diff --git a/docs/integrations/weatherapi.md b/docs/integrations/weatherapi.md
index ef12b3c5..dc94896 100644
--- a/docs/integrations/weatherapi.md
+++ b/docs/integrations/weatherapi.md
@@ -57,14 +57,15 @@ An absent `data` member is treated as a missing source. For ordinary sources,
 `data: null` is also missing. The active-alert exception is listed above: its
 explicit `null` payload represents an empty alert result.
 
-Hourly forecast data must be present and contain at least one `period`; a
-missing, malformed, or empty hourly product fails collection. The remaining
-sources follow the configured missing-source policy. Under `error`, collection
-fails; under `warn`, the source is omitted and an inspectable warning is
-recorded; under `none`, the source is omitted without a warning. A per-source
-policy overrides the default. See [Configuration](../config.md) for policy
-settings and [Weather data internals](../internal/weather-data.md) for recorded
-source metadata.
+Hourly forecast data must be present and contain at least one `period`. Every
+hourly period needs nonzero `startTime` and `endTime` values, with `endTime`
+after `startTime`; a missing, malformed, empty, or invalidly bounded hourly
+product fails collection. The remaining sources follow the configured
+missing-source policy. Under `error`, collection fails; under `warn`, the
+source is omitted and an inspectable warning is recorded; under `none`, the
+source is omitted without a warning. A per-source policy overrides the default.
+See [Configuration](../config.md) for policy settings and [Weather data
+internals](../internal/weather-data.md) for recorded source metadata.
 
 Malformed top-level JSON envelopes and HTTP failures are direct request errors.
 Malformed `data` for an optional source follows its missing-source policy.
diff --git a/docs/internal/collect.md b/docs/internal/collect.md
index 5fbd7d8..dd50e5e 100644
--- a/docs/internal/collect.md
+++ b/docs/internal/collect.md
@@ -20,6 +20,9 @@ Fetch failures retain Weather API endpoint context but do not project upstream
 response bodies into application-facing errors. Oversized response bodies fail
 collection before source decoding.
 
+The required hourly product must contain one or more periods with usable time
+bounds. An invalid hourly product fails collection before derivation begins.
+
 ## Application Composition
 
 `internal/app` owns the narrow `Collector` interface used by workflow tests;
diff --git a/docs/internal/weather-data.md b/docs/internal/weather-data.md
index 3c6e19d..485f6f3 100644
--- a/docs/internal/weather-data.md
+++ b/docs/internal/weather-data.md
@@ -45,6 +45,10 @@ marked missing only when the adapter's missing-source policy treats the
 response or parsing failure as unavailable. The policy itself belongs to the
 [configuration reference](../config.md).
 
+Accepted hourly forecast periods always have nonzero start and end times, with
+the end after the start. Collection rejects a required hourly product that does
+not meet those bounds before it enters downstream derivation.
+
 ## Warning semantics
 
 `SourceWarning` has a source name, stable code, severity, explanatory message,
diff --git a/internal/adapters/weatherapi/client.go b/internal/adapters/weatherapi/client.go
index 148ab27..28461bc 100644
--- a/internal/adapters/weatherapi/client.go
+++ b/internal/adapters/weatherapi/client.go
@@ -224,6 +224,11 @@ func (b *bundleBuilder) fetchHourly(ctx context.Context) error {
 	if len(hourly.Periods) == 0 {
 		return fmt.Errorf("hourly forecast from %s contains no periods", source.Endpoint)
 	}
+	for i, period := range hourly.Periods {
+		if !period.HasUsableTimeBounds() {
+			return fmt.Errorf("hourly forecast from %s has unusable time bounds for period %d", source.Endpoint, i+1)
+		}
+	}
 	source.IssuedAt = &hourly.IssuedAt
 	source.UpdatedAt = hourly.UpdatedAt
 	b.bundle.Hourly = &hourly
diff --git a/internal/adapters/weatherapi/client_test.go b/internal/adapters/weatherapi/client_test.go
index b66ddea..fda6401 100644
--- a/internal/adapters/weatherapi/client_test.go
+++ b/internal/adapters/weatherapi/client_test.go
@@ -501,6 +501,66 @@ func TestRequiredHourlyForecast(t *testing.T) {
 	}
 }
 
+func TestRequiredHourlyForecastValidatesPeriodBounds(t *testing.T) {
+	tests := []struct {
+		name    string
+		body    string
+		wantErr bool
+	}{
+		{
+			name: "valid period",
+			body: `{"data":{"periods":[{"startTime":"2026-05-29T13:00:00Z","endTime":"2026-05-29T14:00:00Z"}]}}`,
+		},
+		{
+			name:    "missing start",
+			body:    `{"data":{"periods":[{"endTime":"2026-05-29T14:00:00Z"}]}}`,
+			wantErr: true,
+		},
+		{
+			name:    "missing end",
+			body:    `{"data":{"periods":[{"startTime":"2026-05-29T13:00:00Z"}]}}`,
+			wantErr: true,
+		},
+		{
+			name:    "empty range",
+			body:    `{"data":{"periods":[{"startTime":"2026-05-29T13:00:00Z","endTime":"2026-05-29T13:00:00Z"}]}}`,
+			wantErr: true,
+		},
+		{
+			name:    "reversed range",
+			body:    `{"data":{"periods":[{"startTime":"2026-05-29T14:00:00Z","endTime":"2026-05-29T13:00:00Z"}]}}`,
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var requested []string
+			server := fixtureServer(t, map[string]handlerOverride{
+				"/forecast/hourly": {status: http.StatusOK, body: tt.body},
+			}, &requested)
+			client := newTestClient(t, server.URL+"/", nil)
+
+			bundle, err := client.FetchBundle(context.Background())
+			if tt.wantErr {
+				if err == nil || !strings.Contains(err.Error(), "hourly forecast") || !strings.Contains(err.Error(), "time bounds") {
+					t.Fatalf("FetchBundle() error = %v, want hourly time-bounds failure", err)
+				}
+				if got := countPath(requested, "/forecast/hourly"); got != 1 {
+					t.Fatalf("hourly requests = %d, want no retry; all requests = %v", got, requested)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("FetchBundle() error = %v", err)
+			}
+			if bundle.Hourly == nil || len(bundle.Hourly.Periods) != 1 {
+				t.Fatalf("Hourly = %#v, want accepted hourly period", bundle.Hourly)
+			}
+		})
+	}
+}
+
 func TestNullAlertsMeansNoActiveAlerts(t *testing.T) {
 	server := fixtureServer(t, map[string]handlerOverride{
 		"/alerts/active": {status: http.StatusOK, body: `{"data": null}`},
diff --git a/internal/weatherdata/bundle.go b/internal/weatherdata/bundle.go
index f00c6ab..3b8d1b3 100644
--- a/internal/weatherdata/bundle.go
+++ b/internal/weatherdata/bundle.go
@@ -132,6 +132,11 @@ type ForecastPeriod struct {
 	RelativeHumidityPercent           *float64  `json:"relativeHumidityPercent,omitempty"`
 }
 
+// HasUsableTimeBounds reports whether the period has a non-empty time range.
+func (p ForecastPeriod) HasUsableTimeBounds() bool {
+	return !p.StartTime.IsZero() && !p.EndTime.IsZero() && p.EndTime.After(p.StartTime)
+}
+
 type AlertRun struct {
 	AsOf   *time.Time        `json:"asOf,omitempty"`
 	Alerts []json.RawMessage `json:"alerts,omitempty"`
diff --git a/internal/weatherdata/bundle_test.go b/internal/weatherdata/bundle_test.go
index f18440d..e589984 100644
--- a/internal/weatherdata/bundle_test.go
+++ b/internal/weatherdata/bundle_test.go
@@ -69,6 +69,30 @@ func TestConvectiveOutlookRunJSONPreservesGeometry(t *testing.T) {
 	}
 }
 
+func TestForecastPeriodHasUsableTimeBounds(t *testing.T) {
+	start := mustParseBundleTime("2026-05-29T13:00:00Z")
+	end := mustParseBundleTime("2026-05-29T14:00:00Z")
+	tests := []struct {
+		name   string
+		period ForecastPeriod
+		want   bool
+	}{
+		{name: "valid", period: ForecastPeriod{StartTime: start, EndTime: end}, want: true},
+		{name: "missing start", period: ForecastPeriod{EndTime: end}},
+		{name: "missing end", period: ForecastPeriod{StartTime: start}},
+		{name: "empty range", period: ForecastPeriod{StartTime: start, EndTime: start}},
+		{name: "reversed range", period: ForecastPeriod{StartTime: end, EndTime: start}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.period.HasUsableTimeBounds(); got != tt.want {
+				t.Fatalf("HasUsableTimeBounds() = %t, want %t", got, tt.want)
+			}
+		})
+	}
+}
+
 func mustParseBundleTime(value string) time.Time {
 	parsed, err := time.Parse(time.RFC3339, value)
 	if err != nil {