EAS 270: What you should learn to deduce from (or recognize on) weather maps/charts
Our textbook (Ross) gives an Environment Canada weather map (more specifically, the preliminary surface analysis, which does not show fronts) as Figure 1.3 (note that CMC stands for Canadian Meteorological Centre, in Montreal, part of the Meteorological Service of Canada; you'll see these charts variously labelled as "EC" or "MSC" or CMC"). Figure 3.1 is a "final" surface analysis, which does show fronts. An analysis depicts the state of the atmosphere based on observations, normally taken at one of the "synoptic times," viz. 00, 06, 12, 18 UTC (=Zulu=GMT). An analysis, then, is to be distinguished from a forecast (also termed a "prognosis" or "prog"). Significant aspects of the forecast charts (progs) are the initialization time and the valid time (whose difference is the forecast "range" or "lead time").
Table 3.2 lists the most commonly seen "upper-air charts", which fall at the "mandatory levels" of pressure (850, 700, 500 and 250 hPa), the surface (also) being a "mandatory level." Figures 3.22 and 3.23 are examples of upper air charts.
The Appendix (starting p478) provides information to help you decode weather information, but you certainly don't have to expect yourself to learn all of this. The list below defines expectations:
Key learning
As term progresses, further elements will be added.
- Translate UTC to local (Edmonton) time (MDT or MST):
- at the start of term we were on MDT, with MDT=UTC-6
- currently, we are on MST=UTC-7
- Know the approximate heights ASL of the four mandatory upper air levels, and the secondary field plotted at that level on CMC analyses (Table 3.2)
Surface Analysis
- recognize/identify isobars and assess the corresponding pressure (based on the 4 hPa spacing and available labels)
- recognize a low or high pressure system
- identify regions where cross-isobar flow is occurring near the surface
- temperature T and dewpoint Td
- surface wind direction in the compass convention (N, NE, E, SE, S, SW, W, NW) or if finer discrimination is needed (N, NNE, NE, ENE, E, ...)
- surface wind speed
- local sea-level corrected pressure (MSLP)
- recognize "present weather" symbols for drizzle (commas), rain (dots) and snow (stars)
- qualitative total cloud amount from station circle: none, 1/4, 1/2, 3/4, full (see p480)
- pressure tendency (qualitative)
- approximate estimate of direction of motion of surface system (inferred from pressure trends)
- estimate possible location of fronts associated with a storm
Upper level isobaric charts at "mandatory levels" (850, 700, 500, 250 hPa)
- understand the meaning of a "height contour" (isoline of height)
- qualitative inference of wind direction and speed from height contours
- height of the constant pressure surface (from contour labels)
- decode T, T-Td
- decode height of the constant pressure surface from coded radiosonde report
- recognize troughs and ridges in the height of the isobaric surface
- recognize dominant pattern of circulation about highs and lows
- recognize thermal (or thickness) troughs and ridges
- recognise zones of strong temperature advection (equivalent to thickness advection)
850 hPa level specifically
- recognise zones of strong horizontal temperature gradient
- recognise zones of strong horizontal temperature advection
700 hPa level specifically
- recognize stippling pattern on EC 700 hPa analyses signifies small T-Td
- recognise zones of strong humidity advection
500 hPa level specifically
- recognize stippled 534-540 dam thickness band (gray on CMC colour charts) and its significance
- by comparing thickness isolines of any pair of 500 hPa charts, deduce thickness change and implied temperature change
- recognize zones of strong thickness advection
- recognize 510-516 dam thickness band (blue on CMC colour charts)
- recognize vorticity maxima on CMC charts, and diagnose whether shear or curvature dominates
250 hPa level specifically
Thermodynamic chart (e.g. skew-T diagram)
- be able to locate or plot any given (P,T) data pair at the appropriate point on the graph, and/or read off the numerical values of (P,T) corresponding to any point
- recognize an inversion
- recognize a layer that is "well-mixed" (i.e. "neutrally stratified" with respect to unsaturated or saturated adiabatic motion)
- identify and name these families of lines: isotherms, isobars, dry adiabats, saturated adiabats, isohumes
- diagnose the static stability of any given layer based on comparison of the ELR in that layer with the benchmarks (dry and saturated adiabatic lapse rates)
- be able to deduce the potential temperature of air at any (P,T) point on the diagram (by tracing down a dry adiabat)
- apply Normand's rule on the skew-T diagram to determine the Lifting Condensation Level, given surface temperature & dewpoint
Last Modified: 30 Nov., 2016