When we get a heatwave in Britain, sometimes it's humid and sometimes it isn't. What makes the difference?

Answer 1

moisture in the air????

Answer 2

It has to do with where the heatwave came from. It could have come from an airmass in the ocean call a Maritime tropical which often leaves moisture behind causing humidity. Or the airmass containing the heatwave could have come from land. This airmass is called a Continental tropical. Because it was formed over land it did not bring any moisture with it.

Answer 3

The humidity of the air is - among other things - determined by where the air comes from. For instance, if the air has travelled a long way over an ocean, it will be more humid than if it has been over a continent for a while.

In Britain, the air should be more humid if the general air flow is from the south-west than if it's from the south-east.

Answer 4

The air that we breathe is made up of numerous gases, including water vapor. The term humidity generally refers to the amount of this water vapor in the atmosphere. Each atmospheric gas has its own vapor pressure, a measure of the number of molecules present at a given temperature. The vapor pressure of water thus measures the amount of water vapor in the air. The saturation vapor pressure is the vapor pressure when liquid water begins to condense. Relative humidity is determined by using the actual vapor pressure divided by the saturation vapor pressure Meteorologists also use dewpoint temperature as a measure of the amount of water vapor in the atmosphere. This is the temperature at which the atmosphere becomes saturated and dew starts to form. It is also defined as the temperature at which the vapor pressure equals the saturation vapor pressure. Thus, as the temperature approaches and equals the dewpoint temperature, the vapor pressure and the saturation vapor pressure become the same. When this occurs, dew starts to form.
This phenomenon is illustrated by a glass of cool iced tea on a warm muggy day. The air around the glass cools and water condenses on the outside of the glass. The temperature at which the water begins to condense is the dewpoint temperature.
There are a few meanings of the term humidity. All deal with the amount of moisture in the air, but differ slightly.
Relative humidity, a term often used by the local TV and radio media, is a measure of the actual amount of moisture in the air compared to the total amount of moisture that the air can hold. Warm air can hold more water than cool air. But if the air (warm or cool) is holding half as much moisture as it can hold when saturated, the relative humidity is 50 percent.
Meteorologists, in contrast, use dewpoint temperature as a measure of the moisture content of air. This is the temperature below which the air can no longer hold the moisture in vapor form and liquid water or dew will form. This phenomenon is illustrated by a glass of cool iced tea on a warm muggy day. The air around the glass cools and causes water to form on the outside of the glass. The temperature at which the water forms is the dewpoint temperature.
Humidity is most often used to describe how a person feels, often in conjunction with heat. If it is hot and humid, a person will usually feel much more uncomfortable. The reason for this is that the body tries to cool itself through evaporation of moisture on the skin. But when the air is humid, evaporating the moisture becomes more difficult and the actual cooling effect is much smaller. It is this that makes the difference.