Intro Guide Part Three: The Dew Point and Feels Like

The first four observations:

The temperature (Temp) is the actual air temperature outside, measured out of the direct sunlight. This is the one we most often hear about.

The apparent temperature (App Temp) is what it 'feels like', when you take the wind and moisture in the air into consideration.

The dew point temperature (Dew Point) tells us how much moisture is in the air. Technically, its is the temperature at which water vapour in the air will condense into liquid water. Or, what temperature you would need to lower the air temperature to, if you wanted saturation - ie. a cloud, dew or fog. 

Relative humidity (Rel Hum) also describes the saturation - so why don't we just use that? I prefer the dew point, because the relative humidity is exactly that, its relative. Relative to what the air temperature is doing.

Let's look at this in real life:

Its 7am in Melbourne, and the temperature is 5C while the dew point temperature is 4C. This airmass produces a relative humidity of 90%. That's really high humidity, but it doesn't feel warm. In fact, when you take the wind and moisture into account, it feels like only 3C!

Its 7am in Brisbane, and the temperature is 25C while the dew point temperature is 24C. This produces a relative humidity of 95% and yes, it feels warm, in fact it feels even warmer than it actually is. The apparent temperature is 28C!

Both situations were 'humid' but one felt frosty, while the other felt oppressive. 

DEW POINT AND FEELS LIKE

The dew point can tell us more about what it should actually feel like:

Tropical regions may use 20+ = humid, and 24+ = oppressive. But southern regions will certainly notice a dew point of 15+, as it's not a regular occurrence.

And this is why an air temperature of 25C can feel so different from city to city. Typically:

• Melbourne - dew point of 5 to 10C = 25C feels like 19C

• Sydney - dew point of 15C = 25C feels like 24C

• Brisbane - dew point of 24C = 25C feels like 28C

WIND CHILL

When you consider both moisture and the wind, it can make it feel very different to what the air temperature states.

A cold outbreak day in Melbourne may have a temperature of 13C, but the wind is a southwesterly blowing at 35km/h, so it feels like only 5C.

Melbourne Airport has often had a temperature of 9C with a southerly blowing at 50km/h, so it feels like -2C -> welcome to Melbourne! 

Meanwhile, up in the mountains, a temperature of 2C with a wind of 80km/h, can make it feel like -16C. This is dangerous, and shows us why we should consider not just the temperature but also the wind speed.

FROST

Another handy hint with the dew point is the potential for frost tonight.

If nothing changes with the weather (ie the airmass remains the same, typically under a high pressure system), then the dew point at 3pm may show how cold the air temperature will drop overnight.

3pm: the temperature is 16C and the dew point is 2C. If nothing changes, the temperature could fall to a minimum of 2C by 8am the next day - the minimum.

This is because the temperature can't go any lower than the dew point temperature, or the air would be more than 100% saturated. 

"Nothing changes" relates to a high pressure system overhead, with clear skies and light or calm winds. 

And yes, 2C can produce a frost, because that's 2C as recorded by the weather station which is 1.5 metres above the ground. The air forms layers overnight as the days heat escapes out to space, so its 2C at chest height, but -2C at the ground. Frost can form with an air temperature as high as 4C. 

The next part of this series is available now: Part Four: How to Read a Forecast.