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.