Surface Weather Elements

Station Plots

• Station "plots" or "model" are used to spatially display the current conditions in a particular region.

Current Plots

Test Yourself

Isoplething

• The analysis process represents only one step in the production of an analysis chart. The construction of isopleths can entail either subjective or objective analysis schemes.

• Subjective analysis refers to a hand-drawn product, where a meteorologist draws isopleths based upon visual interpolation between the irregularly distributed data points, coupled with continuity from previous charts, experience and intuition.

• Objective analysis typically refers to computer generated products, where the isopleths are generated by numerical interpolation schemes involving an organized grid representation of the given field.

• On the surface map there are many more stations to provide information and the following information is given to you:
• Wind speed and wind direction (wind barb at each station)
• Isobars drawn at 4-mb intervals
• Areas of relatively High (H) and Low (L) pressure.

• As one studies the sequence of the three surface maps one sees how both the surface fronts and the areas of low and high pressure move. These features are:
• Transient
• Linked to what's going on at 500-mb! Dynamic!

• Matter of fact, the activity associated with upper-level troughs (movement, location, development) dictates the movement, location and development of the surface low- and high-pressure systems!

• One thing to do in any sequence is pick a location, let's say St. Louis and examine how the surface characteristics change with time. 

Sea-level pressure, surface winds, and frontal positions at 00, 09, and 18 UTC 10 Nov 1998. The dashed blue line denotes the cold front. The contour interval for sea-level pressure is 4 hPa. [Atmospheric Science, Wallace & Hobbs]

Surface air temperature (in degrees celsius) and frontal positions at 00, 09, and 18 UTC 10 Nov 1998.  [Atmospheric Science, Wallace & Hobbs] 

Winds and Pressure

• Wind is plotted vectorially, showing the direction from which the wind is coming from.

• It tends to blow across the isobars going from higher pressure to lower pressure at and near the surface.
• This flow across the isobars is due to friction.

• As you examine the maps you will see that areas of high pressure are separated by troughs of lower pressure.

• A surface trough represents an area of convergence or a line of confluence where air is flowing together in the wind field. The confluence line is generally represented by some type of front.

• Areas of surface low-pressure develop along the confluence line.

• Generally ahead (or to the east) of the surface low pressure the confluence line moves northward (with southerly winds) while behind (or to the west) of the surface low pressure the confluence line moves southward (with northerly winds).

• Counterclockwise (cyclonic) circulation develops around the surface low

Station Models. Wind entries. [Aguado and Burt 411]

Temperature and Fronts

• Surface temps can be plotted.  Once plotted these values represent an excellent way to define frontal location.

• As one can clearly see the temp. north of the convergence or confluence line are much cooler than those south of it.  This is typically the case in the N.H. (weaker differences in the summer than winter).
• Gulf air (warm, moist) -- mT
• Canadian air (cool, dry) -- cP

• If we compare temps to where the wind is blowing from, the relationship between temps and wind direction can be drawn.

• In the warm air region south of the confluence line the temps appear horizontally homogeneous. This is expected in a air mass.

Idealized cross sections through frontal zones showing air motions relative to the ground in the plane transverse to the front. Colored shading indicates the departure of the local temperature from the mean temperature of the air at the same level. (a) Warm front. (b) Stationary front. (c) Cold front. Heavy arrows at the bottom indicate the sense of the frontal movements. [Wallace and Hobbs 321]

Surface air temperature (in degrees celsius) and frontal positions at 00, 09, and 18 UTC 10 Nov 1998.  [Wallace and Hobbs 324] 

Fronts

• Because the confluence line, a boundary, separates relatively cool air from warm air we call it a front.

• The region of strong thermal contrast on the "cold air side (poleward)" of the confluence line is called a frontal zone or baroclinic zone.
• It is the zone, not the front, that separate the cool and warm air masses.  The front represents the warm air boundary of the frontal zone and coincides with the line of confluence in the wind field.

• Fronts are named based on the direction of movement:
• Cold air advancing → cold front (triangles on front)
• Cold air retreating → warm front (semi-circles on front)

• The direction of the front movement is based on the side of the front the symbols are located and direction they are pointing.

• Stationary fronts are denoted by alternating cold and warm front symbols on different sides.

• A transition zone between two air masses of different densities (temp) and humidities.

• They are important not only for temp. and humidity changes they bring but also for the uplift they cause (frontal “wedging”).

• Four primary types:
• Cold
• Warm
• Stationary
• Occluded

Frontal symbols used on surface weather maps. [Robert et al. 166]

• Identifying Fronts:
• Sharp horizontal temperature change
• Sharp horizontal dewpoint/humidity temperature change
• Shift in the wind direction
• Presence of clouds & precipitation
• Change in pressure

• Frontal Characteristics
• Why do fronts move?  Examine the vertical cross sections normal to the fronts which are moving.
• Air within the frontal zone is "trapped" in the shallow wedge beneath the frontal surface and thus can not move relative to the front, or conversely, the front can not move relative to it.
• The direction and speed of movement of the front is determined by the winds within the frontal zone.
• In warm front situation -- cold air retreats poleward and warm air tied to southerly winds tries to push up the frontal surface -- providing for the development of layered clouds (aka stratus).
• In cold front situation -- cold air (associated with northerly winds) advances southeastward forcing the warm air to rise vertically -- giving way to vertically developed clouds.

• Temperatures and Fronts
• Role of fronts in mediating surface temps is important, but other factors influence surface temps including: time of day, sky cover, altitude of the station, and proximity to water
• These can exert an equally important role of influencing temps

• Fronts are often difficult to locate because...
• Over oceans, temps are strongly influenced by underlying water (sea surface temps do not change much over a short distance)
• In mountainous terrain, large difference in station elevation mask the temp gradients
• Terrain effects, nocturnal inversions, convection, and UHI can all affect temps.

Moisture and Dew Point

• Similar to the steep temp. gradients located along lines of confluence, we see horizontal gradients of dew point.
• Steep gradient (continental vs. maritime air)
• Weak gradient (cool maritime vs. warm maritime air)

• In some situations the dew point gradient is a more reliable indicator; especially in summer when temp. differences near fronts are small.

• Dryline is the boundary between  marine and continental air masses.
• Forms due to land-sea geometry and terrain features.
• Typically form during warm season and are situated meridionally across the southern and central Great Plains.

Precipitation

• The distribution of fog and precip. is very much related to the location of the front.

• Early on, the precip. is located north of the stationary front because the warm air overruns the sloping frontal surface.  The snow continues along the front-range of the Rockies as easterly surface winds push the air up the mountain range, lifting mechanism.

• Eventually, the precip will become of greater intensity and will increase in areal coverage due to the surface low and upper air trough deepening causing more surface convergence.

• Fog exists along the warm front and north of the stationary front. Fog is common when warm, moist air passes over a colder, underlying surface.

• The band of precip. associated with the cold front is generally more intense but in narrow bands.  Often t-storms will occur along the cold front

Precipitation Tendency

• Passage of warm front is associated with a leveling off of the pressure (warm, moist air).

• Passage of cold front triggers a rise in the surface pressure.

• Pressure rises after a cold front are usually greater than the pressure falls which occur prior to the passage of a warm front.

• In surface analysis, lines connecting points at which the same pressure tendency occurs are called isallobars.

• The pressure falling in the vicinity of the low pressure indicates that the low is deepening as it moves.  With this comes stronger winds in the circulation around the low.

• When interpreting small changes in pressure, the diurnal cycle in solar heating produces small but noticeable pressure fluctuations that have nothing to do with synoptic situation.  These are referred to as tidal fluctuations and should be removed before a true synoptic assessment.

Works Cited

Aguado, Edward, and James E. Burt. Understanding Weather and Climate. 7th ed., Pearson, 2014.

Rauber, Robert M., et al. Severe and Hazardous Weather: An Introduction to High Impact Meteorology. 4th ed., Kendall Hunt, 2014.

Wallace, John, and Peter Hobbs. Atmospheric Science: An Introductory Survey. 2nd ed., Academic Press, 2006.

Thermal Wind

            Thermal Wind is the vertical shear of the geostrophic wind cause by a horizontal temperature gradient—it “blows” parallel to the thickness contours, leaving low thickness to the left. The Thermal Wind Equation states that the vertically averaged shear of the geostrophic wind (within the layer between any two pressure surfaces) is related to the horizontal gradient of thickness of the layer, in the same manner in which geostrophic wind is related to geopotential height.

Expressed as a linear relationship between vertical wind shear of the geostrophic wind and the horizontal temperature gradient,

            In a barotropic atmosphere—where density is only a function of pressure—the slope of the isobaric surfaces are independent of temperature thus, the geostrophic wind doesn’t increase with height. In other words, there is a complete absence of the horizontal temperature (thickness) gradients such that on constant pressure surfaces. However, the slope of the isobaric surfaces and the speed of the geostrophic wind may vary from level to level due to those thickness variations.

            In an Equivalent Barotropic Atmosphere, isobars and isotherms, on a horizontal surface map, have the same shape.

            In a Baroclinic Atmosphere—where density is a function of both pressure and temperature—the height and thickness contours intersect such that the geostrophic wind exhibits a component normal to the isotherms (or thickness contours). In other words, the horizontal temperature gradients cause the thickness of the layers between isobaric surfaces to increase with higher temperatures. When multiple layers are stacked on each other the geostrophic wind and the slope of the isobaric surfaces increase with height.

Gradient Winds

Gradient Wind (or flow) develops only in the absence of friction, when considering curved flow and flows perpendicular to the contours, for the same reason as in geostrophic flow. However, gradient wind is not truly geostrophic because it is constantly moving, thus undergoing an acceleration. Nonetheless, this time, in order for the air to follow parallel to the contours there must consider the effects of the centrifugal force as well as the pressure gradient force and the Coriolis force.

Subgeostrophic Flow is when V < V_g, air curves cyclonically (counter-clockwise), and the CF needs greater than in the geostrophic case in order to balance the PGF.

Supergeostrophic Flow is when V > V_g, air curves anti-cyclonically (clockwise), and the CF does not need to be as great as in the geostrophic case in order to balance the PGF.

Putting it all together…

Geostrophic Wind (and Equations)

Geostrophic Wind is a nonaccelerating flow occurring only in the upper atmosphere (due to the lack of friction) and when the winds are considered at “steady-state” (when the PGF counterbalances the CF). In fact the geostrophic flow is simply a special case of gradient flow that arises when the wind flows parallel to the isobars.

Chinook, Santa Ana and Katabatic Winds

Chinook, Santa Ana and Katabatic winds are those that flow downslope in response to the distribution of high- and low-pressure systems over and near large mountain areas, where compressing of descending air leads to adiabatic warming.

Chinook winds, off the eastern slopes of the Rocky Mountains in North America, form due to air flowing across the range. Low-pressure systems east of the mountains cause strong winds to descend the eastern slopes. Although, sometimes the presence of a large mass of cold, dense air near the base of the mountain range may prevent a chinook from flowing all the way down the slope.

Santa Ana winds, contrary to what people believe, occur in response to a large area of high-pressure which descends toward lower elevations and warms by compression causing air to flow out of the Rockies, they are not warm because they pass over hot desert surfaces. When Santa Ana’s develop, the combination of hot, dry winds, low humidity, and an abundant source of fuel can set the stage for an extensive fire that destroys a great deal of land or property.

            Katabatic winds, on the other hand, originate when air is locally chilled over a high-elevation plateau, where the air becomes dense due to its low temperature and flow downslope. These very strong gusts and lulls of winds cover much of coastal Antarctica and Greenland. They also flow out of the Balkan Mountains towards the Adriatic coast, where they are called boras; whereas, in France, they flow out of the Alps into the Rhone River Valley and are called mistrals.

Buys-Ballot Law

In the Northern Hemisphere, with the wind at your back, low pressure is to your left (and high pressure is to your right) because winds travel counterclockwise (cyclonic) around low pressure zones, again, in the Northern Hemisphere.

Coriolis Force Motion on a Latitudinal Circle

Coriolis Force—the force per unit mass that results from the rotation of the earth and acts on a moving particle with respect to the earth to deviate it.

•      Perpendicular to the axis of rotation and the velocity vector.
•      Directed to the right (in the Northern Hemisphere) and radially outward from the axis of rotation. 
•      Can only change direction, not magnitude.

Components of the Coriolis force (CF) due to relative motion along a latitude circle. Ra is the apparent radius, Re is the radius of the earth., and omega is Earth's rotational frequency.

Weather Station Model

Temperature, pressure, moisture, and wind measurements are reported hourly at the surface (most are usually made 2 meters above the ground).

77: Temperature.

68: Dewpoint.

998: Pressure, to the nearest tenth of a millibar. Add either a 10 or 9 in front based on which would bring the value closer to 1000. The pressure here is 999.8 millibars (mb).

-03: Pressure tendency the last 3 hours, to the nearest tenth of a millibar. The pressure here has fallen .3 mb the last 3 hours.

Middle Circle (filled in w/ mostly black): Cloud cover. It's mostly black showing that this station is mostly cloudy. Technically, this represents a broken sky with 7/8 of the sky covered with clouds.

Black line, extending from circle: Wind barb. It points to where the wind is coming from. The wind here is from the southwest, hence a southwest wind. The two lines extending represent 20 knot winds with each line representing 10 knots.

Symbol between 77 and 68: This is the present weather field and in this case shows that there is a thunderstorm occurring at the station.

Symbol next to -03: That line is the pressure tendency. The 1st hour the pressure was steady, then fell the last two hours.

Triangle (with a dot above it): Previous weather, or the weather one hour ago. In this case it was a light rain shower.

How does the upper air station model differ from the surface station model?

─     Temperature is given in Celsius

─     Dewpoint depression is given not dewpoint temperature

─     Altitude of the pressure surface is given instead of pressure

─     Cloud cover is not noted

─     Circles indicating station locations are often omitted

Physical Principles: The Movement of Rossby Waves

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt 

Focus on the Environment and Social Impacts: Radiation Inversions and Human Activities

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Physical Principles: The Coriolis Force

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) 7th Edition by Edward Aguado (Author), James E. Burt (Author)

Thunderstorm: Variables & Ingredients

Ø  Variables needed for Severe thunderstorms:

1.    Moisture

2.    Instability

3.    Lift

4.    Wind shear

Ø  Bob, from Texas

̶        Launches weather balloons (radiosonde)

̶        Radioing back temperature, dewpoint, etc…

̶        Thermodynamic diagrams

Ø  Wind shear

1.    Speed Shear

o   Winds increasing speed with height

2.    Directional Shear

o   Winds changing direction with height

Ø  Development

̶        Cumulus Humilius

̶        Cumulus Congestus

̶        Towering Cumulus – not precipitating

̶        Cumulonimbus (Cb) – precipitating

Ø  Texture

̶        More “cauliflower” the stronger the updraft

̶        “rock hard towers” implies that most of the cloud is in the liquid phase

̶        Updraft liquid weakens or reaches high in the troposphere = liquid freezes = giving cloud a “glaciated” texture (considered fairly weak)

Ø  Anvil

̶        Crisp

̶        Fuzzy

Ø  Vertical Shear

̶        Increases longevity and organization

̶        Strong shear = storm-scale rotation by tilting horizontal vorticity into vertical vorticity

̶        Too much shear = the storm cannot organize (“orphan anvils”) = CAPE is too weak and shear is too strong

Ø  Flanking Line

̶        Flanking line leading into the main updraft

̶        Main cell SW is tilted due to the environment shear

Ø  Boundaries (pg. 307)

̶        Describes fronts

̶        The leading edge of thunderstorm outflow

̶        Leading edge of the sea breeze

̶        Any other lines marking the junction of 2 airmasses

Ø  Creating Boundaries

̶        Differential heating of air either over surfaces with different properties, such as water, and lands, forests and fields, urban and rural landscapes, or over surfaces heated differently  (land over cloudy versus clear skies)

Ø  Occlusion = Cold air rapping around a cove

Ø  WER = Not a lot of precipitation/ at all

Ø  Anvil à Sinus Cloud à Made from ice crystals

Ø  More evaporation = High LCL’s = Relative humidity is lower towards the ground

Ø  LCL = Helps indicate the relative humidity of the sub-cloud layer

Ø  Wet Bulb Zero = Sleet = Frozen Rain

Ø  Verga = Rain that evaporate before hitting the ground

Ø  BRN (Bulk Richardson’s Number): CAPE is too weak and the shear is too strong

̶        Sweet spot: 10-45 BRN

                 BRN = CAPE / Shear

Ø  What 3 influences does dry air have on severe weather? 

̶        More evaporation = Stronger downdraft

̶        Dryer air in Mid-level of atmosphere tends to promote large hail growth

̶        Connectivity unstable (will learn in unit 2)

Ø  Single Cell Thunderstorms

̶        Single cell storms are dominated by buoyancy processes

̶        Sometimes called “air mass” t-storms, these storms are poorly organized and pose relatively little threat to the public (lightning and hail)

̶        Typical of afternoon thunderstorms

̶        Updrafts form in relatively random locations

̶        The dominant forcing feature is instability since they form in a low-shear

̶        Goes through the cycle within 30-60mins

̶        Severe weather threats minimal

̶        Pulse Severe Storm

Ø  Severe single cell thunderstorm

̶        Forms in a low shear environment

̶        Taller updraft/More instability

̶        More intense reflectivity/More intense core

̶        Longer lasting

̶        Precipitation takes longer to descend to the ground/Stronger updraft

̶        Vertical Integrated Liquid (VIL) is larger

̶        “Popcorn Severe”

Ø  What are the differences with ordinary thunderstorm and a “pulse” severe thunderstorm?

1.    Taller updraft/More instability

2.    More intense reflectivity/More intense core

3.    Longer lasting

4.    Precipitation takes longer to descend to the ground/Stronger updraft

̶        Both form in a low shear environment

Ø  Land Spouts

̶        Single cell thunderstorm can create them

̶        Horizontal shear causing vertical vorticity that stretches and causes a tornado

̶        Tend to have a double vortex (thin core and translucent on the outside)

̶        Usually weak, not always