Snow forecasting can be a daunting task, even for seasoned weather forecasters. There are many variables to consider, and just slight adjustments in these variables can make the difference between snow and rain. Snow forecasting is particularly challenging with the temperature is between 0C and 2C, because at these temperatures, a small adjustment in temperature can switch the precipitation type from snow to rain and vice versa.
The factors we will discuss here include:
- Dew point temperature
- Evaporative cooling
- Upper air temperatures
- Bring it all together
It’s a lot more complicated than just temperature – look to the dew point. The dew point is an additional atmospheric parameter that tells us the temperature at which water condenses at in the surface environment. If the temperature is 2C however the dew point is 0C, that means that any condensed water, or surface water, is likely to be closer to 0C. This means that precipitation has the potential to freeze when it reaches the surface if the dew point is at or below freezing – irrespective of air temperature.
This also means that if the dew point is 0C, then the temperature of the precipitation itself is likely to be 0C. So, if the air temp is 3C, however the dew point is 0C, then if you were to stick a thermometer on the precipitation it would measure somewhere between 3 and 0C, – probably closer to 0C. Of course, this means that the precipitation will more likely be snow when the dew point is at or below freezing. An air temp of 3C and a dew point of -1C will almost guarantee snow, because remember, the temperature of the precipitation is likely to be slightly higher than the dew point temperature. An air temp of 3C and dew point of 2C will definitely give rain, for this reason. So, you have now learnt that air temperature isn’t really too important. Look to the dew point. Low dew points are often indicative of a cold, continental air mass, which is often found in sunny conditions. This can heat up the surface layer, often to around 4-6C, especially in Spring. However, the dew points provide more information about the temperature of the air mass and likelihood for rain or snow, for scientific reasons I will not go into during this blog post.
Another factor is evaporative cooling. This is a mechanism whereby the intensity of precipitation can allow in-situ cooling of the air mass and subsequent lowering of the snow line. It can reduce the air temperature by several degrees within a short time frame, i.e. an hour, thus turning the rain to snow. This is quite common during frontal snow events. During these events, calculate the “wet bulb” temperature, and this is the temperature that will shortly be found following around 2/3mm of precipitation, although light winds and adequate precipitation intensity is required. Evaporative cooling will not be effective in a relative humidity above 90%, because the air is already saturated so further evporative cooling is not possible. During this process, moisture evaporates from the snowflake, the kinetic energy within the evaporated molecules derives latent heat from the atmosphere.
Next is upper air temperatures.
This isn’t a particularly useful one. Mainly because we aren’t looking for a specific temperature here, unlike explained in the dew point example. It can rain with an upper air temperature of -4C, and snow with an upper air temp of 0C. The upper air temperature is the temperature at 850millibars. In low heights, the upper air temperature level is brought closer to the surface because the surface pressure starts off lower. Think of it as moving the pressure gradient downwards. This means that for a particular flow, say an easterly, -4C uppers may be fine in very low pressure, however will result in rain in a higher pressure flow because the upper air temperature is raised by around 400ft. This is why I would shy away from using upper air temperatures unless you’re experienced. They can be very useful when one is aware of the particular upper air thresholds which exist, these are dependent on the atmospheric setup and factors in place. For example, ascertaining that the sea level upper air temp requirement is -7C, so with -6C uppers we can be confident that snow will fall to roughly 150m on the coast and 50m in land. This gradient is because inland the requirements are usually a bit lower because we can factor out heating from the sea.
The freezing line is much better for those with less knowledge, because it is a fixed value. A freezing line of 300m will produce snow to heights between 50 and 150m, because snow generally will make it relatively intact to heights between 150 and 250m below the freezing level because the dew point will remain at or below freezing for this extent.