An isotherm is a line drawn on a map joining places of equal

Step 1: Analyze the question The question asks what an isotherm joins on a map. The key is the word itself.

Step 2: Explain the concept Break the word into its Greek roots:

• iso = equal
• therm = heat or temperature

So an isotherm is a line joining places with equal temperature. Isotherms are usually drawn after temperatures have been reduced to sea level, so that altitude does not distort the pattern.

Step 3: Check each option

• A. Pressure: Places of equal pressure are joined by isobars (baros = weight or pressure). ✗
• B. Cloud cover: Lines joining places of equal cloud cover are called isonephs. ✗
• C. Temperature: Joined by isotherms. ✓
• D. Rainfall: Places of equal rainfall are joined by isohyets (hyetos = rain). ✗

Step 4: Answer The correct option is C. temperature.

Exam tip: Learn the Greek roots and you can answer many "iso-" questions:

• Isotherm → Thermometer → temperature
• Isobar → Barometer → pressure
• Isohyet → Hyetal (rain) → rainfall
• Isohaline → Haline (salt) → salinity
• Isoneph → Neph (cloud) → cloud cover

Another memory trick is to link "therm" to a thermos flask, which keeps things hot or cold.