An isobar joins places with equal atmospheric pressure. Its proposed Greek source is isobarēs, of equal weight, whose components reach isos, equal, and baros, weight or pressure.
Merriam-Webster qualifies the borrowing from isobares as probable. The weather-map sense is separate from the nuclear-physics sense of isobar.
Sources
- Merriam-Webster: isobar
Word History qualifies the Greek borrowing and supplies baros; definition 1 gives the weather-map sense.
- Merriam-Webster: is-
The combining-form definition and history supply Greek isos, equal.
- Merriam-Webster: isotope
Word History gives the shared is- element with Greek topos, place.
- Online Etymology Dictionary: isotope
Isotope: from Greek *isos* “equal” + *topos* “place”