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.