Why rain?

[Andy Bown]

I guess it'll be to do with upper temperatures, but had to ask!

The min last night was -2c, it was still below freezing with frosty surfaces at 06.15, so how come it has only rained since about ten minutes after that thermometer check? Temp now is nudging +1c with heavy rain.

[johnholmes]

your own asnwer is right Andy.

You need to look at the t-phi nearest to you for what the upper air is doing. If not wanting to do that then a quick scan of the 850mb temp on GFS or the Extra chart will show why. 850mb temp and the zero isotherm.

hope that helps

John

[Somerset Squall]

it was the same here, -1C and raining at 7am.

[Thundery wintry showers]

I'm not 100% certain of the specifics, but John is certainly right.

As far as I'm aware, if the temperatures in the upper atmosphere are high enough to cause falling snow to melt and turn to rain, or the precipitation starts off at above freezing and thus falls as rain, it will rain at low levels regardless of the temperature. Surface temperatures close to or below freezing may give rise to 'freezing rain'.

With regards the temp at which snow can fall, if there is a strong temperature contrast between the upper and lower atmosphere, snowflakes have a shallower layer of warm air to pass through before they reach the ground, so snow can fall at temperatures well above freezing. This is why unstable "sunshine and showers" conditions often produce snow at higher temperatures than persistent frontal bands, and also why snow falls at high temperatures in spring than in winter (localised surface heating from the sun)

[Andy Bown]

Thanks to John and TWS for your help. Interesting to have my obs backed up by SS.

[BrickFielder]

If the air higher up is above freezing then precipitation may fall as rain even if temperatures below are freezing or below because it takes time for the rain to freeze. It just may not have enough time to turn to snow by the time it reaches the ground. This is typically where icing (ice storms) can happen with rain falling and then freezing when it comes into contact with wires roads etc. Sometimes the ice build up can get very thick damaging wires and roofs with the weight.

[WhiteFox]

Always an interesting question!

In the Valentine's Day storm over here, most stations were reporting surface temperatures of between -7 and -5oC, yet we had rain (albeit freezing rain) falling from the sky.

The reason was that we had very dense cold air in situ, with a low pressure system moving up the coast. The low pressure system was drawing in much warmer air at higher levels, so at the 850 height the temperature was about 2-3oC above freezing for long periods of the storm. The chronology was thus:

First, powdery snow fell. Upper air temperatures were around -5 to -7oC and surface temperatures about -8oC. Early forecasts suggested that the surface temperatures would rise above freezing once the storm really wound up and a closed pressure system was formed.

Massive Warm Air advection on the Eastern side of the storm drew warmer air into the core. However, at the surface the colder air was too intense to be shifted and did not mix with warmer air; basic principles here I think with the much colder air of greater density remaining at low levels. I think some weather stations at higher elevations, say about 1000 feet were reporting temperatures close to or even above freezing, whereas at sea level temperatures never rose above -5oC.

When the storm intensified, the warmer air aloft wrapped around the system and fed into the North Easterly flow. The result was that in the upper levels of the storm, rain was falling. Of course, once it hit the extremely cold air in the lower 1000 feet, it froze. So, the snow turned to sleet and then to freezing rain.

When the storm pulled away, colder Canadian air fed down which turned the final ppn back to snow.

The final results were about 5 inches of snow followed by about an inch of sleet and then an inch of freezing rain. A total mess and remarkable for the fact that higher elevations experienced rain where lower elevations had freezing ppn.

The weather can be a strange beast!

I'd guess with a very shallow layer of cold air it wouldn't turn to ice..