WeatherBY MR. PINKEPANK (MR. P.)

Thunderstorms and Convection

Convection- the transfer of heat by mass movements Happens in buildings when heat rises and cold air sinks Happens in stove pots when boiling water rises to the top

A thunderstorm is a convective process Warmer air from near the surface rises upward and cooler air from

higher up sinks downward Warmer air rising upward- updraft Cooler air sinking downward- downdraft

Thunderstorms

More on Thunderstorms

Downdraft can produce strong winds in 2 ways: Cold air hits the ground, and spreads out rapidly in all directions When winds are strong in upper levels, rain and/or hail brings higher

momentum air down to the surface Lines/clusters most likely to produce large scale wind damage

Especially bow echoes (picture next slide) Hail caused by strong updraft moving water drops to freezing

levels Single “supercells,” with rotating updraft, most likely to produce

violent tornados (more later)

Bow Echoes

Backwards “C” Strongest winds in center of the

bow Bowing out shape from stronger

winds outrunning rest of line

Tornadoes In supercells, form from wall cloud, which is intersection of

warmer air from updraft and cooler air from downdraft Can also form along front edge of line of storms (less commonly) Pre-existing rotation around a horizontal axis (must already be

present!) is tilted up into the vertical axis by storm updraft

Supercell and Wall CloudWall cloud- base is lower than elsewhere. Updraft rotates in wall cloud. To the right is a rain/hail shaft.

Forecasting Thunderstorms/Convection

Lapse rate- the rate at which air cools with height Typically 6.5-9.8 Celsius/kilometer

Steeper lapse rates = more rising motion, due to a stronger convective circulation Makes steeper lapse rates vital for thunderstorms, with stronger storms requiring even

steeper lapse rates Instability- the steepness of the lapse rate

More instability= steeper Precipitation can still form with low instability, but instability must be high enough for a

thunderstorm Storms can not only form from surface-based instability, but also from instability

occurring higher up in the atmosphere Storms and precipitation need something to make air rise (trigger)

Examples of processes that cause rising motion: fronts, strong daytime heating, topography (next slide)

Mountain Air Circulation

When air goes over a mountain, it rises on the windward side, and sinks on the leeward side Makes climate much wetter on windward side and drier on leeward

side Water vapor condenses into clouds and rain as air cools Air on leeward side is dried out from moisture having already been

squeezed out Air cools as it rises, and warms as it sinks (even discounting lapse

rate) Makes mountain tops cooler than surface Makes leeward side warmer than windward side

Mountain Air Circulation

Fronts Cold air is more dense and less thick than warm air Cold front- colder air moving into warmer air

Due to density, colder air moves in under warmer air, eventually extending to the top of the troposphere (lowest layer of atmosphere)

Warm front- warmer air moving into colder air Due to density, warmer air moves in on top of colder air, eventually

extending to the surface Fronts can be large or small scale

Large scale can be from a cyclone Small scale can be from thunderstorm outflows (coming picture)

Cool downdraft air spreading in different directions, flowing outward Causes local temperature changes- sometimes significant!

Leads to formation of local front May form new storms

Fronts

Warm Front:

Cold Front

Fronts

Outflow boundary

Drylines

Dryline- horizontal moisture boundary, separating warm, moist air, from hot, dry air Tend to be shallow, compared to warm fronts and cold fronts Warm, moist air is from Gulf of Mexico

To the east of dryline boundary Hot, dry air is from elevated terrain of southwest U.S. and northern

Mexico To the west of dryline boundary

Can be a trigger of severe weather in the warm, moist airmass Storms would not form behind the dryline

Air behind it would be too dry to form precipitation

Drylines

Temperature Inversions Area where temperature becomes warmer, not colder, with height Also called an atmospheric “cap”

Air is too stable for rising motion Air chunks/parcels rising upwards from below would be colder than

environment Forcing them to sink back downward

Major effects on severe thunderstorms At least initially, thunderstorms cannot form in an atmospheric cap

Remember, thunderstorms require rising motion If cap is too strong, may entirely prevent storm formation Provided enough forcing for rising motion, moderately strong cap can

make storms more severe When a storm would then form, instability would be more favorable

More time for lower-level heating, before clouds and precipitation would cool the lower levels

Troughs and Ridges

Trough- lowering of pressures (and pressure surface heights) higher up in atmosphere Associated with more active weather Colder air underneath, at surface

Colder air more dense and less thick- in other words, more compact

Ridge- increasing of pressures (and pressure surface heights) higher up in atmosphere Associated with calmer, quieter weather Warmer air underneath, at surface

Warmer air less dense and more thick- in other words, less compact

Troughs and Ridges Trough

Ridge

Local Lake Impacts In winter, Lake Michigan gives LaPorte more snow, called lake effect

snow Forms from cold, northerly air moving over warmer water

Results in a convection response In summer, Lake Michigan weakens thunderstorms that cross over it

Lake water is colder, weakening the instability, sometimes even producing a near-surface temperature inversion

When storms come from the northwest or north, makes them weaker in Michigan City and the northern outskirts of LaPorte

Impact no longer exists when storms reach downtown LaPorte By then, instability has been back for a long enough time period, that storms re-

intensify Storms coming from the southwest or south do not result in this impact

These storms would not cross the lake before reaching local areas

Dewpoint Dewpoint- the temperature at which air would be fully cooled to

saturation, when kept at a constant pressure At times, can be used as an approximation of low temperature

Most valid approximation under these assumptions: Clear skies Calm winds No cold or warm fronts moving through No precipitation No mountain air circulations

Importance of Exact Surface Low Pressure Track

Surface low pressure system track is very important Determines if temperatures will be warmer or colder

Location of fronts depend on surface low position Even more critical when temperature differences are large

Determines nature of precipitation North of low- more likely to see steady rain/snow South of low- more likely to see thunderstorms

Just how complicated can weather prediction be?

Pretty darn complicated! Computer forecast models work with equations such as these:

Third semester Calculus is required to understand the equations Even these are simplified, making some approximations and assumptions This is why computer models and meteorologists can’t always get it right!

Quiz You may use your notes, but must be individual, silent work Explain- as long as is necessary to demonstrate full understanding

1. For Michigan City and the north side of LaPorte to be the most impacted, from what direction(s) would a severe thunderstorm need to be coming from? Explain.

2. Under what conditions will the dewpoint be a better approximation of the nighttime low temperature? (Make a list).

3. Explain how an atmospheric cap could make thunderstorms stronger or weaker.4. Explain how a warm front will move into a colder air mass. Why does the process

happen as it does?5. Steeper ________ is a requirement for thunderstorms.6. Explain how a thunderstorm updraft can make a tornado.7. What are the air masses like to the east and west of a dryline? In the U.S., where

do these air masses originate from?8. Think about it: In the winter, would lake effect snow happen more easily earlier,

or later, in the season? Explain.