The figure above shows five environmental lapse rates (ELRs). Which ELR shows an isothermal layer (i.e., temperature remaining constant with height)?

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The figure above shows five environmental lapse rates (ELRs).  Which ELR shows an isothermal layer (i.e., temperature remaining constant with height)?

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The question asks us to identify which environmental lapse rate (ELR) corresponds to an isothermal layer, meaning temperature remains constant with height (zero lapse rate).
Option by option reasoning:
- ELR 2: If ELR 2 shows a decreasing temperature with height (a negative or positive lapse rate), this is not isothermal. A nonzero slope indicates temperature change with altitude, so this cannot be isothermal.
- ELR 4: An isothermal layer has zero lapse rate, i.e., horizontal temperature profile with height. If ELR 4 is drawn as a flat line (no change in temperature with altitude), this matches the definition of an isothermal layer exactly.
- ELR 1: If ELR 1 indicates temperature changing with height (positive or negative lapse rate), then it is not isothermal. Any slope away from zero means a change in temperature with altitude.
- ELR 5: Similarly, ELR 5 would be nonzero slope unless it coincidentally is a horizontal line; but typically in such figures only one ELR is flat. If ELR 5 shows a slope, it’s not isothermal.
- ELR 3: Again, unless it is a zero-slope line, this would imply temperature changing with height, not isothermal. A nonzero lapse rate disqualifies it.
In summary, the defining feature of an isothermal layer is a zero lapse rate (temperature unchanged with height). Among the options, the one that shows no change in temperature with altitude corresponds to the isothermal layer, which is ELR 4 in this figure.

WX 301 WX301 Fall 2025 Exam 2 - Requires Respondus LockDown Browser

The figure above shows five environmental lapse rates (ELRs). Which ELR shows an isothermal layer (i.e., temperature remaining constant with height)?

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