diff --git a/src/main.zig b/src/main.zig
index f001823..92620e1 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -10,7 +10,7 @@ const Airports = @import("location/Airports.zig");
 const Resolver = @import("location/resolver.zig").Resolver;
 const GeoLite2 = @import("location/GeoLite2.zig");
 
-pub fn main() !void {
+pub fn main() !u8 {
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
     defer _ = gpa.deinit();
     const allocator = gpa.allocator();
@@ -67,7 +67,10 @@ pub fn main() !void {
     });
     defer rate_limiter.deinit();
 
-    var metno = try MetNo.init(allocator);
+    var metno = MetNo.init(allocator, null) catch |err| {
+        if (err == MetNo.MissingIdentificationError) return 1;
+        return err;
+    };
     defer metno.deinit();
 
     var server = try Server.init(allocator, cfg.listen_host, cfg.listen_port, .{
@@ -77,6 +80,7 @@ pub fn main() !void {
     }, &rate_limiter);
 
     try server.listen();
+    return 0;
 }
 
 test {
diff --git a/src/weather/MetNo.zig b/src/weather/MetNo.zig
index 26a85fb..a455b32 100644
--- a/src/weather/MetNo.zig
+++ b/src/weather/MetNo.zig
@@ -7,6 +7,8 @@ const zeit = @import("zeit");
 
 const MetNo = @This();
 
+pub const MissingIdentificationError = error.MetNoIdentificationRequired;
+
 const MetNoOpenWeatherEntry = struct { []const u8, types.WeatherCode };
 // symbol codes: https://github.com/metno/weathericons/tree/main/weather
 // they also have _day, _night and _polartwilight variants
@@ -63,10 +65,24 @@ const WeatherCodeMap = std.StaticStringMap(types.WeatherCode);
 const weather_code_map = WeatherCodeMap.initComptime(weather_code_entries);
 
 allocator: std.mem.Allocator,
+identifying_email: []const u8,
+
+pub fn init(allocator: std.mem.Allocator, identifying_email: ?[]const u8) !MetNo {
+    const email = identifying_email orelse blk: {
+        const env_email = std.process.getEnvVarOwned(allocator, "METNO_TOS_IDENTIFYING_EMAIL") catch |err| {
+            if (err == error.EnvironmentVariableNotFound) {
+                std.log.err("Met.no Terms of Service require identification. Set METNO_TOS_IDENTIFYING_EMAIL environment variable. See https://api.met.no/doc/TermsOfService", .{});
+                std.log.err("See \x1b]8;;https://api.met.no/doc/TermsOfService\x1b\\https://api.met.no/doc/TermsOfService\x1b]8;;\x1b\\ for more information", .{});
+                return MissingIdentificationError;
+            }
+            return err;
+        };
+        break :blk env_email;
+    };
 
-pub fn init(allocator: std.mem.Allocator) !MetNo {
     return MetNo{
         .allocator = allocator,
+        .identifying_email = email,
     };
 }
 
@@ -100,12 +116,20 @@ fn fetchRaw(ptr: *anyopaque, allocator: std.mem.Allocator, coords: Coordinates)
 
     var response_buf: [1024 * 1024]u8 = undefined;
     var writer = std.Io.Writer.fixed(&response_buf);
+
+    const user_agent = try std.fmt.allocPrint(
+        self.allocator,
+        "wttr/1.0 git.lerch.org/lobo/wttr {s}",
+        .{self.identifying_email},
+    );
+    defer self.allocator.free(user_agent);
+
     const result = try client.fetch(.{
         .location = .{ .uri = uri },
         .method = .GET,
         .response_writer = &writer,
         .extra_headers = &.{
-            .{ .name = "User-Agent", .value = "wttr.in-zig/1.0 github.com/chubin/wttr.in" },
+            .{ .name = "User-Agent", .value = user_agent },
         },
     });
 
@@ -145,7 +169,7 @@ fn deinitProvider(ptr: *anyopaque) void {
 }
 
 pub fn deinit(self: *MetNo) void {
-    _ = self;
+    self.allocator.free(self.identifying_email);
 }
 
 fn parseMetNoResponse(allocator: std.mem.Allocator, coords: Coordinates, json: std.json.Value) !types.WeatherData {