Freezing Drizzle and Fog Create Havoc
Posted: Dec 18, 2014 10:58 PM CDT | Updated: Aug 5, 2015 2:23 PM CDT
At 10:51am, the National Weather Service in Sullivan, Wisconsin issued a Freezing Rain Advisory:
URGENT - WINTER WEATHER MESSAGE NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI 1051 AM CST THU DEC 18 2014
FREEZING DRIZZLE CAUSING ICY ROADS...
FREEZING DRIZZLE OCCURRING OVER WASHINGTON...OZAUKEE...WAUKESHA AND MILWAUKEE COUNTIES HAS RESULTED IN ICY ROADS.
THE MILWAUKEE NEWS MEDIA REPORTS NUMEROUS ACCIDENTS DUE TO ICY ROADS ACROSS NORTHERN MILWAUKEE AND SOUTHERN OZAUKEE COUNTIES.
THESE CONDITIONS ARE EXPECTED TO LINGER INTO THE EARLY AFTERNOON HOURS.
So how did this develop and why was there ice with temperatures below freezing? Well, it is a little complicated, but first we need two variables - freezing temps and moisture. We had both Thursday and they can be seen in this forecast sounding over Milwaukee, specifically over Mitchell International Airport. What is a sounding, you ask? A sounding is a representation of the atmosphere above a specific point - from the surface all the way up to sixty or seventy thousand feet. Since this is a forecast sounding, it is an estimation of the atmosphere, not a recording, above Mitchell International Airport. Here is a cut from the sounding:
Ok, there is a lot going on there. Let me define and explain just part of what you see. The orange line is the freezing or, more accurately the melting point, of water. Everything to the left of the orange line is below freezing. The red line is the temperature. The green line is the dewpoint and the blue line is the wet bulb temperature. The most important issue is that the entire column of air is left of the line, or below the melting point from the surface up.
So, if it is below freezing, how did we have liquid water? Well did you know that water can remain liquid below 32°F or 0°C? It can and does. That is why it is more accurate to say that 32°F is the melting point of water and not the freezing point. In fog and stratus clouds, water vapor can condense into super cooled water droplets. That is; water droplets that remain liquid even when the temperature is colder than 32°F. When this happens, the water droplet freezes on contact to any surface it touches that is below freezing; surfaces such as roads, trees, cars and even airplanes. On Thursday, roads were the biggest issue. It also didn't help that rain, earlier in the week, washed salt off the roads. So, the thin layer of ice collected on untreated roads.
So what happened this morning? Well, we meet the first two criteria to make snowflakes: 1. colder than melting point and 2. the air is moist/ saturated. Well, it also takes hexagonal ice crystals that grow off of condensation nuclei. Hexagonal ice crystals form at temperatures colder than 10° F. If you look at the sounding above, the horizontal white area is cloud cover. The freezing mist and drizzle came out of this area. That cloud layer never gets colder than -12°c or 10°F. Meaning that ice crystals never developed in great numbers to create snowflakes. In this absence, the water vapor condensed out as liquid mist or drizzle. Our forecast sounding saw this:
As you can see, there is a warning on the software: No ice crystals in Clouds. Consider Freezing Drizzle. That looks straight forward. How can that be missed? Well, that same software also called for no precipitation. So, what amounted to a trace amount of water, created some problems on area roads Thursday. By the afternoon, most of the freezing drizzle and fog ended.You now know that water doesn't always freeze at 32°F and to get snowflakes is actually very technical. In a future article, I will explain how temperature also determines the type of snowflake and why this is critical in forecasting snow amounts.As always, let me know if you have any questions and feel free to like, share, recommend or forget this article. Mark McGinnis