What about those Cold Fronts?
When we fly aircraft for either a living or a hobby there is no doubt about the fact that we “are in the air” and we need to know what it means to be “living in the air”. To any type of aviator it is important to watch the weather and know what to expect. My Will-ism is to always “look at the sky” and watch what is going on up there. Which way are the winds blowing, what is the temperature and dew point spread, is the barometric pressure changing, and what type of clouds are we looking at? These are all clues as to what is going on in the environment (air) just by watching. It is also a good operating practice to watch the Weather Channel (regularly) to see what they are showing and compare their information to what you are seeing.
Before we look into what types of clouds to expect prior to a Frontal Passage (FROPA) let us look at a few generalizations about the weather (wx) that we see every day. Most of the wx we experience in the US comes from the west (southwest through northwest). Prior to a cold front approaching we get a southerly flow of wind. As the cold front gets closer, wind velocity and gusts increase; then it begins to rain, with lightning and thunder (a by-product of lightening). The temperature begins to drop as the front moves through and the winds shift and flow out of the north, marking the Cold Frontal Passage (C-FROPA). Quite often the next morning the temperature and dew point cool and as they get closer together we get ground fog. Our typical C-FROPA!
Let’s dissect this common weather phenomena and fly through a C- FROPA. The wx map we are looking at above shows a typical cold front approaching from the NW. So let’s takeoff and fly west into an approaching cold front to see what is going on inside the front.
Takeoff: Clear skies, light wind from the SW, atmospheric pressure about 30.00 inhg, temperature dew point T/Dp 10 degrees apart, (T is always higher than the Dp). Clouds are the typical high altitude cirrus formations.
Climb: Cloud formations continue to build and become overcast at altitude, Altostratus. Winds become stronger from the S with gusts developing between light to occasional moderate bumps. Outside Air Temps (OAT) stay warm even as we climb.
Initial Cruise: As we near the front the winds become very strong from the S-SW, gusts build to moderate levels, and light rain begins to fall. OAT begins to drop and our flight condition turns from a VFR flight into an IFR condition as we enter into an area of nimbostratus clouds.
Cruise: As we near the front we enter into cumulonimbus clouds. Now it’s raining very hard and we are flying through heavy to possibly extreme turbulence. Hail is possible as well as very strong updrafts followed by downdrafts. The OAT has dropped dramatically and airfoil icing is a real possibility (even during summer months).
Back Side: (C-FROPA) We have now passed through the cold front. The winds are shifting to the N, still in moderate turbulence, and rain is beginning to diminish. The OAT Is dropping as well as the DP. Clouds begin to dissipate and the rain is ending.
Descent: We start to see higher stratocumulus clouds decreasing to stratus. Visibility is returning to VFR conditions, winds are veering to the NW and turbulence is light to the occasional moderate bump.
Approach and Landing: Clear skies, OAT and DP much cooler than at takeoff, 15 to 20 degree difference in the T/DP spread. Wind is out of the NW steady with occasional light bump and great visibility and barometric pressure is at 30.15 or higher. We made it!
If and when you, as an aviator run into this wx condition, make a decision early to change your flight. Some of the options are:
Land at the nearest airfield, and let the front pass through (grab a sandwich and drink). Strong fronts usually move through quickly.
Turn around and land at your original airfield and fly out west tomorrow.
Observe the wx map and know it’s not a good idea to go west today and make the trip later.
A quick note: If you are using the NEXRAD radar in your aircraft be aware that the wx information you are observing may be as much as 15 to 20 minutes old, and lead you into a flight condition and into “No Man’s Land”.
Fly Safe, Check That Weather, File a Flight Plan……Will