Field guide: read the alien clues

Each transmission gives a constraint on the same city. A strong guess satisfies every message so far. Use this guide to distinguish a useful geographic inference from an assumption the data does not support.

Temperature is a climate clue

The first clue uses the player’s local calendar month when the mission starts. It describes the target’s long-term monthly mean air temperature from 2001–2025, including day and night. It is not today’s temperature or the daytime high. A warm October clue can suggest a tropical city or a springtime city in the Southern Hemisphere, but elevation can make either cooler.

A range such as 15–20 °C normally includes 15 and excludes 20. Follow the limit note shown beneath the clue. The unit toggle converts the original boundaries; rounded numbers do not change which cities match.

Distances describe mapped features

Coastline and international land-boundary clues use the nearest feature in the selected Natural Earth data. They describe surface distance in kilometers or miles, not driving routes. The map geometry is simplified: tiny islands and local shoreline details may not appear. A city near water is not necessarily near a mapped ocean coastline.

Equator, tropic, polar-circle and International Date Line clues constrain a different part of the world. Hemisphere clues identify the north/south and east/west quadrant. Combine these with climate before assuming that two cities on a similar latitude are equally plausible.

Elevation, rain and seasonal contrast

Elevation is sampled from NOAA’s ETOPO surface model at the city location. It is not an average for the whole metropolitan area. Annual precipitation is derived from the stored climate profile. Seasonal temperature difference compares the warmest and coolest monthly mean temperatures; it does not measure the difference between a single hot day and a cold night.

A coastal location can moderate seasonal contrast. A high-altitude city can have a surprisingly cool climate despite being near the equator. These are useful hypotheses, but the numeric clue remains the deciding evidence.

Tree cover and population use surrounding areas

Tree cover is the fraction of the area within 20 km classified as trees in ESA WorldCover 2021. It does not mean all vegetation or a city’s official park percentage. Population density is derived from the GHS-POP E2020 grid within 25 km of the city location. It is not the population divided by a municipality’s administrative area.

A worked deduction

Imagine a mission that starts with a mild monthly-temperature band. Keep several continents in consideration. A later Southern Hemisphere clue removes northern candidates. A short coastline distance favors coastal possibilities; a high-elevation clue may then move attention to elevated coastal regions. Every step intersects the evidence rather than replacing the previous guess with the latest clue.

This example explains the method without prescribing a particular city. Actual missions choose useful clues dynamically, and some cities become identifiable before the last round. Later messages may explicitly confirm what earlier evidence already established.

Shield coverage is a map rule

A shield has a fixed radius in the game’s projected map coordinates. It keeps that radius as you zoom, and it is not a constant kilometer radius everywhere on Earth. Look at the covered city markers to judge protection. You can move fields to any map position in the first five rounds; the final answer must be an exact city.

Read the source citations and measurement limitations for the datasets behind these clues.