Fronts and Clouds
Pilots preparing for a VFR flight are expected to read aviation weather maps, METARs, TAFs and interpret this information to assess if their flight can be successfully completed. We refresh some of these once learned subjects and discuss, among other things, warm- and cold fronts and the effect on the flight they have in terms of visibility and cloud base.
All of this while avoiding to become a meteorological textbook, there are enough websites doing that so were are not going too. In the menu on the left you will find pages with weather maps, radar and satellite images suitable for aviation weather flight planning.
The transition zone between two air masses is known as a front. You will notice a front passage by changes in temperature, wind direction, wind intensity and a change in barometric pressure.
Pilots are expected to to recognize these indications.
Reading Weather Fronts
The severity of the weather in a front depends on the stability, temperature and moisture differences in these air masses. These air masses can originate from different sources: polar, maritime and continental. Each source has different properties which depends on the region and how long the air mass has been there.
When looking at weather maps you will notice these blue lines with triangles and red lines with half solid circles. These are colder or warmer air masses and they move generally from west to east. An air mass moving in with a higher temperature than the air mass it is displacing is called a warm front, see image to the right.
Warm fronts move slower than cold fronts and as warm air is less dense than cold air the front tends to slope over the cold air many miles (up to 500) ahead of the surface front. Indicators of a approaching warm front are: high cirrus clouds followed by a lowering base of Cirri-, Alto- and Nimbostratus clouds.
Flying into a warm front you may notice lower stratiform clouds, ice pellets (depends on freezing level), freezing rain and rain. If the air mass is unstable and contains enough moisture embedded Cumulus or even Cumulonimbus (Cb) can be found. With moist and stable air masses you will find an extensive layer of clouds well ahead of the surface front. Precipitation will be zero if the air is dry and stable.
As the front moves through, the wind veers, temperature rises (warm front) and the barometric pressure slows or stops dropping. When the front has passed stratus clouds and or fog can remain with poor visibility. Its the warm sector between warm front and following cold front.
On the contrary, air masses with lower temperature overtaking a warmer air mass are known as cold fronts. See figure on the right. The cold air is denser, heavier than warm air and moves under the warm air. Fast moving cold fronts move the warm air up and forward, the weather along and up to 150 - 200 miles ahead of the surface front.
This creates extremely hazardous weather. These fronts creates lines of thunderstorms aka squall lines. As the fronts pass the temperature drops, lowers dew point and the wind veers (changes direction clockwise, anticlockwise in southern hemisphere)
Slow moving cold fronts are shallower and have less hazardous weather. The air is lifted slowly as the front approaches. If the air is stable, you will see a broad band of weather behind the front. Stratus clouds formation is likely with possible fog or rain. With unstable air Cumulus and Cumulonimbus clouds with localized rain are more likely. Visibility is usually pretty good after a cold front passes through and temperatures will drop.
Clouds over the Indian Ocean as seen from space
At one point in time the faster moving cold front is going to catch up with the warm front. When this happens we have an occluded front and this is were the colder air lifts up the warmer air between the fronts. There are two type of occluded fronts, warm front occlusion and a cold front occlusion. A flight through an occlusion may result in flying through severe possibly very severe weather.
When there is no or very little movement between air masses we speak of a stationary front (how convenient). The weather will be like a warm front but it will stay longer in one place due to the lack of motion.
Clouds form when the water vapor in the air changes to visible form as tiny droplets or ice crystals. When the temperature is below freezing and water droplets are still present they are said to be supercooled. There are four families of clouds: high, middle, low and the vertical types (think of towering cumulus (TCu) and Cumulonimbus (Cb). The table shows at which altitude range you will find them. Some type of Cumulus clouds seem to have no limit. In tropical zones Cumulonimbus can reach altitudes of 40000 feet or more.
|HIGH CLOUDS||MIDDLE CLOUDS|
|(16500 to 45000 feet)||(6500 to 23000 feet)|
|Cirrus (Ci)||Altostratus (As)|
|Cirrostratus (Cs)||Altocumulus (Ac)|
|Cirrocumulus (Cu)||Nimbostratus (Ns)|
|LOW CLOUDS||VERTICAL CLOUDS|
|(surface to 6500 feet)||(1500 and higher)|
|Stratus (St)||Towering Cumulus (TCu)|
|Stratocumulus (Sc)||Cumulonimbus (Cb)|
|Fair weather Cumulus|
For more information about clouds we refer to the numerous meteorological websites about this interesting subject.