As newcomers arrive they spend some time here while finding a more permanent abode. Their biggest response when they first arrive is “We can see the sun and breathe the air.” Changes will happen in China for that reason alone.
Weather is the day-to-day, month-to-month and year-to-year change in the temperature, precipitation, wind and humidity that we experience. Weather cycles come and go from hot to cold and dry to wet.
Too many of the modern news reports talk about day-to-day weather and put it down to climate change. In the Canadian Prairies we have had at least four major ice ages producing glaciers that wiped the slate clean.
The last ice age faded about 10,000 years ago, so we are blessed with youthful soils developed since then. So, in the grand scheme of things we are still recovering from the last major ice age.
A mile or so of ice melted as a result of a major long-term climate change event- and not a fossil fuel in site. There are many textbooks written on the topic of “causes” but there is still much room for debate and uncertainty.
At the University of Saskatchewan library this winter I stumbled on an interesting book: Climate and Culture Change in North America AD 900 – 1600, written by W.C. Foster in 2012. He started with a graph that showed warm and cold periods from 4000 BC to the present.
Apparently the decline of the Roman Empire was not all a political problem. As W. C. Foster traced the effects of the climate changes on North American Cultures from 900 -1600 AD, his conclusion was clear.
As we look at the time since we broke the sod around 1900 and have built a thriving AG industry we see the following temperature trends (10 year averages). Snow is such a big issue for agriculture and the measurements are shaky at best.
This website is designed to be an educational resource for teaching a unit on the Earth's atmosphere. This unit is embedded in the Earth Science curriculum in the 6th grade of most elementary schools.
These webpages incorporate both text and diagrams that are related to a specific topic in Atmospheric Science. All topics introduced on the website were chosen based on material covered in a standard 6th grade Earth Science textbook.
In the United States, pressure is commonly expressed in millibars (MB) or inches of mercury (Hg). It is actually a measure of the average kinetic energy or speed of the molecules in a substance (air).
The more kinetic energy (speed) the molecules have, the higher their temperature and vice versa. The Kelvin scale is convenient for scientific calculations, but is not used to report the air temperature.
As one moves away from the earth's surface (the heat source), the air becomes cooler. At about the altitude where jet aircraft fly (˜30,000 ft), the air temperature becomes geothermal.
The bottom of this geothermal layer marks the end of the troposphere and the beginning of the stratosphere. The reason for this warming is that ozone in the stratosphere absorbs ultraviolet (UV) radiation.
The air temperature decreases with height because there is little ozone at those altitudes to absorb the UV radiation. The final layer is the thermosphere, which is separated from the mesosphere by a boundary called the menopause.
Air temperature increases again in this layer, due to the absorption of solar radiation by oxygen molecules. Dew point temperature is a measure of the moisture content in the atmosphere and is the temperature to which air must be cooled (at constant pressure, with no change in water vapor content) for saturation to occur.
The dew point temperature is a good indicator of the actual amount of water vapor in the air. The sun's radiation warms the earth's atmosphere and surface and becomes heat energy.
Solar radiation mostly passes through the atmosphere and is absorbed by all objects, such as humans, trees, flowers, roads, etc. The higher the substance's temperature, the shorter the wavelength of the emitted radiation (think of how a burner on an electric stove turns from black to red as it heats up).
Also, the higher the substance's temperature, the greater the emission rate of radiation. Remember that temperature is just the measure of the average kinetic energy or speed of the molecules in a substance.
The rate of heat transfer through conduction also depends on whether the substance is a good conductor. Convection is the transfer of heat through the movement of a fluid, such as water or air.
This type of heat transfer can occur in liquids and gases because they move freely, making it possible to set up warm or cold currents. Convection occurs naturally in the atmosphere on a warm, sunny day.
These rising “bubbles” of warm air, called thermals, act to transfer heat up into the atmosphere. When the cooler air reaches the surface, it is warmed and it too eventually rises as a thermal.
These “bubbles” or thermals can result in cloud formation, which will be discussed more in the Weather section. Evaporation is very important because it is how water vapor, which is needed for clouds and precipitation, enters the atmosphere.
As air containing water vapor rises into the atmosphere, it will expand and cool. If it cools to its dew point temperature, the air will become saturated and condensation will occur.
Condensation can be observed in the atmosphere as clouds, fog, dew, or frost form. Eventually, the droplets can grow large enough that they will not be able to stay suspended in the cloud.
Precipitation is water, either liquid or solid, that falls from the atmosphere to the surface. The vertical component of the wind is generally quite small, except in thunderstorm updrafts.
Air that is rising cools, which may cause it to reach saturation and form clouds and precipitation. Conversely, air that is sinking warms, which causes clouds to evaporate and produce clear weather.
On weather maps highs and lows are surrounded by lines called isobars. When isobars are packed close together, the pressure is changing rapidly over a small distance.
Also, notice that (in the Northern Hemisphere) the wind blows clockwise around a high pressure system and also slightly outward from its center. Around a low pressure system, the wind blows counterclockwise and slightly in towards its center.
As the PGF becomes stronger (i.e. pressure changing rapidly with distance), the wind speed increases. If the PGF and the Coriolis Force are exactly equal and opposite, the wind would blow parallel to isobars, with high pressure on the right.
Friction is the force that causes air to slow down and spiral into lows and out of highs. This is why low pressure systems are often associated with adverse weather conditions.
Conversely, high pressure systems are generally associated with fair weather. Sinking air warms and tends to evaporate any clouds that may be present.
High-level clouds are typically thin and white, but may display an array of colors when the sun is low on the horizon. Since these clouds are located lower in the atmosphere, they are primarily composed of water droplets.
Some clouds can span the depth of the troposphere and therefore cannot be classified as high, middle or low. Cumulus clouds are characterized by a flat base and can grow to heights exceeding 39,000 feet (12,000 meters).
They can contain both liquid droplets and ice particles because they cover a large depth of the troposphere. If air remains over a source region long enough, it will acquire the properties of the surface below.
Examples include central Canada, Siberia, the northern and southern oceans and large deserts. Air masses that originate over land will be dry and are designated with a lowercase “c” for continental.
Air masses that originate over water will be moist and are designated with a lowercase “m” for maritime. Once formed, air masses can move out of their source regions bringing cold, warm, wet, or dry conditions to other parts of the world.
Fronts are classified by which type of air mass (cold or warm) is replacing the other. Cold fronts have a steep slope, which causes air to be forced upward along its leading edge.
This is why there is sometimes a band of showers and/or thunderstorms that line up along the leading edge of the cold front. Cold fronts are represented on a weather map by a solid blue line with triangles pointing in the direction of its movement.
This gradual rise of air favors the development of widespread, continuous precipitation, which often occurs along and ahead of the front. Warm fronts are represented on a weather map by a solid red line with semi-circles pointing in the direction of its movement.
Stationary fronts can also produce significant weather and are often tied to flooding events. The boundary between the two cold air masses is called an occluded front.
Occluded fronts are represented on weather maps by a solid purple line with alternating triangles and semi-circles, pointing in the direction of its movement. The figure below shows the average number of days that thunderstorms occur over the United States.
The greatest occurrence of thunderstorms occur in the southeastern United States, with a secondary maximum over the Colorado Rockies. The southeastern United States can tap into moisture from two of these sources (Gulf of Mexico and Atlantic Ocean).
This is one reason why this region has the greatest frequency of thunderstorms in the United States. Conversely, air is considered to be stable if it returns to its original position after being “pushed” upward.
Another ingredient that must be present is a lifting mechanism to give the air the initial “push” upward. The warm, moist air rises and cools, eventually condensing into a cumulus cloud.
During this stage, updrafts keep the water droplets and ice crystals suspended in the cloud. As the cloud builds to altitudes where the temperature is below freezing, large raindrops and even small hail begin to form.
Eventually, the raindrops and small hail become heavy enough that the updraft cannot keep them suspended in the cloud, and they begin to fall as precipitation. These falling particles, and evaporation and cooling of air near the cloud boundaries, create a downdraft, which signifies the beginning of the next stage.
Lightning, thunder, heavy rain and possibly small hail are produced during this stage. Sometime after the storm enters its mature stage, it eventually begins to dissipate.
During this final stage, the updrafts weaken and the downdrafts dominate the thunderstorm. Without the warm, moist air, cloud droplets stop growing and only some light precipitation remains.
Many times, the lower portion of the cloud evaporates and the only thing left of the thunderstorm is the anvil. A tornado is a violently rotating column of air that originates within a thunderstorm and is in contact with the ground.
Vertical wind shear induces a “rolling” effect in the atmosphere, similar to the diagram below. In the diagram to the left, a rotating column of air, which was produced by the speed shear, is lifted vertically by the updraft of a developing thunderstorm.
Due to the counterclockwise rotation of the mesocyclone, supercells often take on a “hook” appearance when viewed on radar. The figure to the right is a schematic of the precipitation associated with a supercell and the area encircled in red is the location of the rotating updraft.
The term “funnel cloud” is used to describe a region of strong rotation where the circulation has not reached the ground yet. The funnel becomes visible when water vapor begins to condense into liquid droplets.
Tornadoes are classified according to the damage they cause, which is related to their wind speed. A tornado's wind speeds are estimated based on the damage caused by the storm, which is assessed after-the-fact.
This new scale uses Degree of Damage Indicators, in order to get a more realistic estimate of a tornado's winds. Enhanced Fajita (EF) ScaleScaleCategoryWind SpeedPossible DamageEF-0Weak65-85 plight: tree branches broken, sign boards damagedEF-1Weak86-110 moderate: trees snapped, mobile homes pushed off foundations or overturned, windows brokenEF-2Strong111-135 significant: large trees snapped or uprooted, weak structures destroyedEF-3Strong136-165 severe: some roofs torn off framed houses, trees leveledEF-4Violent166-200 devastating: roofs and some walls torn off well constructed houses, car thrown or overturnedEF-5Violent>200 incredible: houses may be lifted off foundation, structures the size of automobiles can be thrown over 100 meters, steel-reinforced buildings highly damagedHurricanes are tropical cyclones that have an organized circulation, with sustained winds exceeding 74 mph.
A tropical storm becomes a hurricane when it reaches maximum sustained winds of 74 mph. Saffir-Simpson ScaleCategoryWind SpeedDamage174-95 damage mainly to anchored mobile homes, shrubbery, and trees.296-110 Mfume damage to roofs of buildings, considerable damage to shrubbery and trees, with some trees blown down and major damage to mobile homes.3111-130 Mfume structural damage to small residences, mobile homes destroyed, foliage blown off trees and large trees blown down.4131-155 extensive damage to doors, windows and roofs, shrubs, trees and all signs blown down, and complete destruction of mobile homes.5>155 severe window and door damage, extensive roof damage to residences and industrial buildings, some complete building failures with small buildings blown over or away. Category 1Category 2Category 3Category 4Category 5 The main parts of a tropical cyclone are the eye, the eyeball, and the rain bands.
The eye is located in the center of the storm (see satellite image to the right) and is a region of generally clear skies and light winds. The size of the eye is typically 20-40 miles across, but can be larger or smaller depending on the storm.
The skies are generally clear in the eye because the air is sinking in this region of the hurricane. At the ground, the transition from the very strong winds under the eyeball to the near calm conditions in the eye can be deceiving.
The eyeball is a wall of deep clouds (see photo to the right) that produce the torrential rainfall that surrounds the eye of hurricanes. The clouds and thunderstorms that swirl in toward the storm's center are called spiral rain bands (see radar image to the right).
Spiral rain bands can produce heavy downpours and wind, as well as tornadoes. When a cluster of thunderstorms develops or moves into environment described above, the disturbance can become more organized, which leads to the formation of a tropical depression.
The warm water is one of the most important contributors to tropical cyclone formation because it acts as the “fuel” for the storm. This warms the atmosphere, making the air lighter and causing it to rise further.
This inflowing air will begin to rotate under the influence of the Coriolis Force. Both weather and climate tend to be quite variable, with short and long timescale variations.
One of these naturally occurring circulations is the El Niño/Southern Oscillation (ENSO) cycle. These winds enhance upwelling (the rising of cold water from the deep ocean towards the surface) in the eastern Pacific off the coast of South America.
Therefore, sea surface temperatures, as seen on the figure below, are cool off the coast of South America and significantly warmer in the western Pacific. This wind reversal brings the warmer water from the western Pacific towards South America.
The following graphics show the effects that a strong El Niño pattern can have globally. Generally, the impacts of La Niña tend to be opposite those of El Niño.
The following graphics show the effects a strong La Niña pattern can have globally. La Niña effect during December through February La Nina effect during June through AugustAtmospheric greenhouse gases (water vapor, carbon dioxide, methane, and other gases) trap some of the earth's outgoing (infrared) energy, which causes the atmosphere to retain this heat and warm.
Without the greenhouse effect, temperatures would be much lower than they are today because the energy (heat) would simply escape to space. However, if the atmospheric concentration of greenhouse gases increases, problems may arise.
Since the beginning of the industrial revolution, the atmospheric concentration of certain greenhouse gases has increased. The atmospheric concentration of carbon dioxide, one of the most talked about greenhouse gases, has increased by nearly 30%.
Scientists generally agree that the increase in the concentration of carbon dioxide is primarily due to the combustion of fossil fuels and other human activities. Another source of carbon dioxide in the atmosphere is from plant respiration and the decomposition of organic matter.
Human activities (i.e. fossil fuels burned to power car and trucks, heat homes, etc.) Have yielded an additional release of carbon dioxide into the atmosphere, increasing its concentration.
The rate of climate change, particularly global warming, is likely to accelerate due to the increasing concentrations of greenhouse gases.