Fundamentals Flashcards
Meteorology
Study of processes and phenomena of the atmophere.
Atmosphere
Big basin full of fluids, such as water vapor and gases (considered fluids in physics).
What keeps the atmosphere active?
The planet’s motion, its tilted axis, and its orbit around the sun. The sun warms a new part of the atmosphere every second.
How much is earth tilted on its axis? What does this tilt cause?
23.5 degrees. This causes some parts of the Earth to get sunlight more directly in parts of the orbit than others.
Tropics
Area between the Tropics of Cancer and Capricorn. They don’t see a substantial change of daylight hours from month to month, thus there isn’t an opportunity for temperatures to cool enough for winter. They get equal sun much of the year.
Season change increases with …
higher lattitude.
Polar Twilight
The sun never rises in the Arctic, but it’s bright enough to see because the sun is just below the horizon. Reversed in Antarctic.
Midnight Sun
When the sun never sets in the Arctic in the summer. Reversed in the Antarctic.
What causes most of our weather?
Earth’s atmosphere trying to reach equilibrium after differential heating, e.g., between night and day.
Do all parts of the planet react the same to sunlight?
No. E.g., bodies of water, especially large ones, are slower to heat or cool than land masses.
What is the effect of the setting sun?
Reduces differential heating, which can spell an end to ongoing thunderstorms, particularly in tropical locations or near the sea.
How often does the moon orbit the Earth? What does this cause for weather?
Every 27 days. Causes the daily ebbs and flows of the ocean tides. Must be aware of tides during heavy rain or tropical systems, as flooding can be exacerbated by the already-high waters.
How tall is the atmosphere? How many levels are there? What’s in it?
6,200 miles. There are five main levels from top to bottom.
Where does most weather occur in the atmosphere?
The troposphere, or bottom seven miles. The top is roughly where commercial airplanes fly.
What is above the troposphere? What is the boundary called? How do you tell?
The stratosphere. The tropopause. During strong thunderstorms, quickly rising clouds rise to it but then flatten. The tropopause and stratosphere includes more ozone which traps more heat, so it’s warmer than upper troposphere.
How does the temperature change in the troposphere?
Temperatures steadily decrease as altitude rises. The thinner the atmosphere, the fewer molecules to deliver heat. Earth’s surface traps heat and radiates warmth into the atmosphere, heating lower levels more effectively.
What is barometric / atmospheric pressure? How is it measured?
Measure the weight of the air above a location. Measured in bars or inches of mercury (displaced by the weight of air above it).
What is standard atmospheric pressure?
29.92inHG (1013mb)
What are the three main variables that can change in an air mass?
Pressure, density, and temperature. All three are interrelated.
What is temperature?
Measure of how energetic molecules are. Warm air is molecules more energized. Cold air is less energized.
How doe temperature affect density?
Warm, active molecules need more space. This is why steam expands in a tea kettle. Cold air is denser. This is why a blown-up balloon will look deflated outside in cold air.
How does temperature affect pressure?
Cold, dense air in a column above a spot means high pressure at the surface. Warm, less dense air in that same column means low pressure.
Why does pressure fall ahead of storms?
The area of a cyclone that is the most favorable for storm development is also the part that is the warmest (the warm sector).
Humidity
The amount of moisture in the air.
Relative Humidity
Amount of moisture in the air relative to a particular air mass’s capacity to hold water.
Dew Point
The temperature at which moisture in the air can begin to condense into water droplets. If the temperature and dew point are close together, you get fog. Indicate how the air will feel because it relies only on the amount of water in the air and is not calculated relative to the temperature.
Condensation
A given mass of air has a maximum amount of moisture it can hold, and this amount depends on temperature. As parcels rise, they cool, and eventually, they become saturated - unable to hold any more water. When this happens, excess moisture looks for anything to condense onto, such as particulates in the air or preexisting ice crystals.
How do clouds form?
Formed by rising air (updraft). There is more air at lower levels of the atmosphere, so there is more water vapor and more particulate matter for water to condense upon. As air rises, that moisture and particulate reach cooler air, the temperature meets the dew point, and the moisture collects on the particulate and coalesces into large drops. A cloud is a collection of these large drops (or ice crystals, if the layer is cold enough).
What impacts the height and type of cloud?
The amount of moisture in the air.
What direction does the air flow in the middle latitudes?
West to east, which is also the direction the Earth rotates.
Where are the strongest air flows?
They follow the strongest temperature gradients.
What is a gradient?
Change in a particular variable over a geographic area.
Jet Streak
Areas with the strongest air flow. These are areas within the jet stream where winds are especially fast - like rapids in a river. They’re important because the entrance region (where air accelerates into the streak) creates rising motion below it. The exit region (where air decelerates) creates sinking motion below it. These areas of rising and sinking air can enhance or suppress storm development.
Jet Stream
Overall course that jet streaks flow through the atmosphere. The jet stream forecast is a great way to get insight into the country’s weather. It’s a river of fast-moving air in the upper atmosphere. Just like a river has bends and meanders, the jet stream can develop waves. These waves help guide our storm systems and can create “dips” (troughs) where cold air moves south and “hills” (ridges) where warm air moves north.
Where are Jet Streaks and Jet Streams found?
Generally between 30-40k ft. Nearer the poles, they can be closer to 20k. Near equator, can be close to 50k.
What happens to airflow in a jet streak or stream?
When airflow slows down, the air to the right of the jet converges (comes together). To the left, it diverges (moves farther apart) as it is pulled toward the pole by the Coriolis force. Air on the left has lower pressure, and air rises to fill in the gap, creating clouds, rain, snow, and thunderstorms. The air on the right converges, sinks, and prevents most clouds and precipitation from forming at the surface level. Divergence creates low pressure at the surface. Convergence creates high pressure.
The strength of the jet stream is dictated by?
How much air masses change. Drastic differences between air masses at the surface lead to higher strength.
Trough
Jet stream makes a U shape. The chance for a stronger storm increases because divergence is increased.
Ridge
Jet stream makes an inverted U shape.
Dome of High Pressure
Refer to pleasant weather across a broad region. This means there is a ridge-like arc of the jet streak to the north of such a region that is deflecting away all the bad weather.
Where does bad weather at the surface lie?
Below a jet. In a trough, it’s more likely on the eastern side and can be just to the east of the trough itself. To the north, one expects to find noticeably colder air. To the south of a ridge, one expects hotter, potentially humid air, with the threat of showers and thunderstorms found on the north side of the jet streak forming the ridge.
When are jet streaks weaker?
In the summer due to less variance in temperature than the winter. The winter jet stream creates stronger divergence and allows for stronger storms, both in terms of geography and intensity.
Cold Front
Dense, cold air masses wedge under warmer air, forcing it upward. They typically bring sharp weather changes and can trigger thunderstorms. Bring crisper air with reduced humidity, and conditions are often much breezier after they have passed. Colder, denser air is unimpeded and more forceful as it trails the front. Tend to be more abrupt and noticeable than warm fronts, with a sharper temperature gradient. Tend to be associated with more violent thunderstorms in summer or heavy snowstorms in winter. Purges an atmosphere of any moisture, leaving cold dry air. In summer, leads to less humidity and sunny skies after the front passes.
Warm Front
Represent the leading edge of a warm ai mass that is moving into a colder region. Think of a gentle ramp. Warm air gradually slides up and over cooler air. Because warm air can hold more moisture, it usually bring steady precipitation and more gradual weather changes. Tends to be scattered, light-to-moderate rain or snow. Moisture starts higher up and doesn’t have as far to travel, and doesn’t travel upwards as quickly. Usually travel from the south or southwest, where it’s warmer. Warm air can’t push cold air out of an area because cold air is denser, which means it stays at the surface. Hence, warm fronts bring gradual change.
Cyclone
Low-pressure system. Flow rotates counterclockwise around the center of low pressure (“cyclonic flow”). Only defined as a cyclone if it is well organized, meaning there’s a 360-degree circulation around a point. There’s divergence aloft, and air flows into the area above the low-pressure point to fill in that gap. The strongest winds tend to be at the center of the system.
Anticyclones
High-pressure system. Rotate clockwise.
Front
Where temperature gradients are sharp - where there is a great deal of temperature variation over a short distance. at the surface. It’s called either a warm or cold front depending on the air temperature behind the front relative to the region its moving into.
Occluded Front
When fronts overtake each other. Warm air can no longer reach the center of low pressure.
Free Convection
Air rises, thanks to its inherent buoyancy as a result of heat and humidity increasing into the afternoon.
Convection
The sun heats the ground. The air near the ground becomes warm and less dense. This warm air rises. As it rises, it cools and becomes denser. The cooled air eventually sinks back down (completing the cycle).
As warm air rises, it cools and can form clouds (when the water vapor in the air condenses). Strong convection can create:
- Thunderstorms
- Scattered showers
- Turbulence for aviation
- In extreme cases, tornadoes
Expect convection on:
- Sunny, warm days (especially in summer)
- Over dark or paved surfaces that heat up quickly
- When there’s moisture in the air
- Often strongest in the afternoon when the ground is warmest
Signs
- Puffy, cotton-ball-like cumulus clouds
- Rising dust devils or thermal waves
- Birds soaring in circles (they use these rising air currents).
Updraft
Rising air. Depending on the magnitude (determined by how warm the air is compared to the air above it), updrafts can lead to more low-level moisturemerging with moisture aloft, leading to fatter raindrops or growing hailstones. Updrafts draw air from all around a thunderstorm. They tend to be stronger on the leading edge of a thunderstorm and so can cause the entire storm to rotate.
Nor’easters
Refers to northeasterly winds that batter the coat as the center of a cyclone moves by. Develops in the southeast, just off the coast. Are some of the strongest storms found outside the tropics. Satellite images often look similar to hurricanes.
When can rain or hail fall?
When it is heavy enough to overcome the strength of the updraft.
Gulf Stream
Channel of warm water that helps to guide weather features that can feed off the uncharacteristically warm water. Moves from the gulf up the atlantic coast.
What do mountains create?
Low pressure. If air is attempting to move through mountains, the air column is stretched out, and some of it will be unable to make it through the range. If there’s a west wind through a mountain range, then east of the mountains, there will be an area of low pressure essentially generated by the presence of a mountain range. These low-pressure areas can give rise to storms.
Alberta Clipper
Develops in the lee (sheltered side from the wind) of the high peaks of the Canadian Rockies. Don’t have access to as much moisture. Provide a quick batch of snow to the Canadian Prairies, Upper Midwest, and Great Lakes. They don’t become major snowmakers until they pull moisture from the Great Lakes and the Atlantic. They move very quickly because they have access to portions of the jet structure with more energy.
Lee
The side sheltered from the wind.
Windward
The side exposed to the wind.
Polar Vortex
A vortex is fluid moving in a circular motion. A polar vortex is the polar jet stream spinning around the North or South Pole. The polar jet stream holds back the coldest air on the planet. It earns its notoriety when the polar jet inks far enough south to impact highly populated areas. Usually, I visit the US, particularly the plains, at least once a winter. Less often, it lingers for weeks.
Bomb Cyclone
Area of low pressure that drops by at least 25mb of pressure over 24 hours. The pressure drops like a bomb. It’s also explosive cyclogenesis - a cyclone that develops extremely quickly. The winds flowing into the feature will be large, leading to a clash of air masses. These storms are typically found during the transition seasons, particularly from late winter through spring. Often lead to prolific snowfall or severe weather, often both. Explosive cyclogenesis is more likely over the seas, as landmasses produce friction that slows the development of cyclones.
Black Lines on a Weather Map
Isobars. “Iso” means “equal,” and bars or millibars are the measure of the atmospheric pressure. An isobar is a line of equal pressure. Lower numbers represent lower pressure, and higher numbers reflect higher pressure.
Blue Line with Flags
Cold front, with triangles pointing in the direction of movement.
Red Line w/ Half Circles
Warm front. Half circles point in the direction of movement.
Alternating line of blue triangles
Stationary front.
Purple line with half circles and triangles all pointing in the same direction
Occluded front
Brown or purple dashed lines
Catchall symbols, either for surface troughs or dry lines
Clouds
Product of rising air, which causes water droplets within the rising air parcel to collide, coalesce, and accrete into larger droplets until they reach the point that their presence can reflect and refract sunlight, making them visible. Ten primary cloud types. Most types feature combinations of the root names and modifiers.
Three basic cloud types
Cirrus, cumulus, and stratus
Two helpful modifiers
Nimbus (convective) and alto (mid-level)
Shelf Clouds
Downdrafts reach the surface and spread away from the initial downdraft. This is a sign that strong straight-line winds are on their way. Pushed forward by the storm’s motion and momentum and by thunderstorm downdrafts, which are caused by falling rain redirecting and overwhelming updrafts. Much longer than wall clouds and cover the full base of a thunderstorm. Move parallel to the base of the ground.
Fractus Clouds (Scud)
Small, dark, fast-moving clouds that flow easily with wind moving to or from the thunderstorm. Slowly rise to the base of the thunderstorm.
Wall Cloud
More localized lowering of the base of the cloud. Rotate around a vertical axis through the wall cloud. Parent clouds to tornadoes. Funnel clouds, which become tornadoes, emerge from the base of wall clouds and descend down toward the ground. They only do this from wall clouds. Wall clouds typically occor at the SW corner of a thunderstorm. This is why most smart storm chasers go to the west or south of the storm.
Types of Preciptation
Rain, snow, and other types of frozen precipitation.
Rain
Most common form of precipitation. When air rises, it cools because temperature is lower at higher altitudes. As air cools, it loses its ability to retain moisture. Moisture condenses out, latching onto any surface it can find - often a tiny particle of dust or debris or an existing ice crystal. This occurs high oup where temperatures are very low, many raindrops start out as snow. Updrafts allow condensation to continue, which causes rain droplets or ice crystals to collide and coalesce. If enough moisture, those droplets/crystals will get large enough to begin to fall, despite updrafts. If surface temperature is above freezing, precipitation will fall as rain. If below, will fall as snow.
Why do clouds remain suspended in the air?
Updrafts of warm air hold them up in the atmosphere. Because of updrafts, condensation can continue.
What type of clouds doe the heaviest rain typically come from?
The darkest ones. Clouds are darker because less light filters through the clouds unless it’s dusk. Because cumulus clouds are taller, they will produce heavier rain and larger drops.
When doe heavy rain form?
Warm surface temperatures lead to strong updrafts.
Why do rainstorms produce more overall moisture than snow storms?
Because warm air can retain more moisture. Roughly, 1 inch of rain = 10 inches of snow.
How do rainstorms sometimesstop storms that create them?
Storms with heavy, dense raindrops can suppress updrafts. Unless storms are on the move, heavy rain works against an active rainstorm or a thunderstorm.
Virga
Rain shaft that doesn’t reach the ground.
Hail
Exclusively a warm-weather phenomenon. Forms in thunderstorms with strong updrafts when droplets fall and are forced back upwards. They collect more layers of liquid in a ping-pong-like process. This continues until the hailstone is large enough that gravity can overcome the force of the updraft. They are the product of violent winds within thunderstorms. The presence of hail 2 in diameter is a decisive indicator that a tornado is possible.
Snow
If the area between the clouds and the surface is mostly below freezing, precipitation will fall as snow. Colder temperatures lead to crystallized flakes falling the whole way to the Earth. Colder snows, unless they are wind-driven, often fall more gently. Lighter snow blows around easily and leads to drifting and visibility issues in open country.
What is happening when you see fat snowflakes?
Snow falls when there’s a layer of air that is above freezing near the surface. This produces slightly liquid flakes that are more capable of merging with other flakes, resulting in fat ones. Individual snowstorms rarely contain as much moisture as even the most generic heavy rainstorm.
What type of clouds produce snow?
Since there’s less convection in the winter, snow does not fall from cumulus clouds. It typically falls from stratus clouds. Nor’easters or other very strong storms are the exception. Snow-producing clouds tend to have lower bases than rain-producing clouds because moisture condenses quickly in cold air.
Mountain Snow
Air can’t move through mountains, so it must go up and over. This is called mechanical or orographic lift. Air rises and cools, and, as a result, moisture is wrung out of the air with great efficiency. Most mountain chains have a dry region on the downwind side of the range.
Freezing Rain
Precipitation falls through the atmosphere as a liquid until it reaches the surface. As it falls, it encounters a thin layer of cold air above the surface, which cools it below the freezing point. Because there’s nothing for the water to freeze onto, it becomes a super-cooled liquid and freezes in contact with the ground. Freezing rain forms when warm air overrides areas with cold air. Even a small amount of freezing rain can paralyze a city.
Sleet
Caused when precipitation falls through a slightly thicker cold layer at the surface than freezing rain, causing it to freeze right before the surface. You hear it bounce when it hits the ground, and if it is big enough, you can see it. If it hits a warm surface, it will melt and then refreeze as ice on roads and sidewalks, but it doesn’t accrete to elevated surfaces like branches or car doors.
Graupel
Starts as snow and falls through a layer with supercooled droplets, which collect on the snow and freeze. It’s like soft hail. It might look like hail, but it won’t be as solid or loud. You might be able to crush it between your fingers.
Dew
It is not technically a form of. precipitation because it doesn’t form in clouds and falls to the Earth. Forms atel when temperatures cool, and the air loses its ability to retain as much moisture as possible. Saturated air deposits moisture on cars, grass, and any other object it can find.
Frost
Same process as dew, but when it’s colder.
Fog
Type of low stratus cloud. It’s so low that it reaches the ground and obscures vision. There are different types.
Radiation Fog
Heat from the surface radiates to higher levels, causing surface temperatures to cool, leading to condensation.
Advection Fog
Seen along coasts, often as part of the marine layer - a daily sheath of fog and low clouds. Warm air moves over colder surfaces, like tropical air moving over the cold Pacific.
Evaporation Fog
Cooler air sitting above a warm body of water. Evaporation from the water immediately saturates the air above it, leading to wispy fog in the early hours above lakes and ponds.
Topographical Fog
Flow that runs into mountains and moves up the slope (upsloping) will create fog or clouds on the mountainside. In valleys, cold air can stagnate, unable to move over or around the nearby hills. Cooler air, in concert with moisture, can lead to stubborn valley fog.
Lightning
Byproduct of aggressive updrafts within nimbus clouds. Updrafts on thunderstorms can rise as fast as 60mph. Water droplets pass past each other, creating static electricity. Electricity, like air pressure, seeks equilibrium, so it needs to find an outlet, either as a lightning bolt to another cloud or to the ground. Lightning takes the path of least resistance to connect positive to negative poles.
Thunder
Lightning is incredibly hot, so it superheats air around the bolt and causes it to expand, creating the crack and rolling thunder.
Tornado
Technically, it’s not a cloud. Instead, they’re a tight circulation of wind that emanates from the cloud. Occur in thunderstorms with very strong updrafts. Rotation on the back side of the storm will start to drop toward the ground, called the rear flank downdraft.
Where will a tornado move?
Hard to predict but generally in the same direction as the thunderstorm.
How fast are the winds in a tornado?
From 85mpg to over 200 depending on Fujita Scale.
Derecho
Storm with forward momentum and winds all moving in the same direction, allowing downdrafts created by rain to develop behind the thunderstorm. Colder downdrafts reinforce the storm’s momentum. Produce long-lasting and very strong straight line winds (120mph or more). Such storms can be 100 miles long and progress over many stakes over 12 hours.
Downbursts / Microbursts
The weight of moisture overwhelms updrafts. At times, the updraft can become locally overwhelmed, creating strong winds.
Flash Floods
Rapid flooding of low-lying areas. Slow-moving rainstorms, tropical features, or a series of thunderstorms that track over the same location can lead to flash flooding. This causes rivers and creeks to overflow their banks, as well as low-lying areas, roads, and valleys. Often, there is a current underground as water continues toward lower ground.
Mudslides
Heavy rain weakens the soils or even bedrock on hills.
Seasonal Flooding
Heavy snow in winter then rapid melting in spring. This can be made worse by an especially warm period at upper elevations or by a wet spring. This can be monitored as meltwater and runoff moves downstream.
Heat Index
The calculation attempts to approximate what the temperature will feel like to the human body and how dangerous it might be.
Blizzard
Used colloquially to mean any strong winter storm, but a Blizzard Warning includes heavy snow and winds over 35mph. Bring whiteouts and visibility down to zero.
Black Ice
A layer of ice, so I think that it can’t be seen on roadways. It can be created by dew, fog, or very light rain.
Wind Chill
Similar to heat index. Combines cold with another variable, in this case wind speed, to estimate the danger.
Green Hue to Sky
Severe weather may be on the way.
Watches, Warnings, and Advisories are issued referencing what geographic entity?
County / parish / borough
Watch
Conditions are favorable for the development of an extreme weather condition. Issues for counties and last a longer period of time than a warning.
Warning
Severe weather is occurring and expected to arrive soon in the warned location, if not there already. Focus on a smaller location and last a shorter period of time.
Severe Thunderstorm
Winds of 58mp or more and hail >0.5in in diameter. Lightning and heavy rainfall not factors.
Tornado Warning
Funnel cloud observed or if radar indicates tight rotation
Flash Flood Warning
Heavy rain expected
Is there a warning for lightning?
No
Blizzard Warning
Heavy snow + 35mph winds lasting three hours or more
Winter Storm Warning
Can be issued for a wide variety of winter conditions. Criteria varies from place to place. Local NWS Office determines what is impactful.
Tornado Emergency
Confirmed tornado moving toward a populated area.
Confirmed Tornado
Tornado observed on the ground by spotters or emergency personnel.
Weather Advisories
Most often seen in winter, although it can be seen in summer as well. Follow the same logic as a warning when forecasters are sure the phenomenon is occurring or about to occur, but the weather is less severe than what would call for a warning. The best example is a Winter Weather Advisory when snow, blowing snow, or other wintry precipitation is expected. Advisories can be issued for different criteria depending on location, e.g., trace snow in Atlanta but not a few inches of snow in Minneapolis. Also, it can issue strong but not severe storms in summer or wind, heat, and fog.
Storm Prediction Center
Issues all tornado and severe thunderstorm watches. NWS Offices issue warnings. Produces fire weather prediction information.
Convective Outlook
Graphical look at where thunderstorms might be expected on a given day, including whether severe and where. Four categories of risk. Issued daily and can extend up to eight days.
Slight
Severe weather might be expected somewhere in the region.
Moderate or High Risk
Severe weather outbreak expected in an area.
National Hurricane Center
Issues tropical outlooks for Atlantic and Eastern Pacific.
National Center for Environmental Prediction (NCEP)
Produce commonly used models by American meteorologists.
Global Forecast System (GFS)
One of two primary forecast models for American meteorologists. Models the entire planet, which means it is better at global systems. For this reason, its better at long-range forecasts.
North American Mesoscale System (NAM)
One of two primary forecast models for American meteorologists. Takes a narrower look, only looking at North America. Takes a higher resolution look at the forecast area, but not as good at long-range prediction.
Mean Sea Level Pressure (MSLP)
Shows where big systems are and where they might be heading. Loop it to get a sense of where systems are moving.
Radar Colors
Precipitation in graduating levels of intensity. Purples and whites often represent hail.
Hook Echo
Hook like shape on the radar display. A tornado usually appears at the SW end of a rotating supercell. Tornadoes can occur without hook echos.
Supercell
Highly organized thunderstorm with a rotating updraft (column of warm air) called a mesocyclone. Can produce all types of severe weather:
Strong tornadoes (most violent tornadoes come from supercells)
Large hail (sometimes bigger than baseballs)
Damaging straight-line winds
Flash flooding
They’re long-lasting (can persist for hours)
They can strengthen and weaken multiple times
Bow Echo
Reflective of strong straight-line winds. Appears as an arc, pushed forward in the direction of the strongest wind flow. The momentum of the continuous thunderstorm downdrafts propel the center of a line forward faster than the ends of the lines, where the dynamics aren’t the same. Looks like a bow on radar.
Dark clouds plus wind blowing TOWARD the storm means
Localized low pressure, strong updrafts, and the threat of hail or even tornadoes.
Wind blowing AWAY from storm
Suggests the storm is collapsing, and the outflow is caused by the weight of falling rain suppressing the updraft and dispersing all that energy - unless the storm is linear, in which case very strong winds will precede the storm
Rapid cloud growth is indicative of
Instability in the area. Indicates that thunderstorm activity is possible and could erupt rapidly.
“Red sky in the morning, sailor’s warning. Red sky at night, sailor’s delight.”
Fairly reliable. If you see a red sky in the AM, that means the sky is clear in that direction, and clouds may be coming from the West.
“Halo around the sun or moon, rain or snow soon.”
Somewhat reliable. May indicate increasing moisture at high altitudes, which causes halos. Can foretell advancing low pressure, which may or may not mean precipitation.
