Precipitation

Precipitation Formation Requires Requires –condensation nuclei (solid particles) –saturation (air at dew point) Result is temperature dependent Result varies geographically Result is temperature dependent Result varies geographically Collision-Coalescence Process = Raindrop Bergeron Ice-Crystal Process = Snowflake

How Precipitation Forms Important Facts: -cloud droplets are tiny (20 micrometers). -many condensation nuclei are present. -tiny particles fall more slowly than large ones. -a cloud droplet’s diameter must grow ~200 times to reach a raindrop’s diameter. -to attain the volume of a rain droplet, the cloud droplet increases a million times in volume.

Where do nuclei come from? Ocean salt Ocean salt Pollution Pollution Wind born dust Wind born dust Cloud seeding Cloud seeding Microbes Microbes

Relative sizes of raindrops, cloud droplets, & condensation nuclei

Mist drizzle rain/sleet mm less than 0.5mm 0.5 – 5 mm Approximate size of types of Precipitation

As droplets fall they collide with smaller droplets and coalesce. after collecting ~1 million cloud droplets the particle is large enough to fall without evaporating. Because there are a large number of collisions needed, clouds with great vertical extent are typically produce precipitation by this process. The Process from Warm Clouds: The Collision-Coalescence Process

Collision & Coalescence: a) warm cloud composed only of small cloud droplets of uniform size; b) different size droplets

Cloud droplet rising & then falling through a warm cumulus cloud by growth and coalescence

Distribution of ice and water in a cumulonimbus cloud

Water droplets and ice crystal are in equilibrium; water vapor molecules > liquid is saturation vapor pressure over water is greater than it is over ice

Types of Precipitation: Rain Rain Freezing rain or Glaze (ice crystals freeze onto a frozen surface) Freezing rain or Glaze (ice crystals freeze onto a frozen surface) Snow (ice crystals have not melted) Snow (ice crystals have not melted) Sleet (ice crystals melt as they fall) Sleet (ice crystals melt as they fall)

Precipitation Types / Properties

Temperature Profile for Rain

Temperature Profile for Snow

Snowflakes and Temperature Snow crystal images from an electron microscope

Sleet and Freezing Rain sleet sleet freezing rain freezing rain Sleet makes a ‘tap tap’ sound when falling on glass.Sleet makes a ‘tap tap’ sound when falling on glass.

Sleet Sleet is a wintertime phenomenon that refers to the fall of small particles of ice that are clear to translucent. Sleet forms when rain passes through a cold layer of air and freezes into ice pellets. This occurs most often in the winter when warm air is forced over a layer of cold air.

Sleet – partially snowflake (cold droplet) freezes into a pellet of ice before reaching the ground

Temperature Profile for Sleet and Glaze

Blizzards & Ice Storms

hail Hail: Pieces of ice either transparent or partially opaque, ranging in size from that of small peas to that of golf balls or larger; biggest size in US 757 g & 14 cm diam.; Hail: Pieces of ice either transparent or partially opaque, ranging in size from that of small peas to that of golf balls or larger; biggest size in US 757 g & 14 cm diam.; Single hailstorm can damage in minutes; annual loss hundreds of millions of $ in US; Single hailstorm can damage in minutes; annual loss hundreds of millions of $ in US; Hail is produced in a cumulonimbus cloud when large frozen raindrops that grow by accumulating supercooled liquid droplets Hail is produced in a cumulonimbus cloud when large frozen raindrops that grow by accumulating supercooled liquid droplets

Hail – contd. For a hail to grow to the size of golf ball, it must remain for 5-10 minutes in the cloud For a hail to grow to the size of golf ball, it must remain for 5-10 minutes in the cloud Ice crystals of appreciable size that can’t be supported by rising air, begin to fall – Hail Ice crystals of appreciable size that can’t be supported by rising air, begin to fall – Hail Largest form of precipitation occurs during the warmest time of the year (due to strong updraft that keeps the crystal to become bigger) Largest form of precipitation occurs during the warmest time of the year (due to strong updraft that keeps the crystal to become bigger)

Accumulation of small hail after a thunderstorm

What about hail? updraft cycles updraft cycles accretion accretion A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.