in normal case.

 

The temperature during nighttime of a place is generally low as compared with that during daytime due to radiation of heat into sky by the earth. The minimum temperature during nighttime can be safely predicted by the meteorologists on the basis of relative humidity of the place during daytime. During change from daytime to nighttime, only the temperature of the atmosphere changes, not its moisture content. At the level of a particular moisture content which is measured by relative humidity, Dew point is the minimum temperature up to which atmosphere can cool. Hence, this temperature becomes the minimum possible temperature of that place during nighttime. Dew point temperature can be known , after recording the relative humidity of the place during daytime.

 

But this process of prediction fails during Winter when the prevailing temperature is around 0°C. In this case temperature outside may drop below freezing point. Hence prediction made on the basis of Dew point will not work because Dew point can not be below freezing point.

 

Hence, meteorologists find it difficult to make prediction of minimum temperature during nighttime when the season is Winter and atmospheric temperature is around 0°C during daytime.

Hence, meteorologists find it difficult to make prediction of minimum temperature during nighttime when the season is Winter and atmospheric temperature is around 0°C during daytime.