high level clouds that are mainly composed of ice crystals
Cirrus cloud Cirrus clouds are formed when water vapor undergoes deposition at high altitudes where the atmospheric pressure ranges from 600 mbar at 4,000 m (13,000 ft) above sea level to 200 mbar at 12,000 m (39,000 ft) above sea level.[13] These conditions commonly occur at the leading edge of a warm front.[14] Because humidity is low at such high altitudes, this genus-type tends to be very thin.[2] Cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets in regions where air temperature is lower than -20 °C or -30 °C.Cirrus usually occur in fair weather.They are formed when it is high enough to be cold and freeze the water drops into ice. They sometimes may be caused by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are like blown out ice-crystals spreading from the top of a dying cumulonimbus.
Warm front As the air mass rises into regions of lower pressure, it expands and cools. As it cools, any water vapor that is present will condense and form extensive cloud cover. The first clouds that indicate an approaching warm front tend to be mostly high cirrus at first, changing to cirrostratus as the front approaches. However, if cirrocumulus also appears, there is greater airmass instability approaching ahead of the front. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 8 hours away.[2] A thickening and lowering of these high clouds into middle-étage altostratus or altocumulus is a good sign the warm front or low has moved closer and precipitation may begin within less than six hours. Once the clouds have thickened to 2,500 metres (8,200 ft) from the earth’s surface, precipitation can begin to fall from heavy nimbostratus. If unstable altocumulus castellanus accompanies or takes the place of the main altostratus layer, cumulus congestus or cumulonimbus producing showers or thunderstorms may follow. Low stratus and stratocumulus commonly form underneath the main precipitating clouds.