Quick Method to Determine Precipitation-type

[Wx4cast]

Thickness is the distance between one (pressure) level in the atmosphere from another level. The greater the distance between levels the greater the thickness and the "WARMER" is the air in that layer. The less the distance between levels the "COLDER" the air between to those layers so the lower the thickness value.

Each layer has a critical thickness. The dirty rule of thumb is a thickness of 5400 meters or 540 decameters along with an 850 hPa temperature of ZERO degrees equates to snow at the surface. This is not really true. A lot can depend on temperatures below 850 hPa and how stably stratified the atmosphere is. This is where checking thickness' in the lower layers of the atmosphere are important.

The following layers and the critical values for them (critical as in rain vs snow) are listed. Again the lower the value the colder the air the better the chance for snow so thickness' does matter.

Layer Critical Thickness

1000-850 1300 meters or 130 dkm

1000-700 2860 meters or 286 dkm

1000-500 5400 meters or 540 dkm

850-700 1540 meters or 154 dkm

The last layer is critical for snow or snow/mix or precip changing to sleet.

The last "critical value" also know as the "MAGIC NUMBER" (wish there really was such a thing with the atmosphere) is: ([ 1000-850 thickness TIMES 2] + [850-700 millibar thickness]). If the value of this is at or below 4140 meters than forecast mostly snow.

Other things to look for are for example an 850-700 millibar thickness > 1540 meters but ALL the other levels are below (cold) the critical thickness. The warm layer betwen 850-700 millibar will either cause sleet to mix in or even have the precip to fall as mostly sleet. Conversely it is possible to have 1000-500 millibar thickness > than 5400 meters but all the other thickness values are at or below (cold) critical you'll probably wind up with all snow.

A final note about critical thickness values: they vary based on both a locations LATITUDE and ELEVATION

[baroclinic_instability]

They vary a lot in the intermountain west. Thicknesses just don't work out there that well at all especially with valleys and the inversions that can develop in winter. Some people rely on 700 temps out there, but really the only way is a combination of soundings and local knowledge of valley inversions (i.e. how deep does it develop, how apt is it to mix out, etc.).

[hudsonvalley21]

Thickness is the distance between one (pressure) level in the atmosphere from another level. The greater the distance between levels the greater the thickness and the "WARMER" is the air in that layer. The less the distance between levels the "COLDER" the air between to those layers so the lower the thickness value.

Each layer has a critical thickness. The dirty rule of thumb is a thickness of 5400 meters or 540 decameters along with an 850 hPa temperature equates to snow at the surface. This is really true. A lot can depend on temperatures below 850 hPa and how stably stratified the atmosphere is. This is where checking thickness' in lower layers of the atmosphere are important.

The following layers and the critical values for them (critical as in rain vs snow) are listed. Again the lower the value the colder the air the better the chance for snow so thickness' does matter.

Layer Critical Thickness

1000-850 1300 meters or 130 dkm

1000-700 2860 meters or 286 dkm

1000-500 5400 meters or 540 dkm

850-700 1560 meters or 156 dkm

The last "critical value" also know as the "MAGIC NUMBER" (wish there really was such a thing with the atmosphere) is: ([ 1000-850 thickness TIMES 2] + [850-700 millibar thickness]). If the value of this is at or below 4180 meters than forecast mostly snow.

Other things to look for are for example an 850-700 millibar thickness > 1560 meters but ALL the other levels are below (cold)the critical thickness. The warm layer betwen 850-700 millibar will either cause sleet to mix in or even have the precip to fall as mostly sleet. Conversely it is possible to have 1000-500 millibar thickness > than 5400 meters but all the other thickness values are at or below (cold) critical you'll probably wind up with all snow.

The latter layer is critical for for snow or mix or snow mixing with or changing to sleet.

A final note about critical thickness values they critical value does vary a bit based on both a locations LATITUDE and ELEVATION

Thanks for the post. Well written and easy to understand.

[Wx4cast]

Thanks for the post. Well written and easy to understand.

I'll try and find the "critical" thickness for a few selected locations around the NEUS. Glad you liked it.

[ORH_wxman]

Andy, I think the 850-700 critical thickness is 1540m rather than 1560m.

[hudsonvalley21]

I'll try and find the "critical" thickness for a few selected locations around the NEUS. Glad you liked it.

I've just read your other post on how to read the Skew-t diag. I mentioned this would be great to have say a weather 101 topic fourm. Thank you for sharing your expertise.

[Wx4cast]

Andy, I think the 850-700 critical thickness is 1540m rather than 1560m.

Good catch thanks Will...

[Tornado Girl]

I just wanted to add to what Wx4cast said about thickness. The basic Idea when making a forecast is this, if the temperature near the ground is above freezing and the thickness is big then it should rain. However, if the temperature near the ground is below freezing and the thickness is small then the precipitation should fall as snow. Like Wx4cast said the thickness that separates rain from the snow is called the critical thickness, which you will remember from above is defined as the 1000-to-500-mb thickness, basically this means that is where there is a 50 % chance of snow and a 50 % chance of rain. Like Andy said, the standard value for critical thickness is 5400 meters, or 540 decameters on the East Coast. He also said, the critical thickness value depends on such things as location and elevation. If you're on the West Coast, The Great Plains, or some mountain critical thickness will be different. Therefore, 5400 meters is not always the critical thickness. Does this "Magic Value" always work? ... In meteorology there are few absolutes ... so the short answer is no. In the late Fall or early Spring. Snow has been known to fall when the thickness is higher than the critical thickness. This is because the temperature near the ground is below freezing and the temperature aloft is warmer. because of this the flakes do not melt before they hit the ground. Of course the reverse is true for rain, if the temperature near the ground is above freezing and the air temperature aloft is lower.