weather forecast

[regulators]

on a weather forecast what does it mean when it says dew point, humidity and visibility

[brachiopod]

Let's start with humidity. This is the amount of water vapour (ie water in its gaseous state) in the air. The higher the air temperature, the more water vapour that the air can 'hold'. For a given temperature, there is a maximum amount of water vapour the air can hold, called the saturation point.

When we call the weather 'humid' or 'muggy'(a hot and sticky summer's day for example) we mean that there is a lot of water vapour in the air or that the air is close to saturation. This is known as relative humidity, expressed as a percentage of the saturation point (eg "85% humidity" would be a really sticky day irrespective of temperature!)

When water vapour cools, it condenses into tiny droplets. At high altitudes, these can coalesce to form rain. At low altitudes, the droplets can be suspended in the air to form mist or fog. This not only happens on Winter mornings, but can occur on summer days when, for example, warm moist air becomes near-saturated with water vapour such as near the sea. ('Sea mists')These conditions will reduce visibility, which is expressed as a distance. (Eg. 50m (thick fog) or 8km (sea haze). Particulates in the atmosphere (smoke particles etc.) can further reduce visibility and can act as condensation nucleii for water vapour to condense upon. (This is what caused the 'pea-soupers' in London in the 50's)

Visibility is always quoted in the shipping forecast, since a slight haze (that wouldn't affect us, say, driving on land), could reduce visibilty to shipping by several kilometres - (significant if you are keeping a lookout for other shipping.)

[Cont..]

[brachiopod]

Dew point, put simply, is the temperature at which water vapour condenses to form dew.

In reality, it is quite complicated, and depends on a number of variables.

So, going back to humidity, we also have absolute humidity, which is the mass of water vapour per volume of air, (g/m^3)

So air with an ab.humidity of  X g/m^3 may have a relative humidity of, say, 70% at 30�C. If this air cools to 15�C, the rel. humidity may be 90%. If it cools further to 7�C, the rel. humidity will pass its saturation point (ie >100%) and the vapour will condense.

If this happens at ground level, the condensed droplets are known as dew. The temperature at which this happens for air of a given abs. humidity is known as the dew point.

The process is further complicated by processes such as;

Super-cooling (where water vapour doea not condense, despite being past the saturation point);

Radiational cooling of the ground, where the ground cools down at a faster rate than the overlying air. On a summer's evening, this means the ground temperature can be 5�C and the overlying air temperature 15�C.

In the Autumn, it means you can get a frost even though the air temperature does not fall below freezing.

 

(Sorry if this is all a bit disjointed - but 1� year-old is running around and pulling mouse cable out of 'pooter at the same time as trying to think of and type answer)

[adydb]

wow brachiopod what an A1 answer.....