AVIATION 120

1. AVIATION 120 Meteorology

2. Today�s Agenda Precipitation Precipitation Types Rain, drizzle, sleet (ice pellets), freezing rain, freezing drizzle, hail Virga, Showers Snow, snow pellets, snow grains, ice crystals Hail Measuring precipitation Doppler Radar

3. Precipitation Types - Drizzle Drizzle is very fine, small droplets of liquid precipitation that has a diameter of 0.5mm or less Drizzle usually falls from stratus clouds Small raindrops may partially evaporate after leaving the bottom of the cloud and arrive at ground level as drizzle Drizzle droplets fall very slowly and sometimes appear as if they are �floating� in the air if they are caught in slight updrafts

4. Precipitation Types - Rain Rain is considered to be liquid precipitation with a diameter greater than 0.5mm Large rain drops can reach 5mm or more in diameter In mid to high latitudes, rain generally forms by the ice crystal (Bergeron) process and then melts as it falls through warm air Continuous rain is usually produced by nimbostratus, showery or intermittent rain can be produced by stratus, TCU or CB As pilots, rain drop size can indicate the strength of updrafts, and therefore turbulence Visibility usually improves after a rainstorm due to the rain removing particles (condensation nuclei) from the air Rain that combines with pollution in the air, acid rain can form

5. Precipitation Types - Snow Like rain in the mid and high latitudes, snow is formed by the ice crystal (Bergeron) process Specifically, snow flakes are formed by ice crystal aggregation (when ice crystals collide and stick together) The falling snowflake will reach the ground as a solid if the air is cold Snow can exist in temperatures warmer than 0�C (see the Focus section on page 179 of Meteorology Today) Ice crystals can exist in several different forms such as plates, columns, needles, and dendrites which are the most common form Ice crystal forms depend on temperature and moisture content of the air Since a snowflake may fall though several different temperature and moisture levels on its way through a cloud, many complex snowflake shapes can be formed as a result of different shape ice crystals aggregating Showers of snow are known as flurries; heavy flurries may be referred to as snow squalls Strong winds may cause blowing and drifting snow

6. Precipitation Types - Snow

7. Precipitation Types � Sleet and Freezing Rain If a snowflake partially melts in an above freezing layer and falls into a deep freezing layer, the partially melted snowflake or cold raindrop freezes into a small ice pellet called sleet Sleet pellets are usually transparent and have diameters of 5mm or less If the freezing layer is not deep, freezing rain will occur as supercooled water drops form in the cold air If the supercooled drops are less than 0.5mm in diameter, they are referred to as freezing drizzle

8. Precipitation Types � Sleet and Freezing Rain

9. Precipitation Types � Snow Grains and Snow Pellets Snow grains are elongated or fairly flat, opaque grains of ice with diameters of less than about 1 mm Snow grains fall in small quantities from stratus cloud as continuous precipitation and never showers Snow grains don�t bounce or shatter when they hit hard surfaces Snow pellets are similar to snow grains, but larger with diameters less than 5 mm Snow pellets are light, snow-like pieces of ice that are brittle, crunch underfoot and bounce or break apart when they hit a hard surface Snow grains and pellets form by riming (accretion) of ice crystals with supercooled water droplets A rimed ice crystal is also known as graupel In a CB with strong updrafts, graupel may form into hail as accretion continues to increase the size of the pellet

10. Precipitation Types � Snow Grains and Snow Pellets

11. Precipitation Types - Hail Hailstones are pieces of ice that can be clear or opaque and are greater than 5 mm in diameter Hail can reach very large diameters with the largest verified hailstone measuring 14 cm across (weighed 1.67 pounds) Hail can be extremely destructive to property on the ground as well as a major hazard to aviation � hundreds of millions of dollars of damage are done annually due to hail Hail if formed in cumulonimbus clouds when graupel or any other airborne particle acts as an embryo and collides with supercooled waterdroplets These collisions cause a hailstone to form as the supercooled water droplets accrete to the hailstone embryo For large hail to form, the growing hailstone must remain in the cloud for long periods of time (5-10 minutes for a golf-ball size hailstone) For the hail to remain in the cloud for long enough periods, the CB must produce strong updrafts

12. Precipitation Types - Hail

13. Hail Damage to Aircraft

14. Measuring Precipitation Instruments Standard rain gauge Tipping bucket rain gauge Weighing-type rain gauge Water Equivalent of Snow The depth of water from a melted column of snow is the water equivalent The water equivalent is variable depending on the type and density of the fallen snow Typical water equivalents for freshly fallen snow are on the order of 10:1 (10 cm of snow melts into 1 cm of water) Can vary from 30:1 to less than 2:1

15. Measuring Precipitation

16. Doppler Radar Doppler radar is used extensively to detect precipitation in cloud When the radar energy strikes a cloud or rain droplet, part of the energy is reflected back to the transmitter, producing a radar echo The time it takes for the energy to be transmitted out and reflected back indicates the distance an echo is from the radar site Using the principle of Doppler shift, new technology radars can measure the horizontal speed of rain Doppler radar is therefore an important tool for detecting tornado activity in CBs Radar images can be viewed at: http://www.weatheroffice.ec.gc.ca/radar/index_e.html

17. Radar Image