Emphasise a Reference Isopleth

Forecasters read a tephigram against a handful of reference lines — the 0 °C isotherm for the freezing level, −20 °C for the cold limit of the airframe icing band, a mandatory pressure level. The emphasis option distinguishes any member of any isopleth family.

The Freezing Level

Map the member value to an empty style. The member keeps its family’s colour and draws at 1.5 pt instead of the usual 0.5 pt — the printed-chart idiom of same ink, heavier line:

import matplotlib.pyplot as plt

import tephpy  # registers the "tephigram" projection

fig, ax = plt.subplots(subplot_kw={"projection": "tephigram"})
ax.isotherms(emphasis={0.0: {}})

Colour and Dashes

Any of color, linewidth, linestyle and alpha overrides that default, so the airframe icing band’s bounds can carry their own styling:

ax.isotherms(
    emphasis={
        0.0: {"color": "tab:cyan"},
        -20.0: {"color": "tab:cyan", "linestyle": "--"},
    }
)

An omitted key falls back to the family’s own style, so {0.0: {"linestyle": "--"}} is a dashed member in the family’s colour at the emphasis width.

Values the Interval Never Lands On

An emphasised member is always drawn, whatever the zoom ladder would select, so the dendritic growth zone’s −12 °C and −18 °C bounds appear even though no isotherm interval includes them:

ax.isotherms(
    emphasis={
        -12.0: {"color": "tab:purple"},
        -18.0: {"color": "tab:purple"},
    }
)

A value outside the diagram’s domain is a no-op — it is simply never in view. That is silent on the analytic families (isotherms, dry adiabats and isobars); the curved moist adiabats and mixing ratios build through MetPy, which can warn about a far-out value before the diagram ever gets to ignore it, so emphasise a value those families actually cover.

Every Family, Every Tier

The option is the same on all five families, so a mandatory pressure level is the same gesture:

ax.isobars(emphasis={500.0: {}})

and it takes the usual precedence — the accessor keyword over tephpy.config over the convention default. A family reads tephpy.config when the axes is created, and re-reads it on ax.clear(), so set the configuration before creating the diagram it should apply to:

import matplotlib.pyplot as plt

import tephpy

tephpy.config.isotherms.emphasis = {0.0: {}}
fig, ax = plt.subplots(subplot_kw={"projection": "tephigram"})

Passing an empty mapping at the accessor emphasises nothing, which is how one diagram opts out of a configured emphasis:

ax.isotherms(emphasis={})

Note

Emphasis reaches a member’s line and its inline label. Where a family labels a diagram edge instead, that edge’s tick marks and tick labels take one colour for the whole family — matplotlib styles ticks per axis, not per tick — so an emphasised member’s edge tick is placed but not recoloured.