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Weather is a response to variations in insolation and resulting energy imbalances in atmospheric motion and atmospheric moisture.
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An air mass is a large body of air with relatively uniform temperature and moisture over large horizontal distances.
- It is formed via patterns of air circulation
- In the narrow zone where the two air masses meet, weather conditions can change drastically across short distances and times.
- The general rule for air masses is that they acquire the characteristics of the regions where it forms (i.e., in a warm area, the formed air mass will be warm)
| Dry | Humid | |
|---|---|---|
| Cold | A (Arctic / Antarctic) cP (Continental Polar) Stable with surface inversions. A masses are inherently continental (dry) because the water is frozen | mP (Maritime Polar) Unstable with cold air overlying warmer air adjacent to oceans |
| Warm | cT (Continental Tropical) Unstable with hot surfaces and cooler air aloft. Lack of moisture. | mT (Maritime Tropical) E (Equatorial) Unstable with abundant moisture. E masses are inherently maritime because air is air is over or near water. |
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Continental Air Masses are dry air masses.
- Arctic / Antarctic masses are formed near the poles. Cold and dry because the water is frozen. They tend to reinforce regions of polar high pressure.
- Continental Polar Masses are warmer than A masses. They are less cold and dry, especially during summer.
- Continental Tropical Masses are hot and dry masses over land. Regions over cT masses tend to be hot and dry throughout the year
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Maritime Air Masses are wet air masses
- Equatorial masses are very warm and moist. They are unstable and characterized by tropical storms
- Maritime Tropical Masses form over warm oceans, typically near the subtropics.
- Maritime Polar Masses are cool and humid, not as cold as cP masses.
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A front is a narrow zone that separates two air masses.
- In a front between two air masses, less dense, warmer air is pushed up over the denser, colder one. This facilitates atmospheric motion.
- A cold front is one where a cold air mass displaces a warmer one upward.
- We typically see cumuliform clouds in this front because the warmer air is pushed higher than in a warm front.
- Precipitation tends to be localized, intense and short.
- Typically associated with cP, but also along A or mP air masses.
- A warm front is one where a warm air mass displaces a colder one.
- We typically see stratiform clouds in this front.
- Precipitation tends to be light, wide-spread, and longer.
- Typically associated with mT or cT air masses.
- A stationary front is one where neither the cold nor hot air masses displace each other.
- Because it doesn’t move, these fronts see longer precipitation.
Cyclones and Anticyclones
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A cyclone is an enclosed area of low pressure.
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Anticyclones are enclosed areas of high pressure.
- Semipermanent anticyclones remain in the same general area, whereas Migrating anticyclones move.
- No fronts trail from anticyclones; The pressure gradients in anticyclones are weak.
- Anticyclones bring fair weather and stable atmospheric conditions.
- They are characterized by descending air.
- The descending air results in cool nights and a large temperature contrast between day and night.
- It may also cause fog.
- It can also cause temperature inversions when it occurs over a mountain. The cold air is trapped on the side as it cannot move up.
- Anticyclones migrate in the direction of large-scale atmospheric circulation.
- Many anticyclones result from convergence in Rosby waves.
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An approaching cyclone or anticyclone induces a change in the wind over an area.
Cyclogenesis
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Cyclogenesis occurs as follows
- A stationary front exists where two air masses meet. The temperature difference creates a pressure gradient with one area having low pressure.
- A small disturbance causes the cold air to move south to one side and warm air to move to the north side of the other. Pressure drops in the center, making it a low pressure area.
- Upper level divergence from the jet stream strengthens the low pressure area. The Coriolis effect induces movement
- Occlusion Stage: The cold fronts catch up to the warm fronts creating an occluded front. This causes the warm air to be lifted upwards.
- This signals the end of the cyclone since the warm air and cool air converge to the same temperature.
- As warm air is forced up, it cools and forms precipitation.
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Cyclones can form on the leeward side of mountains as a colder air mass is stretched vertically.
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They also typically form off shore of cold land during the colder times of the year.
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Times when the Rossby waves are most accentuated (curved) are favorable for the formation of mid-latitude cyclones.
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An upward flow of air into the upper atmosphere helps strengthen a cyclone.
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Cyclones are guided by large-scale patterns of atmospheric circulation and so follow similar paths called Storm Tracks.
Tropical Cyclone
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Some common names for tropical cyclones are Hurricanes (Eastern Pacific / Atlantic), Typhoons (Western Pacific), and Cyclones (Indian Ocean).
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They traverse across oceans and spread energy as they move across the globe.
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Tropical Cyclones are characterized by:
- Low atmospheric pressure
- Large areas of strongly rotating winds
- Locally elevated sea levels
- High wind-driven waves.
- Coastal and inland flooding.
- Erosion.
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Rainfall within tropical cyclones is caused by the low atmospheric pressure which causes air to rise and condense.
- Tropical cyclones form with a feedback loop — warm water vapor from the ocean rises, cools and produces clouds, releases energy during condensation, which warms more air around it and intensifies the updraft effect that draws more warm air upwards.
- Tropical cyclones dissipate when passing through cold waters, over land, or against dry air masses.
- Tropical cyclones begin in the tropics as relatively weak disturbances in normal patterns of air pressures and wind directions but which gradually strengthen
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Sea levels during a tropical cyclone can buckle as a result of differing atmospheric pressure.
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The path a tropical cyclone takes is influenced by air pressure and air currents in the atmosphere. Its rotation is also influenced by the Coriolis effect.
- A tropical cyclone forms under specific conditions limited to certain regions and then travels along a path called a storm track.
- Tropical cyclones cannot form in the equator due to the lack of the Coriolis effect.
- Cyclones rotate clockwise and turn south and southwest in the Southern Hemisphere, but rotate counterclockwise and turn north and northeast in the Northern Hemisphere
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The eye of the storm is a region of calm, clear air. It is the result of dry air flowing towards the center of the cyclone, which evaporates the clouds in the region.
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The eye wall is the region surrounding the eye that has the strongest winds.
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A tropical cyclone can be strengthened or weakened based on certain environmental conditions.
- Tropical cyclones strengthen when passing through warm waters due to additional heat and moisture. Similarly, they weaken when passing through colder waters due to less heat and moisture.
- Cyclones strengthen when passing through wetter air masses due to additional moisture. Note that typically the cyclone will not gain moisture as the air mass supporting the cyclone will typically be saturated.
- Similarly, cyclones weaken when passing through drier air masses. The mass is incorporated into the cyclone which means that the air mass loses moisture and heat.
- Land weakens cyclones as the land provides less moisture and can even remove moisture and energy from the cyclone.
- Topography affects cyclones. Mountains remove moisture but also increases friction which weakens the cyclone.
- Fast winds and strong vertical wind shear can both assist cyclogenesis and halt it. Strong winds can shear the top of the cyclone or spread the cyclone over a large area, dissipating it.
Thunder Storms
Other Weather Phenomena
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An elve is a flat disk of dim reddish light that forms due to intense lightning.
- It’s an acronym for Emissions of Light and Very Low frequency perturbations due to Electromagnetic pulse sources.
- The light is generated due to the excitation of nitrogen molecules.
- Typically, it occurs within the thermosphere.
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A sprite pertains to ghostly red lights that shoot straight up from the top of a thunderstorm cloud.
- Typically it occurs near the top of the mesosphere.
- This happens because when lightning strikes, the top of the cloud has a strong negative charge. This combined with the low pressure of the mesosphere causes excitation of the nitrogen atoms.
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A blue jet is a dim blue streak of light resembling puffs of smoke that burst out of a thunder cloud, arc upward and fade away.
- They form within the stratosphere.