Weather

What Weather is and Why it is Important Weather is the short-term daily characteristics of the atmosphere in a given region. Human activities are governed by the weather – what we wear, what we will do. Since weather is so important for humans, many components or elements of the atmosphere are monitored by many weather stations around the globe.

The Elements of Weather The elements of weather include air temperature, pressure, humidity, precipitation, visibility, cloud cover and wind. Numerous weather stations around the globe monitor these elements on a continuous basis.

Instruments Used to Measure Air Elements A barometer measures air pressure, a thermocouple or thermometer measure air temperature, an anemometer measures air speed, a wind vane measure wind direction and wet-and-dry bulb thermometers can measure humidity (water content of air by percent).

Winds and Air Masses Weather is transported from place to place by winds and winds depend on air masses and pressure differences. An air mass is a three-dimensional parcel of air that has uniform or homogenous temperature and humidity characteristics. Air masses are named according to where they originate. North America has 7 major air masses.

Characteristics of the Continental Air Masses Arctic air is cold and dry. Continental polar air is cool and dry. Maritime polar air is cool and wet. Maritime tropical is warm and wet. Continental tropical is warm and dry. These air masses have their characteristics because of the water or land that they form over.

Fronts A front is the boundary where two different air masses meet. The polar front is the boundary where cold, dry polar air meets warmer, humid sub-tropical air. The frontal wave theory of cyclonic development helps explain the weather in mid and high latitude regions of the world (United States and Canada).

Symbols for Fronts A line designates a front. Blue icesickles indicate an advancing cold front in the direction of the points. Red semicircles indicate an advancing warm front in the direction that the round points to. A line with alternate red semicircles and blue icesickles pointing in opposite directions indicates stationary warm and cold air masses. Semicircles and icesickles on the same side of a line indicate an occluded front of warm air lifted high above cold air masses which is moving in the direction of the points/semicircles.

The Overtaking of a Moving Cold Front to Warm Air When a cold front overtakes warm air, it forces the warm air upward because warm air is less dense. As the warm air rises it cools and forms towering storm clouds called Cumulo-nimbus clouds that rise high and produce heavy rains below them (in the cold air region). Fast winds in the upper atmosphere blow the upper cumulo-nimbus ahead of the cold front to make cirrostratus and cirrus clouds. Cirrostratus and cirrus clouds foretell a coming cold front.

Cold Front Symbols A front is the boundary where two air masses meet. The symbol for a cold front is blue icesickles and the direction of the front’s movement is in the direction the blue triangles point.

Fronts Help Explain Weather Changes Since World War I, the Frontal Wave Theory of Cyclonic Development has been used to explain how weather changes as warm and cold air masses bump up against each other. When warm air meets cold air, it rises because it is less dense. As the warm air rises, two things result – 1) the warm air cools and releases water as precipitation and 2) a region of lower pressure is created as the air rises. Thus rain and low pressure are associated weather conditions.

Cyclone Development When air rises in the Northern hemisphere it also generates a circular surface wind movement in a counterclockwise direction because of the coriolis effect and wind shear, the effect where surface winds speed up as they rise due to less friction.

Anticyclone Development Falling air creates a high pressure region and this generates circular surface winds that rotate in a clockwise direction in the Northern Hemisphere.

Southern Hemisphere Cyclones and Anticyclones In the Southern Hemisphere, the direction of cyclones and anticyclones is reversed due to the coriolis effect and wind shear.

How Surface High and Low Pressure Regions Interact Winds flow in spirals from high pressure regions to the nearest low pressure region. For high pressure regions, note the increasing pressure in hectopascals from outside to inside and the reverse for low pressure regions.

When Cold Air Overtakes Warm Air When a cold air mass overtakes a warmer air mass, it lifts the less dense warm air over itself which causes towering cumulnimbus clouds to form. These clouds produce thunder showers that falls behind the front into the cold air mass. The tops of the towering cumulonimbus clouds push into upper atmosphere layers which have fast-moving winds. These winds push the tops of the Cb clouds ahead of the front to form first cirrostratus then cirrus clouds ahead of the front. The presence of CS and Ci clouds predicts the coming of rain (to be caused by the coming cold front).

When Warm Air Overtakes Cold Air When a warm air mass overtakes cold air, it rises over the cold air forming an incline. As the warm air rises it forms nimbostratus and the altostratus clouds which produce steady rains. Eventually as they reach upper levels with fast winds they form cirrus clouds. Seeing cirrus clouds followed by raining alto stratus and nimbostratus is a prediction that a warm front is advancing with fair weather ahead.

Types of Clouds and Rain Patterns Associated With Fronts

Stationary Front A stationary front is the boundary between a cold and a warm front that are not moving or moving sideways past each other. The symbol for this is a line with alternating warm and cold markers in opposite directions.

An Occluded Front When a cold front overtakes a warm front, it may push the warm air up over its cold air and the cooler air the warm front was moving over. An occluded front is the boundary of cold air with cool air over top of which lies a mass of warm air that rains into the cold air mass. The symbol for an occluded front is a line with triangles and semicircles pointing in the same direction and indicating the general motion of the boundary.

Forming an Occluded Front When a cold front overtakes a warm front, an occluded front is formed.

Cloud Formation As water evaporated from the earth rises in warmed air, it cools (adiabatic cooling) as it rises. As the rising air cools, its relative humidity rises since cooler air can hold less water (relative humidity = actual water in air/total water the air could hold at a given temperature). When the air’s relative humidity reaches 100% (the dew point), water vapour (gas) begins to condense on microscopic dust or salt particles to form droplets of liquid water or ice if at a high altitude. These droplets are seen as clouds. All weather clouds form in the stratosphere, the lowest region of the atmosphere.

Cumulus Clouds Cumulus clouds are found in fair weather. They do not produce rain. They are located in the lower regions of the stratosphere. They have a “cottony or fluffy” appearance. They are composed of liquid water droplets. Cumulus from an airplane 

Cirrus Clouds Cirrus clouds are whispy, high-altitude clouds. They are composed of ice crystals and do not form precipitation.

Cumulonimbus Clouds Cumulonimbus clouds are thunder storm clouds that produce much rain (any cloud with the word, nimbo or nimbus is a rain cloud). These towering clouds reach from the lower to the upper statosphere. They produce thunder and lightning and have a typical anvil shape at the top of the cloud. These clouds signal the approach of a cold front. They are composed of ice crystals at the top and water droplets at the bottom.

Nimbostratus Clouds These rain clouds are composed of ice crystals at the top and water droplets at the bottom. These are lower to mid altitude clouds in the stratosphere. They are associated with an approaching warm front.

Comparison of Cloud Types

Weather and Cloud Types

Three Causes of Rainfall Orographic rainfall is caused by warm air being forced up to higher altitudes by mountain ranges. Convectional rainfall occurs when warmed, moist air rises then cools and condenses into water vapour. The base of these clouds is where the condensation point/dew point occurs. As rain falls, heat is released (vapour condensing to liquid), and this extra energy makes the storm more active with lightning and thunder. Continental (Ex: Prarie) storms have this kind of rainfall. Cyclonic or frontal rainfall occurs when cold and warm fronts interact with each other.

Jet Streams Jet streams are narrow, meandering streams of upper air at altitudes between 9-12 km (at the border of the troposphere and stratosphere) moving up to 450 km/h. In the northern hemisphere there are two jet streams, the polar and sub-tropic jet streams which move seasonally south (in winter) and north (in summer).

Jet Streams And Cyclones Jet streams movement often triggers the development of low pressure cells or cyclones. Regions under a jet stream will likely experience storms and changeable weather. Regions north of the polar jet stream will experience colder, drier air while regions south of this jet stream will experience warmer, moister air. Meandering jet streams create Rossby waves that generate cyclones and anticyclones.

Jet Airplanes and Jet Streams Jet airplanes make use of jet streams when travelling in easterly directions since the plane’s speed (900 km/h) is increased by the jet stream’s speed. For a 10 h flight travelling at 900 km/h, if the plane gets in a slower jet stream of 125 km/h, the flight time reduces to 8.8 h due to the plane’s increased speed (1025 km/h). For a fastest jet stream of 450 km/h, the flight time is reduced to 6.7 h. When travelling west, jetliners avoid jet streams.

Weather Station Notation Weather stations provide weather information by using special notation.

Weather Station Cloud Cover

Weather Station Cloud Types

Weather Station Wind Speeds and Direction One knot is km/h or mi/h

Reading Weather Maps A

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