Weather

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Weather

Questions that you should be able to answer at the end of the Weather lesson:

What are the parameters of weather? What conditions do we consider when we talk about "weather"?
What's the difference between "weather" and "climate"?
What conditions lead to the development of hurricanes?
What 4 specific atmospheric processes cause rain to occur?
HOW do they cause rain to occur?
Why does Florida so often get afternoon thunderstorms?
What are "air masses" and what are the specific air masses that tend to affect the US?
What are the characteristics of these air masses?
What are meteorological "fronts"? What do they look like and what kind of weather events do they cause?
What is a "mid-latitude cyclone"?
What is the history of development of a typical mid-latitude cyclone?
How do you read a weather map?

What do we mean by Weather? By Climate?

What parameters of weather can you think of? In other words, when we talk about weather, what can we use to describe it? Basically, the state of the Atmosphere:

Temperature
Humidity
Wind

speed
direction

Clouds (an interesting discussion in its own right)

% cover
cloud types - MANY different types

Precipitation

type:

rain
sleet
hail
freezing rain
snow
grauple
dew
fog
frost
and probably others

amount

Air Pressure

So what's the difference between Weather and Climate?

Weather is momentary
Climate is long term weather patterns
Climate is averaged over a 30 year period, and recalculated every 10 years for the previous 30.
Weather can vary wildly, but climate is much slower to change
Because of this, arguments that climate IS or IS NOT changing based on:

a single hot summer
a single cold winter
a sudden late snowstorm
a warm winter day
or even a few years of warmer or colder temperatures

are only a demonstration that the person making the argument has a political axe to grind or is ignorant. Or both.

We don't know everything about climate but we do know a lot. Click here for a link to a teach-In I gave several years ago.
And the agreement among climate scientists and other scientists around the world is nearly unanimous:
The planet IS warming due to human activities. Climate IS changing.

So how do we get our particular weather? An illustration:

Hurricanes:

Terminology:

Any low pressure systems rotating under the influence of the Coriolis Effect are generally known as "Cyclones".
A cyclone forming over warm oceans is known as a "Tropical Cyclone". (there are NON-"tropical" cyclones as well. See farther down in this web page.)
These have specific names depending on which ocean they form on:

In the Atlantic Ocean: Hurricanes.
In the Pacific Ocean: Typhoons.
In the Indian Ocean: Cyclones.

And yes, this means that the term "Cyclone" can mean a rotating low pressure zone rotating under the influence of the Coriolis effect AND a cyclone over a tropical ocean AND a tropical cyclone in the Indian Ocean. Deal with it.
And high pressure systems rotating under the influence of the Coriolis Effect are generally known as "Anticyclones".

How do Cyclones form?
They always form over warm oceanic areas. In the Atlantic:

Tropical North Atlantic near Africa
Caribbean

Gulf of Mexico

which results in:
Humid, warm air - low density

H₂O - molecular weight of 18 - much lower molecular weight than the main components of air:
N₂ - molecular weight of 28
O₂ - molecular weight of 32
Ar - molecular weight of 40
CO₂ - molecular weight of 44

Starts a sequence of events:
1. Low Pressure
2. Air rises
3. Temperature drops
4. Relative humidity rises, eventually reaching 100%
5. H₂O condenses
6. H_v = 540cal/gm - is removed from water vapor when it condenses. It goes into the atmosphere.
7. Atmosphere warms
8. Strong updrafts
9. which returns to step 2 above.....
giving us a vicious positive feedback that makes more and more uplift, giving more and more rain.
HEAT ENGINE:
whose fuel is condensation of water vapor.
How do we stop this process (in other words, what kills hurricanes?) Usually 2 things do it.

The hurricane moves over land. Cut off from the source of water vapor (the ocean), eventually the storm runs out of water. But in the mean time, it can drop a LOT of rain.
The hurricane moves into high latitudes, where the oceanic temperatures are so cold that the amount of water vapor in the atmosphere (the specific humidity) decreases, again cutting off the source of water vapor.

So why does it rain?

In order for it to rain, we need to reach the "Dew Point" when Relative Humidity reaches 100%.

Easiest way to do that? Cool the atmosphere (review RH. Remember when you usually get fog? Late in the evening and early morning, after it's cooled off all night.)

So how do we cool the atmosphere (other than waiting for nightfall)? - by lifting it. In almost all cases where air masses rise, you tend to get rain.

Conversely, anywhere air sinks (and warms), you tend to get dry areas and even deserts.
Examples:

downward moving convection cells in global atmospheric circulation such as at the poles and at 30^o of latitude.
downwind of mountain ranges (see below under "rain shadow deserts".)

Remember that the troposphere cools at higher altitudes? Raise the atmosphere and you cool it.
How do you raise the atmosphere?
Four ways to raise air and cause enough cooling to initiate precipitation:
Clockwise from top left: Convective uplift, Convergent uplift, Frontal uplift, and Orographic uplift.
Let's look at each:

Convective uplift - in rising side of a convection cell. We discussed this in Winds, along the equator and at 60^o latitude, so we won't address it here.
Convergent uplift - this is what happens in part in Hurricanes, and it's also the reason that FL gets thunderstorms on summer afternoons:

Orographic uplift ("Oro" means mountain)

A. Wind blows in from ocean, high in humidity.
B. Hits a mountain range and air is forced upward (Orographic uplift).
C. As wind rises up the windward slope, it cools.
D. Relative humidity increases.
E. When Dew Point is reached, condensation occurs, clouds form, and rain falls.
F. As this happens, even though relative humidity stays at 100%,

specific humidity decreases
(water vapor is being removed through condensation)

G. By the time the air reaches the top of the mountain, it is cold and dry - very dense.
H. Pushes over the top of the mountain on to the leeward slope

and is denser than surrounding air,
and therefore flows downhill.

I. As the air sinks, it compresses and heats up. As temperature rises, saturation humidity rises, and relative humidity decreases.
J. Precipitation does NOT occur.

These are called rain shadow deserts
and are some of the driest places on Earth.
See Altacama Desert

K. Forms warm, dry winds (see below):
Results in Santa Anna, Chinook winds. Can on occasion be very strong winds >60mph.
These winds tend to dry out land, increasing wild fire threat.

Results in: Rain shadow Deserts
And hot, dry winds:

Santa Ana Winds - southern California
Chinook Winds - eastern slopes of the Rockies
Other situation where dense air sinks, producing winds in COLD areas:

Katabatic wind - air over ice caps cools, flows downhill
(Read the above link from a novel. It's a bit overdramatic, but accurately characterizes what can happen in a katabatic wind.)
Williwa - a similar wind in Alaska

Last, Frontal uplift

Results From collisions between air masses

What is an air mass?

An "air mass" is a large mass of air that stays in one place long enough to pick up the characteristics of that region.

What kind of "characteristic"? Mostly characteristics of humidity and temperature.

Warm air masses form near the tropics

Cold air masses for near the poles

Humid air masses form over oceans

Dry air masses form over continents

Examples commonly affecting the US:

Air Mass Name	Abbreviation:	Typical Property:
Tropical	T	warm
Polar	P	cold
Maritime (oceanic)	m	humid
Continental	c	dry

These air masses can combine in various ways:

Conditions:
Cold
Warm

Dry
cP
cT

Humid
mP
mT

cP refers to "continental Polar" air masses, which will be dry and cold, for instance.

When Air Masses Collide:

The primary factor affecting density is temperature.

Fronts (see drawing below):

Imagine we start with either warm or cold air on the left, and a vertical boundary between the air masses.

The air mass on the left begins to move towards the right.

Warm Front (left):

When warm air moves towards cold air:

warm air is less dense than cold air, so it gradually slides up and over the cold air

warm air slides up at a small angle, gradually cooling off

results in a wide area of steady rain

caused by a layered (stratus) cloud that produces rain (stratonimbus)

may take a few days to pass by.

Cold front (right):

When cold air moves towards warm air:

cold air is more dense than warm air, so it is forced underneath the warm air

warm air is forced to rise rapidly at a large angle, cooling quickly

results in a narrow area of intense rain storms, lots of lightning

caused by a vertically rising cloud (cumulus) producing that rain and lightning (cumulonimbus)

may pass in only a few hours

Where would you expect these to occur?

Possibly not where you'd expect:
Warm front occur when warm air masses move towards cold. In other words, in colder areas.
Warm fronts typically occur in the northern part of the US.
Cold fronts occur when cold air masses move towards warm. In other words, in warmer areas.
Cold fronts typically occur in the southern part of the US (including Florida).

Map Symbols:
Fronts are always shown on a map with bumps pointing in the direction of movement.
The air mass for which the front is named is on the side away from the bumps:

Warm front (left) - warm air is to the West, moving to the East.
Cold front (middle) - cold air is to the West, moving to the East.
Stationary front (right - warm air is to the West, cold air is to the East. Neither is moving.

Mid Latitude Cyclones - Another major factor affecting the continental US.
from interaction of Polar Easterlies with Westerlies

A. Stationary Stage: Near northern part of US, cold and warm air meet. On either side of the boundary, air moves towards the boundary and turns right due to the Eoriolis Effect.
B. Wave Stage: This system is unstable, and if the air is slightly less dense in between, the warm air pushes ahead to the east while the cold air pushes ahead to the west.
C. Open Stage: Rising warm air in each case creates warm and cold fronts, leading to condensation, and this begins to add energy to the system, creating a Mid-Latitude Cyclone with a low pressure zone in the center.
D. Occluded Stage: The Cold Front typically travels faster than the Warm Front, and catches up with the warm front. This forces warm air above the cold air in an Occluded Front.
E. Dissipation Stage: As warm are rises above the cold air and stabilizes, horizontal temperature contrasts dissipate, and the storm dies out.
From start to finish, this process typically lasts about 1-2 weeks,
and the system returns to A as southern regions warm and northern regions cool in another 2 weeks to a month.
This process happens repeatedly over the course of a year, and you can watch weather maps and see these.
Click on the link below to see an "animation" of a cross section of a Mid-Latitude Cyclone.
Below that is the map symbol for an Occluded Front.

Click HERE for "animation" of cross sections.

Occluded Front Map symbol

Note that in the map symbol to the left, a cold front from the west has caught up with a warm front, and they've merged. In this situation cold air is on BOTH sides, and warm air between is being pushed upwards (see the animation link above)

Because of that, the Occluded Front Map Symbol has warm front (i.e. rounded) AND cold front (i.e. rectangular) bumps on the symbol, pointing towards the east, where both the cold front and the warm front were headed.

Also note that all 4 of the map symbols shown are in color, as shown on your computer or on a color TV. But on paper, they will be in black and white. Make sure you understand the symbols that designate the various map symbols.

What kind of fronts can you see?