Met. Chap. 8, 9, & 10

Common constant pressure charts used in weather forecasting are the: 850, 700, 500, 300, and 200 mb charts. Identify the average height, in feet above sea-level, for each pressure level.

850 mb 5,000 ft 700 mb 10,000 ft 500 mb 18,000 ft 300 mb 30,000 ft 200 mb 40,000 ft

What is a "haboob"? How do they form?

A haboob is a spectacular desert storm composed of dust or sand. They form as cold downdrafts along the leading edge of a thunderstorm lift dust or sand into a huge, tumbling dark cloud that may extend horizontally for over 90 miles and rise vertically to the base of the thunderstorm.

Define "Jet Stream

A jet stream is a region of relatively strong winds concentrated within a narrow band in the atmosphere. Wind speeds in the central core of a jet stream often exceed 100 knots and occasionally exceed 200 knots. Jet streams are usually found at the top of the troposphere near breaks in the tropopause level. They can can vary in height from 10 to 15 km (33,000 to 49,000 ft MSL).

What is considered standard sea-level atmospheric pressure in millibars (mb) and inches of mercury ("Hg)?

Answer: 1013.25 mb and 29.92 "Hg are the standard atmospheric pressure readings at sea-level.

If you are standing directly south of a smoke stack and the wind from the stack is blowing over your head, what would be the wind direction?

Answer: North Winds are named from which they came.

An wind direction is reported as 315˚. From what compass direction is the wind coming from?

Answer: Northwest A wind from 315° would move from northwest to southeast. Remember: Winds are named from which they came.

Explain how each of the following influence the Coriolis force: (CF = 2ΩV sin Φ) 1) Rotation of the earth (Ω) 2) Wind speed (V) 3) Latitude (Φ)

Coriolis force: (CF = 2ΩV sin Φ) v Rotation of the earth (Ω) v No rotation, no Coriolis force v Wind speed (V) v The greater the wind speed, the greater the Coriolis force (Speed of zero - no Coriolis force) Latitude (Φ) Coriolis force is zero at equator and increases towards the poles (sin of 0° is 0, sin of 90° is 1)

What is a gradient wind?

• By definition, the balance of Coriolis force and pressure gradient force requires the wind to be straight (Geostrophic Wind) • In the atmosphere, we have cyclonic and anticyclonic systems that have winds that rotate or move around high (ridge) and low (trough) pressure systems • This flow around curved contours is called the 'gradient wind'

Name the four scales of motion and give an example of each.

1) Microscale → eddies, downburst (microburst) 2) Mesoscale → thunderstorms, land and sea breeze 3) Synoptic scale → low and high pressure systems, weather fronts 4) Global (Planetary) scale → jet stream, long wave trough and ridges

Name the two dominant jet streams and identify their approximate location and height above sea level

1) Subtropical Jet Stream → located above the subtropical high at ≈ 13 km (43,000 ft) 2) Polar-Front Jet Stream → located near the polar front at ≈ 10 km (33,000 ft)

What three factors contribute to the depth of mixing and, hence, frictional influence in the planetary boundary (friction) layer?

1) Surface heating → produce a steep lapse rate and strong thermal turbulence 2) Strong wind speeds → produce strong mechanical turbulent motions 3) Rough or hilly landscape → produce strong mechanical turbulence

What three factors determine the height of "wind created waves" over a body of water?

1) The wind speed 2) The length of time that the wind blows over the water 3) The fetch, or distance, of water over which the wind blows

What are the four forces that affect the horizontal movement of air?

1. Pressure gradient force 2. Coriolis force 3. Centripetal force 4. Friction

Four weak shallow thermal pressure systems form over the Northern Hemisphere that are not considered semi permanent features. What are their names, and when and where do they form?

1.Canadian thermal high → forms in the winter over west-central Canada (Responsible for our cold winter days in North Dakota) 2.Siberian thermal high → forms in the winter over eastern Russia, can become rather strong (Responsible for the dry Monsoon Season from December - February in India/SW Asia) 3.Southwestern US thermal low → forms in the summer over the desert southwest region of the United States (Responsible for the weak Monsoon Season in July -August) 4.Iran thermal low → forms in the summer over the Iranian plateau (Responsible for the wet Monsoon Season from June - September in India/SW Asia)

What winter weather patterns typically set up across North America during a La Nina event?

1.Colder from western Canada to the Dakotas. 2.Wetter and colder over the Pacific Northwest 3.Drier across the far southern regions of the US from the desert SW to Florida 4.Warmer and wetter across the Ohio River Valley

Name the major ocean currents that flow off of the east and west coasts of the United States.

1.Gulf Stream → warm southerly current along the east coast that flows from Florida to the Mid-Atlantic states 2.Labrador Current → cold northerly current along the New England states that flows from Canada to the Mid-Atlantic states 3.California Current → cold northerly current that flows down the entire west coast.

What winter weather patterns typically set up across North America during an El Nino event?

1.Warmer conditions from Western Canada to the Great Lakes region. 2.Drier in the Ohio River Valley 3.Wetter and colder across the far southern regions of the United States and northern Mexico.

What is a "monsoon"? Where is the best know monsoon in the world? Does any location in the U.S. experience a monsoon?

A monsoon is a wind that changes direction seasonally. The best known monsoon occurs in India. The winters are dry due to a northerly wind and the summers are wet due to a southerly wind. Changes in surface pressure systems from winter to summer are responsible for the seasonal change in the wind direction. The U.S does not have a location with a well defined monsoon but the southwestern desert region of the U.S. does have a rainy spell in the late summer and early fall that is associated with southerly winds off of the waters surrounding Baja, California . Surface wind pattern during winter: High pressure develops over the land mass due to the cooling of the land area during the winter season. This will cause offshore flow which will cutoff the moisture. Surface wind pattern during summer: Low pressure develops over the land mass due to the warming of the land area during the summer season. This will cause onshore flow which will provide the moisture.

How can a coastal area have heavy waves which cause beach erosion on a clear, non-stormy day?

A stationary storm system centered somewhere over the open sea is capable of creating waves with heights from 50 to 100 ft. These waves will be reduced in height as they travel away from the storm changing into large "swells". The swells contain energy which will be released when they break onto shorelines located far away from the initial storm, especially if they coincide with high tide.

How is a surface weather map created? What features can you find on a surface weather map?

A surface weather map is an analysis of sea-level pressure readings from within the domain being analyzed. All station pressures must be adjusted to sea-level pressure before any isobars are drawn on the surface map, therefore the surface weather map has a common height of 0 meters (sea-level). Once the isobar analysis is completed, the location of surface high and low pressure systems can be identified. In future chapters we will use the surface weather map to analyze the location of weather fronts

Explain how a thermal circulation develops.

A thermal circulation is produced by the heating and cooling of the atmosphere near the ground. Air movement is initiated aloft where warm air from the high pressure ridge moves toward the colder air associated with the low pressure trough. The transfer of air away from the high pressure ridge will cause a low pressure system to form at the surface and a high pressure system will form at the surface above the region where the air is moving to.

What is the difference between a "valley" and "mountain" breeze? When do they form?

A valley breeze is a gently upslope wind while a mountain breeze is a gently downslope wind. The valley breeze will form with the onset of surface heating on the valley slopes and the mountain breeze will start when the valley slopes experience radiational cooling. This daily cycle of wind develops best during clear summer weather when prevailing winds are light. Valley breezes are associated with cumulus cloud development, if sufficient moisture is available. The valley breeze will be the strongest at the time of peak heating which typically occurs during the afternoon. The mountain breeze will be the strongest at the time of peak radiational cooling which typically occurs right around sunrise.

Define "El Nino"

An extensive warming of the ocean surface temperatures that begins along the coast of Peru and Ecuador in South America. Major El Nino events occur once every 2 to 7 years and covers a large area of the tropical Pacific Ocean.

What is "upwelling" and what causes this to occur along the western coastline of the United States?

As surface water drifts away from the coast, cold, nutrient-rich water from below rises to replace it. This rising of the cold water from below is called "upwelling". Off the California coast the winds flow parallel to the coastline. The net transport of surface water is at right angles to the wind (Ekman transport), Therefore surface water is moved out to sea and must be replaced by cold water from below near the coastline of California.

On an upper-level chart, what is the relationship between air temperature within a column and the height of a constant pressure surface?

Because of the changes in air density, a surface of constant pressure rises in a warm column of air and lowers in a cold column of air. Therefore, we can conclude that higher heights on a constant pressure chart or associated with warmer air temperatures and low heights on a constant pressure chart or associated with colder air temperatures.

Why is the polar-front jet stream stronger and found further south in the winter than the summer?

Because the north-south temperature contrast along the front is strongest in winter and weakest in summer, the polar-front jet stream shows seasonal variations. In winter, the winds blow stronger and the jet moves farther south allowing the leading edge of the cold air to extend further south. In summer, the polar jet stream is weaker and is mainly located over more northern latitudes. With the polar jet stream further south in the winter, Grand Forks will experience more cold days than warm days during our winter season

Why and how do the average surface pressure systems shift from summer to winter?

Because the sun's declination migrates towards the Tropic of Cancer in the Northern Hemisphere during June and the Tropic of Capricorn in the Southern Hemisphere during December, the zone of maximum surface heating shifts seasonally. In response to this shift, the major pressure systems, wind belts, and ITCZ shift toward the north in June - August and toward the south in December - February.

List the semi permanent high and low pressure systems that affect the weather patterns over the Northern Hemisphere.

Bermuda high → located over the mid Atlantic ocean Pacific high → located off the western U.S. coastline Icelandic low → located over southern Greenland and Iceland Aleutian low → located over the Aleutian Island chain of Alaska

Explain why on a surface weather map, closely spaced isobars indicate strong winds and widely spaced isobars indicate weak winds.

Closely packed isobars indicate the area of highest horizontal pressure change over some distance, Therefore, this is the region of highest pressure gradient (PG).

What is dust devil? How do they form?

Dust devils are spinning vortices that are visible due to the dust, sand, and debris they are able to pick up. Dust devils generally form on clear, hot days over a dry surface where most of the incoming solar radiation goes into heating the surface. The surface air becomes super- adiabic and starts to rise rapidly. Air aloft is unstable allowing the heated surface air to rise to a higher height. The wind is deflected by an object which will set the rising air into a spinning motion either clockwise or counter clockwise.

Identify the global circulation features from the equator to the poles on the three cell model.

Equator → Equatorial low, Doldrums, Intertropical Convergence Zone (ITCZ) 0 to 30˚ → Hadley cell, Trade winds (NE in Northern Hemisphere (NH), SE in Southern Hemisphere (SH) 30˚ → Subtropical high (known as Horse latitudes in NH) 30 to 60 ˚ → Ferrel cell, Westerlies (SW in NH, NW in SH) 60 ˚ → Sub polar low, Polar front 60 to 90 ˚ → Polar cell, Polar easterlies (NE in NH, SE in SH) 90 ˚ → Polar high

How does a 'katabatic' wind differ from a 'chinook' wind?

Even though both are descending winds, the 'katabatic' wind is called a cold down sloping wind vs the 'chinook' wind which is a warm down sloping wind. The difference occurs in the formation process. A 'katabatic' wind develops when cold air develops on a high plateau that eventually grows to a thickness that will allow it to flow down the mountain slope through gaps and saddles in the hills. The sinking air will warm due to compression but it is so cold to start with that it will still be colder than the air it replaces once it reaches the base of the slope. Also note, no latent heat release due to condensation is involved in a 'katabatic' wind.

How do winds move around a surface low and high pressure system in the northern hemisphere?

In the northern hemisphere, the wind will move counter- clockwise (CCW) and toward the center of surface low pressure system and clockwise (CW) and away from the center of a surface high pressure system.

Why does air pressure decrease with height more rapidly in cold air than in warm air?

It takes a shorter column of cold air to exert the same pressure as a taller column of warm air. Since the colder air is more dense, you would go through more mass (density) in a shorter vertical distance.

What is the major role of "Jet Streams" in relation to global circulation

Jet streams play a major role in the global transfer of heat by directing colder air equator ward and warmer air pole ward. This helps to equalize the latitudinal energy imbalance with about 40% of the total heat transport coming from the movement of air

What causes wind gusts?

Near the surface, wind flows over the irregular landscape and objects. The wind breaks into a series of irregular, twisting eddies that can influence the air flow for hundreds of meters above the surface. Within each eddy, the wind speed and direction fluctuate rapidly, producing the irregular air motion known as "wind gusts".

Starting at the equator and moving towards the poles, how does the global circulation pattern help to explain zones of abundant and sparse precipitation.

Low pressure at the equator 0° will cause surface convergence and upward rising air which aids in cloud and precipitation development (Location of Tropical Rain Forests). High pressure located at 30° will cause surface divergence and sinking air which inhibits cloud and precipitation development (Location of major Deserts in the world). Low pressure at 60° aids in cloud and precipitation development (Location of the Polar Front). High pressure centered over the poles at 90° inhibits precipitation development (Location of cold Deserts).

Explain the effect surface friction has on wind speed and direction.

Near the surface, friction reduces the wind speed, which in turn reduces the Coriolis force. The weaker Coriolis force no longer balances the pressure gradient force, and the wind blows across the isobars. Friction acts to reduce the wind speed This reduces the Coriolis force The Coriolis force and pressure gradient force are not in balance This causes the wind to blow across the isobars Wind will move towards the center of a low pressure system and away from the center of a high pressure system

What is a geostrophic wind?

Once the Coriolis force (CF) balances with the pressure gradient force (PGF), the wind no longer accelerates because the net force is zero. Here the wind flows in a straight path, parallel to the contours at a constant speed. We call this a geostrophic wind.

Wind direction is given as the direction from which the wind is blowing. Some localized winds may be expressed differently relating to the wind's interaction with land vs water or terrain. Identify a few of these winds.

Onshore wind→ wind blowing from water onto land Offshore wind→ wind blowing from land to water Upslope wind → air moving uphill Downslope wind → air moving downhill

Explain how shelterbelts protect crops from wind damage.

Shelterbelts consist of a mix of conifer and deciduous trees that are planted in rows perpendicular to the prevailing wind flow. The shelterbelt will cause the wind flow blowing through them to break down into small eddies. This small eddies have little mixing effect on the air near the surface, minimizing the damage to a sensitive crop or reducing soil erosion on a barren field.

Describe the direction of vertical air motions above a surface low and high pressure systems.

Since air is moving into the center of a surface low the air must rise or have an upward vertical motion in the center of the low. Air is moving out from the center of a surface high therefore the air must descend or have a downward vertical motion in the center of the high.

What is the difference between a trough and a ridge on a constant pressure chart?

Since we are looking at a constant pressure chart, troughs will indicate where the pressure level is lowest and ridges will indicate where the pressure level is highest in elevation. Ridges are associated with warmer air aloft and troughs are associated with colder air aloft. If you flew on a constant pressure surface from a trough axis to a ridge axis, you would have to increase in altitude to stay on the constant pressure surface. The regions of lowest elevation (troughs) are associated with the coldest air (low pressure) and the regions of highs elevation (ridges) are associated with the warmest air (high pressure)

The continental United States is located between 25° and 49° north latitude. What area of the Global circulation model does the United States fit into?

The Westerly wind belt at the surface, and the Ferrel circulation cell aloft since both of these are located between 30° and 60°north latitude.

How does station pressure differ from sea- level pressure (SLP)? Can the two ever be the same?

Station pressure is the actual atmospheric pressure measured at any given location. So the higher the elevation of a given location, the lower the atmosphere pressure will be since pressure decreases with height. To make a direct horizontal comparison between various locations, station pressure is adjusted to sea- level pressure in order to have all locations on a constant height chart (sea-level). This will allow the observer to locate high and low pressure systems on a surface weather map. The two are equal when the location of the atmospheric pressure measurement, or barometer, is located at sea-level.

How does a steep pressure gradient appear on a weather map?

Steep pressure gradient (PG) is associated with the areas where the constant height lines are closest together. On this 500 mb constant pressure chart a steep PG is associated with strong winds and a weak PG is associated with light winds.

How does the 'Santa Ana' wind differ from the 'chinook' wind?

The 'Santa Ana' wind affects southern California while the 'chinook' wind affects regions along the front range of the Rocky Mountains from New Mexico to Canada. Both are descending warm and dry winds but the 'Santa Ana' winds descend from an elevated desert plateau. Therefore, heating is due to compression alone and not the release of latent heat.

What does the Coriolis force do to moving air? Is there a difference between the Northern and Southern Hemispheres?

The Coriolis force is an apparent force that causes the wind to deflection to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern Hemisphere. The Coriolis force is a result of the earth's rotation and only influences the wind direction, not the wind speed.

Give an example where the development of a thermal circulation cell can influence the weather.

The development of the land and sea breeze across the state of Florida during the summer months is one good example. The interaction between the temperature differences of the land and surrounding water during the day and night will cause a reversal of the daily wind pattern which can influence the development of thunderstorm activity across the Florida peninsula. The diagrams on the following two slides will help to explain this daily phenomenon.

How is horizontal pressure gradient calculated between two locations? How do you determine when the surface winds are the strongest?

The formula for Pressure Gradient (PG) is: Pressure Gradient = difference in pressure distance GFK - Grand Forks MSP - Minneapolis/St Paul When the difference in pressure between GFK and MSP is 40mb over a distance of 350 miles the PG would be 11.4mb/100 mi, but if the pressure difference between GFK and MSP decreased to 20mb, then the PG would be reduced to 5.7mb/100 mi. Hence stronger surface winds would be expected between GFK and MSP when the PG is 11.4mb/100mi versus 5.7mb/100 mi.

Explain the relationship between the circulation of air and the circulation of ocean currents.

The general wind flow around the globe interacts with the surface of water due to the force of friction. This will start the movement of the major ocean currents. The major ocean currents generally correlate with the flow around the semi permanent high pressure systems.

What is the height of the 'planetary boundary' or 'friction' layer"?

The height is normally from the surface to 1000 m (3,330 ft) AGL. This height can be higher in areas where the terrain is rough, or where the winds are stronger, since both of these factors increase the height of the frictional force above the surface.

What is the name of the instrument used to measure atmospheric pressure?

The name of the instrument is a barometer. There are two basic barometers in use. They are described below. The aneroid barometer is the most common type of home barometer. It contains no fluid. Inside is a small, flexible metal box called an aneroid cell. Before the cell is tightly sealed, air is partially removed, so that small changes in external air pressure cause the cell to expand or contract. The mercury barometer utilizes a long glass tube closed at one end and open on the other. Air is removed from the tube before the open end is submerged into a dish of mercury. The height that the mercury rises into the glass tube is the value of the air pressure in inches of mercury.

What major role do ocean currents play in the scheme of global circulation?

The ocean currents, along with wind, help to balance the world's imbalance in heating. Heat from tropics is moved pole ward and cold air from the poles is moved equator ward. This helps to equalize the latitudinal energy imbalance with about 40% of the total heat transport coming from the movement of ocean currents.

What is "La Nina"?

The opposite of El Nino. The waters off of Peru and Ecuador in South America become colder than normal. La Nina can also effect global weather patterns but they are not as pronounced as those during a strong El Nino event

How does the polar front influence the development of the polar-front jet stream?

The polar front is a boundary separating the cold polar air to the north from the warm subtropical air to the south. The greatest contrast in air temperature occurs along the frontal zone. This rapid change in temperature produces a rapid change in pressure which sets up a steep pressure gradient that intensifies the wind speed and causes the jet stream.

What is the force that initially sets the air in motion?

The pressure gradient force is the force that causes the wind to blow. Movement will be from high to low pressure. The greater the pressure difference over a uniform distance, the greater the wind speed.

What is a "prevailing wind"? What is the prevailing wind in the state of North Dakota?

The prevailing wind is the name given to the wind direction most often observed during a time period over one year. The prevailing wind in North Dakota is from the northwest.

Explain why a "Chinook" wind is warm and dry down sloping wind. Where would you expect the development of a chinook wind event in the United States?

The warming occurs when latent heat of condensation is added to a rising parcel of saturated air on the windward side of a mountain. Chinook winds are dry because moisture is removed from the parcel on the windward side of the mountain when precipitation falls out of the parcel as rain or snow. The best region for the development of a 'Chinook' wind would be along the front range or on the leeward side of the Rocky Mountains in the United States.

How do you convert station pressure to sea- level pressure?

There is a simple formula to use when converting from station pressure to sea-level pressure. Add 10mb to the station pressure for every 100 meter change from sea-level. Example: Devils Lake, ND is located 450 meters above sea-level. Devils Lake recorded a station pressure of 960mb. What is Devils Lake's pressure after converting to sea-level? 960mb + 45mb = 1005mb (Add 45mb for the 450 meter change in elevation from the station level to sea-level)

Why are winds near the surface typically stronger and more gusty in the afternoon?

Thermal turbulence increases with increasing surface heating and instability. Greater instability also leads to a greater exchange of faster moving air from upper levels with slower moving air at lower levels. This exchange increases the average wind speed near the surface during the afternoon since the daytime heating of the earth's surface increases the instability of the lower atmosphere during the afternoon hours.

Define "wind shear":

Wind shear is a sudden change in wind speed or wind direction (or both). The greater the change of wind speed or direction over a uniform distance, the greater the wind shear value.

How do winds move around troughs and ridges in the mid and upper-levels of the atmosphere in the northern hemisphere?

Winds move Counter-Clockwise (Cyclonic flow) around a trough and Clockwise (Anticylonic flow) around a ridge in the northern hemisphere The winds move fairly parallel to the lines of constant height since the frictional force becomes a minimal factor in the mid and upper- levels of the atmosphere. Note: The wind direction would be opposite in the southern hemisphere.

What is a "Jet Streak"?

Within a meandering jet stream you will have areas of weaker and stronger wind speeds. The relatively small region of greatest wind speed within a jet stream is called a "jet streak" or "jet maximum".

Can a "Jet Stream" exist below 10,000 ft MSL?

Yes, but the correct term is "low-level jet". On clear nights when the air is calm near the ground, winds called "low-level jets" can be roaring faster than 50 mph only 1,000 ftAGL. This happens when air near the ground cools and becomes heavier than warmer air above. Winds blow across the top of the heavier air, with little friction to slow them down. This wind shear phenomenon has been detected in many countries and can be a hazard to pilots unaware of it. In the U.S. they occur most frequently in the summer through early fall over the central plains. They can, and frequently do, trigger nocturnal thunderstorm activity.

The high altitude winds called the Jet Stream meander around the world. What is the difference between 'Zonal' and 'Meridional' flow within a Jet Stream flow pattern?

Zonal flow is mainly a west to east flow pattern with the movement of the wind fairly parallel to the west - east lines of latitude. Meridional flow is mainly orientated north to south with large meanders located within the flow pattern. This type of flow is mainly aligned with the north - south lines of longitude (meridians).