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Insider Insights: Articles

CAPE is a summer term you need to know

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By Scott Sutherland
Meteorologist/Science Writer
@ScottWx_TWN
Tuesday, August 2, 2016, 11:26 AM

Want to impress friends and party guests with your weather smarts? Here's a handy guide to weather words like bombogenesis, adiabatic and cumulonimbus, why terms like CAPE, shortwave and positive vorticity advection are so important to forecasters.

Adiabatic Lapse Rate
(AY-dee-ah-ba-tik)

Look up into the sky on a cloudy day and all the clouds appear to start at the same height above the ground. This is no accident. It's due to the very specific rate at which warm air expands and cools as it rises from the ground, which is called the Adiabatic Lapse Rate.

As warm air starts its rise up into the sky, the humidity of that air is less than 100 per cent. This air will expand and cool at the Dry Adiabatic Lapse Rate (DALR) - 1 degree Celcius for every 100 metres it rises.

As soon as the air cools to the point where it becomes saturated - where water vapour begins to condense out of the air - this changes the rate at which the air cools (this height above the ground is known as the lifting condensation level).

The condensing water vapour releases heat, slowing the rate of cooling to just over 1/2 of a degree Celsius, which is known as the Moist Adiabatic Lapse Rate (MALR). This can happen as low down as just above the ground (in the case of fog) or very high up (cirrus clouds).

Bombogenesis

You may have heard the term "weather bomb," referring to a particularly strong storm. Rather than being media hype, this is a real meteorological term.

"Meteorologists gauge how strong a storm system is by how low the pressure is at its centre. The lower the pressure, the stronger or deeper the storm," says Weather Network meteorologist Dayna Vettese. "The term weather bomb is a meteorological term used when a low pressure system deepens at least 24 mb in 24 hours."

Explosive cyclogenesis or bombogenesis refers to the same thing - the increase in cyclonic rotation of a storm, so that the core surface pressure in a mid-latitude storm drops by at least 24 millibars (or 24 hectopascals, if you prefer) in 24 hours.

While these kinds of storms typically occur in cooler months - mid-fall to mid-spring - when the temperature differences between land and ocean are greater, it's always an interesting one to bust out in a conversation.

CAPE


Skew-T plot with temp (red) & dew point (green)
of the atmosphere, and the temp (yellow of a parcel of air
in the cloud. CAPE is in orange.

This has nothing to do with superheroes. CAPE stands for Convective Available Potential Energy.

This is a way for meteorologists to determine exactly how unstable the air is and how strong a storm may become.

Forecasters determine CAPE by potting the temperature and dew point on a special graph - either a tephigram or a skew-T - which shows how those factors vary with height (similar to the lapse rate image above).

If temperatures inside the cloud layer are warmer than the atmosphere around the storm, it means that there is a lot of buoyancy and instability inside the cloud. This can drive air parcels higher and faster, creating stronger updrafts, resulting in stronger downdrafts and generally making the storm more violent.

Having high CAPE values doesn't guarantee severe storms are going to develop, though. Low CAPE values doesn't guarantee no severe storm, either. As an example, some of the worst storms can occur when a warm layer of air 'caps' the storm, keeping the CAPE in the lowest levels and preventing air parcels from pushing higher up. If the air under that warm layer warms up enough that suddenly parcels are able to rise freely, this can cause the explosive development of severe storms. On the other hand, if the 'cap' remains, this will limit the strength of storms, regardless of how much potential energy is waiting to burst forth.

Cumulonimbus
(KYOO-myoo-loh-nihm-bus)


Cumulonimbus. Credit: Morna Lonergan, Aug 29, 2013

Storm cloud. Anvil cloud. Thunderstorm. If you see these towering heaps of cloud in your area, you can be sure that it's going to be an interesting day.

All of these, in one form or another, qualify as cumulonimbus, which translates from Latin as 'heap' and 'rainstorm'.

These storms form when there is a lot of moisture 'packed' into the air, and there's a generous amount of heat driving convection to get that moisture further up into the atmosphere.

The result: heavy rain, strong winds, lightning and thunder, and the possibility of hail and even tornadoes.

Fractus

If you've ever watched passing rain clouds or storm clouds, and noticed those little grey wisps of cloud moving along under the cloud base, looking like sad little hangers-on, those actually have a name - fractus.

A little like cotton candy that's been pulled off the stick, these tiny clouds (sometimes called scud) have been pulled away from the cloud by strong wind shear.

Mesoscale Convective Complex

This term is a name that basically means "really big storm."

Sometimes a particularly active weather front will spawn thunderstorms that become organized on a larger scale. It's not just one storm, or a bunch of pop-up thunderstorms scattered about. These are grouped together, being fed by the same atmospheric conditions, and are typically so large that you can't see the entire system from the ground. You need a satellite perspective to see the entire thing.

Any widespread collection of storms - a squall line or even a tropical cyclone - can qualify as a mesoscale convective system. If satellite images reveal that the resulting system is mostly round, that it's large enough and its cloud tops are high enough (and thus cold enough), it can qualify as a mesoscale convective complex.

These immense, powerful storm systems typically develop and persist overnight, with heavy downpours, powerful winds and frequent lightning, and they can even include tornadoes. This can make them especially dangerous, considering that people on the ground won't be able to see them coming (satellites and weather radar are extremely important in tracking them).


A Mesoscale Convective Complex stretches across four US states on July 8, 1997. Credit: NASA/NOAA/UWM-CIMSS

In addition to the trouble they can cause overnight, MCCs can even allow stormy conditions to persist from one day to the next. After sustaining all the activity through the night, when they dissipate in the morning, their churning, rotating core - the mesocale convective vortex - can continue on to cause another flare-up of thunderstorms later in the day.

Positive Vorticity Advection

For this one, we'll go in reverse order.

Whereas convection is transporting something vertically, advection is the movement of something - mass, heat, energy, whatever - from one place to another, horizontally.

Vorticity describes a small scale rotation (or vortex) inside a fluid flow. Picture the small eddies in the current of a stream or river and that is an excellent example of vorticity. Positive vorticity, at least in the northern hemisphere, is rotation in a counterclockwise direction.

Put this all together and positive vorticity advection (PVA) is how small counterclockwise vortices, high up in the atmosphere, move along in the air flow, typically into areas with less positive vorticity. As higher values of PVA move into regions of lower PVA, it forces air lower down to rise. This causes low pressure systems near the surface to deepen, making the system stronger.


A radiosonde used by NOAA.

Radiosonde

Have you been wondering how meteorologists measure the temperature, dew point and pressure that go into those Skew-T graphs? They use a balloon with a special sensor attached, known as a radiosonde. If they also want wind speed and direction, they use a rawinsonde and track the balloon with radar.

Shortwave

When looking at a map of the upper levels of the troposphere (the lowest layer of the atmosphere, where all of our weather happens), there are long waves present, with ridges and troughs stretching across wide swaths of land and ocean. However, look closer and you can see smaller 'bumps' and 'valleys' along those longer waves. These smaller wiggles are known as shortwaves.

Why do they matter? Whereas the larger waves dictate the general pattern of weather - stable or unstable, active or calm - these smaller scale waves cause air to rise ahead of them in the 'flow', influencing smaller scale weather patterns. This can produce storms in a generally calm pattern or make storms stronger in a more active pattern.


Shortwaves analyzed by Weather Network meteorologist Brad Rousseau. Image credit: WeatherBell

Virga
(VUR-ga)

You see the rain clouds gathering. It may even look like rain is falling, but not a drop touches the ground.

What you're seeing there is virga.


Virga over Kelowna, BC, May 23, 2012. Credit: Dave Morgan

Virga is what happens when rain falls from clouds into relatively dry air. The rain is visible leaving the cloud base, but it evaporates on the way down - converting back to water vapour before it ever reaches the ground. 

This kind of precipitation is seen as pretty harmless, but it can touch off some troublesome effects. As the rain evaporates, it sucks up heat, causing a localized cold spot under the storm. This can trigger a dry microburst - a small column of rapidly descending air, that can be very dangerous to aircraft that might fly through them. Also, storms that exhibit virga can still produce lightning, and since the precipitation never reaches the ground, this can touch off wildfires in dry regions.

Sources: UWM-UCMSS

More by this author
The 12 types of clouds you meet in the spring and summer
'Weather bomb': Why the explosive name?
CAPE, LI's and the dreaded MCS: Decoding thunderstorm lingo
The Know-It-All guide to winter weather terms

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