Add some lerping for the raining effect and fixed area detection for whether it's raining at any particular spot

src/main/java/com/minelittlepony/unicopia/client/UnicopiaClient.java

@@ -20,6 +20,7 @@ import com.minelittlepony.unicopia.entity.player.PlayerCamera;
 import com.minelittlepony.unicopia.entity.player.Pony;
 import com.minelittlepony.unicopia.network.handler.ClientNetworkHandlerImpl;
 import com.minelittlepony.unicopia.server.world.WeatherConditions;
+import com.minelittlepony.unicopia.util.Lerp;
 
 import net.fabricmc.api.ClientModInitializer;
 import net.fabricmc.fabric.api.client.event.lifecycle.v1.ClientTickEvents;
@@ -37,6 +38,10 @@ import net.minecraft.util.math.BlockPos;
 
 public class UnicopiaClient implements ClientModInitializer {
 
+    @Nullable
+    private Float originalRainGradient;
+    private final Lerp rainGradient = new Lerp(0);
+
     public static Optional<PlayerCamera> getCamera() {
         PlayerEntity player = MinecraftClient.getInstance().player;
 
@@ -64,12 +69,6 @@ public class UnicopiaClient implements ClientModInitializer {
         return 0.6F;
     }
 
-    @Nullable
-    private Float originalRainGradient;
-    private float prevRainGradient;
-    private float rainGradient;
-    private float targetRainGradient;
-
     @Override
     public void onInitializeClient() {
         InteractionManager.INSTANCE = new ClientInteractionManager();
@@ -99,20 +98,35 @@ public class UnicopiaClient implements ClientModInitializer {
         BlockPos pos = MinecraftClient.getInstance().getCameraEntity().getBlockPos();
         float tickDelta = MinecraftClient.getInstance().getTickDelta();
 
+        Float targetRainGradient = getTargetRainGradient(world, pos, tickDelta);
+
+        if (targetRainGradient != null) {
+            rainGradient.update(targetRainGradient, 2000);
+        }
+
+        float gradient = rainGradient.getValue();
+        if (!rainGradient.isFinished()) {
+            world.setRainGradient(gradient);
+            world.setThunderGradient(gradient);
+        }
+    }
+
+    private Float getTargetRainGradient(ClientWorld world, BlockPos pos, float tickDelta) {
         if (WeatherConditions.get(world).isInRangeOfStorm(pos)) {
             if (originalRainGradient == null) {
                 originalRainGradient = world.getRainGradient(tickDelta);
             }
-            world.setRainGradient(1);
-            world.setThunderGradient(1);
-        } else {
-            if (originalRainGradient != null) {
-                targetRainGradient = originalRainGradient;
-                world.setRainGradient(originalRainGradient);
-                world.setThunderGradient(0);
-                originalRainGradient = null;
-            }
+
+            return 1F;
         }
+
+        if (originalRainGradient != null) {
+            Float f = originalRainGradient;
+            originalRainGradient = null;
+            return f;
+        }
+
+        return null;
     }
 
     private void onScreenInit(Screen screen, ButtonList buttons) {

src/main/java/com/minelittlepony/unicopia/server/world/WeatherConditions.java

@@ -137,7 +137,16 @@ public class WeatherConditions extends PersistentState implements Tickable {
 
         public boolean inRange(BlockPos pos) {
             final StormCloudEntity cloud = this.cloud.get();
-            return cloud != null && cloud.getBlockPos().isWithinDistance(pos, cloud.getSizeInBlocks());
+            if (cloud == null) {
+                return false;
+            }
+            BlockPos cloudPos = cloud.getBlockPos();
+            if (pos.getY() > cloudPos.getY() + cloud.getHeight()) {
+                return false;
+            }
+            float radius = cloud.getSizeInBlocks();
+            return (cloudPos.getX() - pos.getX()) <= radius
+                    && (cloudPos.getZ() - pos.getZ()) <= radius;
         }
 
         public boolean shouldRemove() {

src/main/java/com/minelittlepony/unicopia/util/Lerp.java

@@ -0,0 +1,47 @@
+package com.minelittlepony.unicopia.util;
+
+import net.minecraft.util.Util;
+import net.minecraft.util.math.MathHelper;
+
+public class Lerp {
+    private long duration;
+
+    private long startTime;
+    private float start;
+    private float end;
+
+    private boolean finished = true;
+
+    public Lerp(float initial) {
+        end = initial;
+    }
+
+    public boolean update(float newTarget, long changeDuration) {
+        if (MathHelper.approximatelyEquals(end, newTarget)) {
+            return false;
+        }
+        start = getValue();
+        startTime = Util.getMeasuringTimeMs();
+        end = newTarget;
+        duration = changeDuration;
+        finished = false;
+        return true;
+    }
+
+    public float getValue() {
+        if (finished) {
+            return end;
+        }
+        float delta = getDelta();
+        finished = delta >= 1F;
+        return MathHelper.lerp(delta, start, end);
+    }
+
+    public boolean isFinished() {
+        return finished;
+    }
+
+    private float getDelta() {
+        return MathHelper.clamp((float)(Util.getMeasuringTimeMs() - startTime) / (float)duration, 0, 1);
+    }
+}