Hardy Tropicals UK

In terms of plant growth these are important, but can also be counter productive in winter. There are benefits and problems with both high and low humidity at any given time of the year.

In short, a dew point can be described as 'the temperature at which dew will form'. What dictates how high or low a dewpoint can get, is the amount of moisture being held in the air, this is usually expressed in actual amounts (gH20/kg Air), or by relatively humidity (the percentage of available water in the air).

In a humid climate the temperature and dewpoint will always be close together. For example in mild, wet periods in winter, or warm, humid summer days the temperature and dewpoint will always be within a few degrees of each other. For example the temperature may be 21C and the dewpoint may be 19C, making it feel very humid.

In times of winter Atlantic depressions it is not unusual to see an air temperature of 11C complimented by a dewpoint of 11C, this means the air is saturated with water, relatively speaking. Conversely in very dry periods, often the dewpoint will be vastly lower than the air temperature, for example a prolonged drought may see a temperature of 25C with a dewpoint of 5C, but in the UK this is an infrequent occurrence.

Relative Humidity is calculated using the air temperature and dewpoint, it is measured in % and it is the measure of humidity people are most familiar with. The standard humidity in the British year is roughly 76%, we are afterall a humid country.

A very broad and crude way of measuring humidity is to multiple the dewpoint by the temperature and divide by 100, but it is very crude and not often correct.

The dewpoint cannot exceed the air temperature, as the dewpoint can only match the air temperature, once the dewpoint reaches the extent of the air temperature, for example if the air temperature and dewpoint are the same, then relative humidity is 100%

Once of the benefits of high humidity, is that it holds more heat so on a clear night where the air is relatively humid, the temperature will drop slower, whereas in an area with very low dewpoints and consequently very low relative humidity, the temperature can fall very rapidly. For this reason, the coldest spots are generally found inland, whilst the warmest spots in winter are often found by the sea in calm conditions.

The highest dewpoint occur under fog and drizzle, whilst the lowest dewpoints occur in strong summer sunshine, or sometimes in continental winter cold spells.

That's pretty much the basics, again feel free to query anything I might have missed out or if you have a general question.

Not strictly related to gardening but......we viewed an empty house recently which claimed to have no damp problems. Imagine our surprise to find every single wall including the internal ones wet for about 2-3ft from the ground up (and to a peak of about 7ft on a chimney breast). The walls were clean (no mold anywhere), there were no mineral deposits in the damp area and there was no blistering to the paint or plaster. It was a detached rural property in a low lying area in Norfolk. Could this be a typical effect of the dew point in an unheated, poorly ventilated house forming dew on the lower, cooler parts of the walls?

Or has it got a monumental rising damp problem?

It sounds like a typical case of the ground being significantly colder than the air. In some cases the difference between the ground and air temperature (a relatively large difference) can cause dampness and humidity lower down.

In fact the best way to describe is is on a morning where it has been cool, the ground about 1 feet high is very cold, but the air above 1 feet high is significantly warmer, you will often see a layer of mist occur, this is basically what has happened in the house (obviously without the mist). A very shallow inversion has set up at ground level, promoting constantly damp below 3ft. The easiest way to combat this is usually to install underfloor heating.