Atmosphere and weather lecture 1

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Nov 3, 2013 (3 years and 9 months ago)


Atmosphere and weather lecture 1

Ch. 16 369
387, skim Ch. 14

First, some definitions:

•Weather is the state of the atmosphere on a local scale at a specific time, while climate
describes the average weather over a period of time (taking weather extremes i

•Meteorology is the study of the atmosphere and the weather.

• Finally, the atmosphere is the thin blanket of gases that surrounds a planet. The
atmosphere of Earth is thick enough to protect the surface against high levels of solar
ion, while thin enough to allow beneficial amounts of solar energy through. It
provides the surface with insulation and also protects against meteorite impacts and other
hazards from space. The atmosphere provides gases that are essential for life to exist


Earth’s atmosphere: The basics

• Earth’s atmosphere extends from the Earth’s surface to space, thinning with altitude.
Although there is no firm line of demarcation where the atmosphere ends, it can be
divided up into four sections, based u
pon temperature profile and composition to a lesser
extent: the troposphere, the stratosphere, the mesosphere and the thermosphere. The
troposphere (up to about 6 miles) is where most weather takes place, though the
stratosphere the level at which ozone e

• Earth’s atmosphere is made primarily of nitrogen and oxygen, along with carbon
dioxide. Also included in the atmospheric makeup are noble gases such as helium and
argon, as well as methane, hydrogen, water vapor and ozone.

• Ozone in part
icular is a crucial atmospheric component since it provides a shield from
intensity ultraviolet radiation.

• Life alters the composition of the atmosphere by taking in gases needed for nutrients
and expelling waste gases. Human activity has altered t
he composition of the atmosphere
as well, with the introduction of carbon dioxide emissions and other chemicals.

• A word on ozone: Ozone (O
) is an unstable molecule that at low levels in the
atmosphere is a component of smog and a serious air pollutant
, but in the stratosphere
fulfils a different function. At high levels in the atmosphere, ozone shields organisms
from high UV radiation exposure because it is absorbed as it reacts with ozone

Atmospheric circulation

• The Earth’s atmospher
e circulates primarily through convection rather than conduction.
In conduction, heat is transferred by collisions between next
door atoms, but this is a
slow process.

• In convection, parcels of warmer air rise, while cooler parcels of air sink. This is
only more efficient, but also drives weather patterns.

• As warm air rises, it expands and cools. This decreases the amount of water vapor the
air can hold. Thus, as air rises, it is more likely to form clouds. Descending air warms as
it compresses. T
his warm air is more able to keep water in its vapor form and so clouds
will either not form or will vaporize.

Conduction is heat transferred by the motion of atoms.

Convection transfers heat through rising and falling packets of material.

Air pre

• Air pressure is the same as air stress

pressure is just compressional stress. Air pressure
is usually measured as the weight per unit area of a column of air, which is why air
pressure is less as you gain altitude.

• In terms of measuring air p
ressure, we typically speak of the amount of mercury that
can be supported in a column at a specific air pressure, which is why air pressure is often
expressed in terms of length. However, the unit millibar is a unit of pressure; this is also
commonly used

• Humid air is less dense than dry air; warm air is less dense than cold air. So high
pressure areas (stable, less likely to form a storm) are those that are dry and mild.

What is the aurora?

• The aurora borealis originates in the ionosphere, a par
t of the upper atmosphere named
for its high concentration of ions. These ions are created when solar radiation strips
electrons from atoms, leaving them as positively charged ions.

The aurora over a village in Norway.

• The fact that the ionosphere
reflects radio signals makes long
distance radio
transmissions possible.

• Aurora are not completely understood, but are caused by interaction between solar wind
and the Earth’s magnetic field. High energy particles from the solar wind react with the
h’s magnetic field (magnetosphere) and create more high energy particles (electrons)
that interact with the ions in the ionosphere. These interactions will cause emission of
radiation, part of which is visible as the aurora.

• The aurora are typically con
fined to high latitudes because the Earth’s magnetic field
channels solar wind particles around the geomagnetic poles. Thus, the activity rarely is
great enough to extend far away from these regions.

• Because it is the Sun’s activity that drives the auro
ra, times of increased solar activity
usually means more Northern Lights.

Moving Air

Atmospheric stability

• Atmospheric stability refers to how likely it is that a parcel of air will rise or fall.
Remember that this motion is driven by density (which is

related to temperature); high
density air will sink while lower density air will rise.

• We judge atmospheric stability by calculating how the temperature of a moving parcel
of air varies as compared to the air it is moving in (the ambient air). When ai
r is stable,
air parcels that rise cool and become denser, which means they will begin to sink;
descending air parcels warm up and their density decreases, which will drive them to rise.

• In unstable air, the reverse is true, and air parcels continue to
rise or fall (increased
vertical motion). This is why unstable air is more likely to form clouds

there is more
vertical motion of air, meaning moist air is more likely to reach the level at which
saturation occurs and clouds form.

The rise and fall o
f air

• The formation of clouds and thus precipitation is driven by the rise and fall of air in
unstable conditions. Air ascends in four ways: in a convective current, up high
topography slopes, along a front, or where moving air (wind) converges.

• Conv
ection requires a rising and a falling branch. For the weather observer, this can be
seen on days where puffy clouds are scattered; the clouds form where air rises and none
form where it descends (blue sky).

• Air, though a gas, cannot move through a

topographic barrier such as a mountain range.
The air is forced to rise over the barrier, which also means that it expands and cools,
forming clouds and often precipitation (if the barrier is high enough). This phenomenon
is the cause of rainshadow desert
s, since air that dumps its moisture on one side of a
mountain is usually too dry to have any moisture left.

• Where air masses that differ in characteristics (temperature or humidity) meet, this is
termed a front. At a front, the warm air in one air mass

has an opportunity to rise over the
colder air mass, lifting the air within the air mass.

• A warm front is one that is advancing behind a cold air mass, while a cold front is one
in which cold air advances behind a warm air mass. So a warm front pushes
over a cold
air mass, while a cold front drives under a warm air mass.

• Wind convergence can be thought of as an air traffic jam. Winds converge as they slow
moving over different types of landscape (where roughness varies). Where winds
air parcels can be driven upwards where they may cool, allowing clouds to
form. This is the driving force behind lake effect snow.

Humidity and Precipitation


• Humidity is the amount of water vapor (water in the form of a gas) that the air holds
For example, at 100% humidity, the air is saturated

it can hold no more water vapor.
This makes it very difficult for the body to cool off through sweating, as the sweat will
not evaporate

so it really is “not the heat but the humidity.”

• When the
air has reached saturation, water condenses. That is, it turns into a liquid (or
solid, depending on the temperature). This is the point at which clouds can form.


• Precipitation is water that falls to the surface of a planet. On Earth, of c
precipitation is in the form of water (solid or liquid).

• The water in clouds is too small to fall to Earth, but is suspended by the air or winds
(updrafts). In addition, unless the air below the cloud is humid enough, any water
droplets that make

it out of the cloud will vaporize.

• For precipitation to occur, water droplets must be large enough to overcome the forces
of the updrafts, as well as be large enough to survive the fall to Earth without vaporizing.
In a complicated process, droplets g
row and fall more quickly inside a cloud, colliding
with other droplets and growing by coalescing. A dense cloud will have more water
droplets to drive this process.

Forms of precipitation

• Rain is simply falling water droplets, and snow is falling ice
crystals (1 cm of rain
equals 10 cm of snow in terms of precipitation). Other types of precipitation include
drizzle, freezing rain, sleet and hail.

• Drizzle is the term for very small water drops, typically originating in thin, low stratus
clouds. The d
rops cannot collide and coalesce, but the distance they have to fall is short.

• Freezing rain is the result of rain falling into a layer of colder (subfreezing) air before
reaching the ground. The rain becomes supercooled and freezes on contact.

• Sleet

is essentially freezing rain that freezes prior to striking the ground, and is usually
the result of a layer of colder air that is thicker than that which produces freezing rain.

• Hailstones are formed in large cumulonimbus clouds. Ice pellets (sleet) a
re transported
into higher levels of the cloud through updrafts, where they are able to collide with water
drops. They then freeze on descent. If this happens frequently, many layers of ice may
build up, creating a hailstone. Powerful updrafts can keep eve
n very large hailstone aloft
before the force of gravity finally pulls them down. Thus, the largest hailstones come
from the most violent storms.

Other forms of precipitation

• Why does dew form? On a clear night, an object on the surface of the Earth wil
l radiate
or lose more heat than the air around it (it is colder than the air). So the surface is cooler
than the air next to it, and if it is cool enough the air may become saturated. Condensation
of water in the air next to the object will form as dew; i
n cold enough weather it will turn
to frost.

• Fog is a cloud on the ground, formed because the air is saturated (unable to hold any
more water in the form of vapor). Fog forms either through the addition of water vapor or
through cooling of the air, whic
h is why fog often forms in wet, cool areas such as


• A cloud on Earth is a mass of water droplets or ice crystals that has condensed from
water vapor.

• Clouds form when water vapor condenses around particles in the atmosphere. Thus,
llutants can drive precipitation because they allow the condensation of water vapor at
less than ideal conditions.

Classification of clouds

• Clouds are classified by temperature, altitude and appearance. We will be most
concerned with the last two types
of classification.

• Stratiform clouds are formed from slow, gentle ascent of air and produce lateral clouds
to which they give their name (stratus). Cumuliform clouds are puffy and are formed
from faster ascent of air (cumulus). Altitude controls cloud t
emperature and yields three
basic cloud types: cirro
type, alto
type or various low clouds formed at high, middle and
low altitudes respectively. These two characteristics give their name to nearly every
common cloud.

• High latitude, very cold clouds in
clude cirrus (thin strands blown by high winds),
cirrostratus (sheetlike but still very thin) and cirrocumulus (thin but with some vertical
development). Cirrus clouds often indicate high altitude winds, and thus a change in the
weather. Storms often follo
w cirrus clouds by a few days.

Cirrus clouds near sunset.

• Middle latitude clouds include altostratus (a uniform sheet lower than cirrostratus
clouds) and altocumulus (rolls or small puffs at lower altitudes, also called a ‘mackerel
sky’). These clou
ds often follow cirrus clouds.

Altocumulus clouds, showing "mackerel" sky.

• Low altitude clouds include fog (condensation of water vapor at or near the Earth’s
surface, also considered stratus clouds), stratocumulus (large, low puffs separated by
r sky), stratus (a uniform grey layer) and nimbostratus (also a uniform but more
bottomed layer from which precipitation is falling).

Stratus clouds nearly hugging the ground.

• Cumulus clouds are the result of diurnal (day
day) convection, a
s stated earlier.
These are typically fair
weather clouds, as the convection that forms them is gentle and

weather cumulus clouds over the prairie.

• Vertically moving air means unstable air, so clouds with more vertical development
mean un
stable air and consequently bad weather. Cumulus congestus clouds, those large,
like cloud masses, are the harbingers of bad weather. The most impressive
are the cumulonimbus clouds, formed when vertical motion is so great that cloud
is driven into the higher layers of the atmosphere, where air motion is so fast
that the clouds shear off at the top (an anvil
shaped cloud).

Storm clouds growing over Texas.