Chapter 7: Climate Weather

Precipitation  Precipitation occurs when a cold air mass meets a warm air mass.  The cold air, being more dense, forces its way under the warm air.  The warm air is forced up quickly.  This leads to large Cumulonimbus clouds giving heavy showers, often with thunder, lightning and hail.

Precipitation: Rain  Rain develops when growing cloud droplets become too heavy to remain in the cloud and as a result, fall toward the surface as rain.  Rain can also begin as ice crystals that collect each other to form large snowflakes.  As the falling snow passes through the freezing level into warmer air, the flakes melt and collapse into rain drops.

Precipitation: Rain

Precipitation: Sleet  In this image the surface temperature is higher, 27°F.  Also as elevation increases, the temperature increases to a point where some of the atmosphere is above freezing before the temperature lowers again below freezing.

Precipitation: Sleet  As snow falls into the layer of air where the temperature is above freezing, the snow flakes partially melt.  As the precipitation reenters the air that is below freezing, the precipitation will re-freeze into ice pellets that bounce off the ground, commonly called sleet.

Precipitation: Sleet

Precipitation: Hail  Hail is a large frozen raindrop produced by intense thunderstorms, where snow and rain can coexist in the central updraft.  As the snowflakes fall, liquid water freezes onto them forming ice pellets that will continue to grow as more and more droplets are accumulated.  Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm.

Precipitation: Hail  As the ice pellets once again fall through the cloud, another layer of ice is added and the hail stone grows even larger.  This can repeat many times!

Precipitation: Snow  In the image (left) the green dashed line is the temperature in respect to elevation.  The surface temperature is 25°F and increases with height before decreasing.  However, since the temperature remains below freezing any precipitation that falls will remain as snow.

Tornadoes: How do they form?  Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.

Tornadoes: How do they form?  Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

Tornadoes: How do they form?  An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

Tornadoes: How do they form?

Strength of Tornadoes  They can be categorized as "weak", "strong", and "violent"; with weak tornadoes often having a thin, rope-like appearance.  About 7 in 10 tornadoes are weak, with rotating wind speeds no greater than about 110 MPH.

Strength of Tornadoes  The typical strong tornado often has what is popularly considered a more "classic" funnel-shaped cloud associated with the whirling updraft.  Rotating wind speeds vary from 110 to 200 MPH.

Strength of Tornadoes  Nearly 3 in 10 tornadoes are strong, such as this twister on the plains of North Dakota.  An important safety consideration is that weak and strong tornadoes by definition do not level well- built homes.  Thus, a secure home will offer shelter from almost 100 percent of all direct tornado strikes.

Strength of Tornadoes  Only violent tornadoes are capable of leveling a well- anchored, solidly constructed home.  Fortunately, less than 2 percent of all tornadoes reach the 200+ MPH violent category.

Hurricanes  Hurricanes are products of a tropical ocean and a warm, moist atmosphere.  Powered by heat from the sea, they are typically steered by high-level easterly winds while in the tropics,generally south of 25° north latitude and by high-level westerly winds north of 25° north latitude.

Hurricanes

Three Conditions for Hurricanes  A disturbance gathers heat and energy through contact with warm ocean waters.  Added moisture evaporated from the sea surface powers the infant hurricane like a giant heat engine.  The hurricane forms a wind pattern near the ocean surface that spirals air inward.

Structure of Hurricane  The center, or eye, of a hurricane is relatively calm with sinking air, light winds and few clouds.

Structure of Hurricane  The most violent winds and rain take place in the eye wall, the ring of thunderstorms immediately surrounding the eye.

Structure of Hurricane  At the top of the eye wall (about 50,000 feet), most of the air is propelled outward, increasing the air’s upward motion.

Impacts of Hurricanes  Storm Surges - is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall.

Storm Surge  Storm surge is the greatest threat to life and property.

Impacts of Hurricanes  Storm Tide: is the combination of the storm surge and the astronomical tide.  Destroys property all along coast.

3 Levels of Hurricanes  Tropical Depression- An organized system of persistent clouds and thunderstorms with maximum sustained winds of 38 mph (33 knots) or less.  Tropical Storm - An organized system of strong thunderstorms with a well defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).

3 Levels of Hurricanes  Hurricane - An intense tropical weather system with a well defined circulation and sustained winds of 74 mph (64 knots) or higher.