Precipitation

1. Precipitation

2. What is precipitation? • Why, it is any form of water that falls from clouds and reaches Earth’s surface. • But not all clouds produce precipitation. • For precipitation to occur, cloud droplets or ice crystals must grow heavy enough to fall through the air.

3. When you listen to the weather report and hear that there is a chance for light rain or that it is currently drizzling, there is actually a scientific method for defining the terminology. Now, if the person making the report is just a local broadcaster looking out the window to render an opinion, then the definitions listed in the next slide are not guaranteed to apply, but if the forecast is coming from a certified weatherman, then there is a pretty good chance that the number of drops per second per square foot really does fall within the ranges listed.

5. One interesting tidbit… What is the technical difference between rain and a shower? Rain refers to participation from clouds like stratus and altostratus, and tend to occur over a large area for a long time. Showers originate from cumulus and cumulonimbus clouds and cover a relatively small area for a short time.

6. Types of Precipitation • Rain • Sleet • Freezing Rain • Snow • Hail

7. In cold air way up in the sky, rain clouds will often form.  Rising warm air carries water vapor high into the sky where it cools, forming water droplets around tiny bits of dust in the air. Some vapor freezes into tiny ice crystals which attract cooled water drops.  The drops freeze to the ice crystals, forming larger crystals we call snowflakes. 

8. Rain • When the snowflakes become heavy, they fall. When the snowflakes meet warmer air on the way down, they melt into raindrops. In tropical climates, cloud droplets combine together around dust or sea salt particles.  They bang together and grow in size until they're heavy enough to fall.

9. Sleet. • Sometimes there is a layer of air in the clouds that is above freezing, or 32 degrees F.  Then closer to the ground the air temperature is once again below freezing. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of precipitation is called sleet.  It bounces when it hits the ground. 

10. Freezing Rain • However, if snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is called freezing rain, and a significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents.  Ice can also build up on tree branches and power lines, causing them to break and our lights to go out.

11. Snow • Snowflakes form when water vapor freezes into ice crystals in cold clouds.  The ice crystals attract cooled water droplets to form various shapes.  They get heavy and fall.  If the air is cold enough, the snow falls all the way to the earth without melting.  If the ground is freezing, the snowflakes stick to the ground. 

13. Hail • There is another kind of precipitation that comes from thunderstorms called hail. • Hail is formed through a sort-of roller coaster ride through intense thunderstorms. Strong convection currents lift small ice pellets high into the middle and upper portions of a cumulonimbus cloud. This is where super-cooled water droplets collide and the ice pellet grows through a process called accretion. Once the pellet is too heavy for the updrafts to keep it within the cumulonimbus, it begins to fall, and if it does not melt completely before reaching ground level, it comes out as hail.