Humidity?

[Backtrack]

Hello all.

I know what humidity is - the dryness of the air I think?

But what impact does humidity have on the temperature decline rate during the night on an Autumn/Winters evening?

Thanks

Edited October 23, 2010 by Backtrack

[vortex_liam]

Humidity is the amount of water vapour in the air. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin hope this helps did this in a few mins so i'll probally be ruled out by another post

[Backtrack]

Humidity is the amount of water vapour in the air. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin hope this helps did this in a few mins so i'll probally be ruled out by another post

Thanks for the reply.

Do you know how this impacts the rate in which a temperature drops?

[Terminal Moraine]

Thanks for the reply.

Do you know how this impacts the rate in which a temperature drops?

Generally speaking dry air cools more quickly than moist air but the rate at which the temperature falls in the evening is also affected by other factors.

For instance the rate of cooling will be greater if the ground is dry as the air spaces between the soil particles lose their heat more quickly than if they were filled with water ( air is a better conductor of heat than water )

The rate of cooling will also be greater in still air as even a light breeze will mix warmer air from above as the cooling air slowly sinks to the lower levels.

If you live on top of a hill, or on a slope, the rate of cooling will be quite rapid shortly after sunset but may then stop or slow down as the cooling air sinks to the lowest levels and warmer air is 'dragged down' from higher levels to replace it. In this situation you'll often find that, after an initial fall in temperature early in the night, there's no further fall in temperature until next morning when the valleys have reached their lowest temperature and the katabatic flow ceases. Once this happens heat is radiated from the air in your locality and the temperature falls again shortly before, or around, sunrise.

As you can see from my brief outline the rate at which the temperature falls in the evening is no simple matter, unless you live in a frost hollow, in which case it's usual for the temperature to fall steadily throughout the night as cooling air gradually sinks to the lowest level and then cools even more.

[Backtrack]

Generally speaking dry air cools more quickly than moist air but the rate at which the temperature falls in the evening is also affected by other factors.

For instance the rate of cooling will be greater if the ground is dry as the air spaces between the soil particles lose their heat more quickly than if they were filled with water ( air is a better conductor of heat than water )

The rate of cooling will also be greater in still air as even a light breeze will mix warmer air from above as the cooling air slowly sinks to the lower levels.

If you live on top of a hill, or on a slope, the rate of cooling will be quite rapid shortly after sunset but may then stop or slow down as the cooling air sinks to the lowest levels and warmer air is 'dragged down' from higher levels to replace it. In this situation you'll often find that, after an initial fall in temperature early in the night, there's no further fall in temperature until next morning when the valleys have reached their lowest temperature and the katabatic flow ceases. Once this happens heat is radiated from the air in your locality and the temperature falls again shortly before, or around, sunrise.

As you can see from my brief outline the rate at which the temperature falls in the evening is no simple matter, unless you live in a frost hollow, in which case it's usual for the temperature to fall steadily throughout the night as cooling air gradually sinks to the lowest level and then cools even more.

Thanks for taking the time to explain this to me TM.

Normally when I am recording night time temperatures the humidity would be around 85%. Probably the reason why the rate in which my temperature declines is slow.

Thanks again.

[Isolated Frost]

Generally speaking dry air cools more quickly than moist air but the rate at which the temperature falls in the evening is also affected by other factors.

For instance the rate of cooling will be greater if the ground is dry as the air spaces between the soil particles lose their heat more quickly than if they were filled with water ( air is a better conductor of heat than water )

The rate of cooling will also be greater in still air as even a light breeze will mix warmer air from above as the cooling air slowly sinks to the lower levels.

If you live on top of a hill, or on a slope, the rate of cooling will be quite rapid shortly after sunset but may then stop or slow down as the cooling air sinks to the lowest levels and warmer air is 'dragged down' from higher levels to replace it. In this situation you'll often find that, after an initial fall in temperature early in the night, there's no further fall in temperature until next morning when the valleys have reached their lowest temperature and the katabatic flow ceases. Once this happens heat is radiated from the air in your locality and the temperature falls again shortly before, or around, sunrise.

As you can see from my brief outline the rate at which the temperature falls in the evening is no simple matter, unless you live in a frost hollow, in which case it's usual for the temperature to fall steadily throughout the night as cooling air gradually sinks to the lowest level and then cools even more.

I agree with Backtrack. Excellent explanation!

[Mesoscale]

how does humidity effect the 'Feels like' temperature? it was 0C and the humidity was 80% i noticed that the feels like temperature was around -3? is that to do with humidity or just wind chill?

Sorry for stealing the post!

Edited October 27, 2010 by Mesoscale

[Terminal Moraine]

how does humidity effect the 'Feels like' temperature? it was 0C and the humidity was 80% i noticed that the feels like temperature was around -3? is that to do with humidity or just wind chill?

Sorry for stealing the post!

I'm assuming the 'feels like' temperature is on your AWS?

Strictly speaking it should take into consideration both wind speed and humidity as both will affect how the temperature actually feels; If your AWS doesn't have an anemometer then it's almost certainly relying on humidity only.

Human skin acts in a similar fashion to the wet bulb in a Stevenson Screen in that water evaporates from it causing cooling. Therefore the drier the air the cooler it will feel relative to the actual air temperature, it's for this reason that people say the heat abroad is more bearable, simply a matter of the humidity being much lower than is usually experienced in Britain during hot weather.

Wind speed plays a greater part than humidity alone because an air flow across the skin increases evaporation and therefore increases the cooling effect. An air temperature of -10c in dry, still air would be much more bearable than one of 1c in a 25 mph wind with 90% humidity.

Having said all that there is a cut off point with wind speed, around 40 mph, beyond which the evaporation from the skin does not increase, or by very little, as it has already reached its maximum.

The 'feels like' temperature is also loosely linked to the general weather in that dry air and a low 'feels like' temperature is also an important factor in the rate of snow melt or in the occurrence of Spring snow showers at a relatively high air temperature.

A clear and sunny winters day with low humidity and an air temperature of 2 or 3 degrees above freezing is much

more conducive to a snow cover lasting throughout the day than a cloudy day with a temperature of 1c and 90% humidity. Similarly, dry air and low dew points in an unstable polar maritime Spring air mass can produce snow showers when the air temperature is as much as 6c above freezing as the 'feels like'( ie the wet bulb ) temperature would probably be around 2c and as the precipitation falls even colder air is dragged down from above.

[johnholmes]

you are doing a grand job in here TM-thank you

there are many wind chill tables, the link below is just one site

http://www.eol.ucar.edu/homes/rilling/wc_table.html

Edited October 27, 2010 by johnholmes