A line on a map joining places that receive the same duration of sunshine is called an:

An isohel is a line on a map that joins places receiving equal amounts (duration) of sunshine over a given period, such as a day, a month or a year. The word comes from the Greek "iso" (equal) and "helios" (Sun).

How sunshine is measured:

• The duration of bright sunshine is recorded with an instrument called a sunshine recorder.
• The common Campbell-Stokes sunshine recorder uses a glass sphere that focuses sunlight to burn a trace on a card; the length of the burnt trace shows how many hours the Sun shone.
• Modern electronic sensors are also used.

Factors affecting sunshine duration:

• Latitude and season, which control the length of the day.
• Cloud cover: cloudy regions, such as those with monsoon rains or frequent fog, receive fewer hours of sunshine.
• Altitude and air pollution.

Distribution of sunshine:

• Hot deserts, such as the Sahara and the Thar Desert, have some of the highest sunshine durations, often over 3,000 to 4,000 hours a year.
• Places in the humid tropics and cloudy mid-latitude regions receive less sunshine.
• In India, western Rajasthan and Gujarat receive the most sunshine, which makes them suitable for large solar power parks.

Uses of isohel maps:

• Planning solar energy projects.
• Agriculture, since crops need sunlight for photosynthesis.
• Tourism and climate studies.

Common isolines and what they join:

• Isobar: equal atmospheric pressure.
• Isotherm: equal temperature.
• Isohume: equal humidity.
• Isoneph: equal cloud cover.
• Isohaline: equal salinity.
• Isonif: equal snowfall.
• Isoseismal line: equal earthquake intensity.
• Contour: equal height above sea level.
• Isochrone: equal travel time.

Why not the other options?

• ✗Isohume joins places with equal humidity.
• ✗Isoneph joins places with equal cloud cover.
• ✗Isotherm joins places with equal temperature.