Dew point

[whiteoaks7]

I understand what dew point is and roughly how it can be measured. What I don't understand is how it can be predicted well in advance (I've just downloaded a grib file and this question occurred to me). My training as a sailor and a pilot tells me that dew point is a consequence of the properties of an airmass so how is it predicted so far in advance? How approximate is the value quoted?

 

[boomerangben]

I understand what dew point is and roughly how it can be measured. What I don't understand is how it can be predicted well in advance (I've just downloaded a grib file and this question occurred to me). My training as a sailor and a pilot tells me that dew point is a consequence of the properties of an airmass so how is it predicted so far in advance? How approximate is the value quoted?

That's an interesting question. I guess if they know what the sea temperatures are and where the air is coming from, they can make a good guestimate for the amount of water a given airflow will pick up. I hope some one who does know answers....

 

[macd]

As I'm sure you know, dew point is a function of humidity and temperature. Both are known for locations where met data is taken, but predictions are based on weather modelling. If you check out a (detailed) grib file, you'll find most of the ingredients there.

 

[sarabande]

you coudl always ask Santa Claus for a psychrometer

 

[southerly]

A download of grib files in Zygrib contains a predicition of dew points 7 days in avance - so there is a relation between weather expectations and its individual data such as dew point - as mentioned already a function between temperature and humidity.

 

[franksingleton]

By definition, the dew point is the temperature at which condensation will occur when the air is cooled,. The air might be warmed by conduction and radiation from the ground but, unless there is some other reason such as a change of air mass, the dew point will stay the same. That is why it is known as a “conservative” property. Dew point is, in fact, a direct measure of the absolute (ie not relative) water content of the air and could equally be expressed in grammes of water per kilogramme of air.

One of its values to sailors is that if the sea temperature is lower than the dewpoint then the cooling by the sea will cause condensation ie into mist or fog. So, if the forecasts indicate that dewpoints will rise then look out for poor visibility. How is it forecast? I will try to give a simple explanation.

All weather forecasts start with numerical weather prediction models. These are mathematical simulations of all the physical processes that create what we know as weather. Among the variables measured in a variety of ways and predicted by the model is the water vapour and liquid water content of the air. At any particular location the water vapour content will depend on where the air is coming from. In other words - is the air coming drier or wetter than what you have now. It will also depend on what happens to the air. If the air is being lifted, it cools. Cloud will form. If the cloud causes rain, then water will fall out and the air will lose water. Evaporation from the sea will increase the water content.

I hope that helps more than it confuses.