Sun, Sky & Daylight

Real-time daylight in Unreal is a team, and knowing each member's job stops the guesswork. The Directional Light is your sun: its rays are parallel (the sun is effectively infinitely far away), so it casts hard, crisp shadows and its rotation - not its position - sets the time of day and the direction light comes from. The Sky Atmosphere actor simulates how sunlight scatters through air, which is what gives you a physically based blue daytime sky, warm horizon, and the reds and oranges of a low sun automatically as you rotate the Directional Light. The SkyLight captures the sky (and distant scene) as an ambient light source and pours that soft, omnidirectional fill into shadows - it is what keeps a shadowed north wall from going black, the outdoor equivalent of GI fill. Finally, an Exponential Height Fog (and optional Volumetric Cloud) adds atmospheric depth, so distance reads and the air feels real.

Epic ships this whole rig as a template - a new Unreal scene often already has Directional Light, Sky Atmosphere, SkyLight and fog placed and linked, and the Environment Light Mixer panel gathers them in one place so you can add or tune them together. The mental model to hold: sun = hard directional light and shadows; sky = soft ambient fill and the backdrop; fog = depth. Get those three doing their jobs and daylight falls into place.

Because the Directional Light's rotation is the sun, controlling daylight is controlling two angles: the sun's height above the horizon (altitude) and its compass direction (azimuth). A high, near-overhead sun gives short, hard shadows and flat, bright light - noon in summer. A low sun gives long shadows, warm colour and raking light that models form beautifully - the reason the golden hour before sunset flatters every building. You can set these by hand in the light's rotation, but Unreal also has a Sun Position Calculator plugin: enter a real latitude, longitude, date and time (and time zone), and it places the sun exactly where it would be for that site and moment. For archviz that matters - shadow studies, checking whether a courtyard gets winter sun, showing a client the west light at 5pm in June - it turns daylight into a genuine analysis tool, not just a look.

A practical workflow: rough the composition with the sun by eye, then, if the project needs accuracy, switch to the Sun Position Calculator with the real site coordinates and the date you care about. Sweep the time and watch shadows travel across the scheme - this is a solar study you can do live and walk a client through. Remember the sun and sky are linked: rotate the Directional Light down toward the horizon and the Sky Atmosphere automatically warms and reddens, and the SkyLight fill shifts with it, so the whole daylight mood moves together. That coupling is what makes it feel like real weather rather than a slider.

In practice you are setting two numbers on the Directional Light's rotation: the pitch (how far above the horizon, the altitude) and the yaw (the compass bearing, the azimuth). A useful reference frame: a pitch near -90 degrees is high overhead noon, a pitch around -5 to -15 degrees is the low, raking golden-hour sun, and once it drops below the horizon you are into dusk and the Sky Atmosphere reddens and dims automatically. One detail that matters for believability: the real sun is not a perfect point, so set the light's Source Angle to roughly 0.5 degrees (its true angular size in the sky) to get soft, real-world penumbra on shadow edges rather than razor-hard CAD shadows. For a long shot, raise the Directional Light's Dynamic Shadow Distance (the cascaded shadow map range) so distant shadows do not fade out, and add an Exponential Height Fog with volumetric fog enabled if you want visible sun shafts raking through the space.

The Sky Atmosphere is physically based - it approximates real atmospheric scattering (Rayleigh and Mie scattering, in the jargon) so blue skies, warm horizons and sunset colour emerge from the physics rather than from you painting them. That is powerful: a scene lit this way tends to sit in believable light automatically, and it stays consistent as the sun moves. For clear-sky daytime and dramatic dawns and dusks, the Sky Atmosphere plus SkyLight plus a Directional Light is usually all you need, and it is fully dynamic.

Sometimes you want a specific sky - a particular cloudscape, a real location's light, a moody overcast - and that is where an HDRI comes in. A high-dynamic-range image is a full 360-degree photograph of a real sky and environment with true brightness values stored, wrapped around the scene as a dome. Unreal's HDRI Backdrop actor (or a SkyLight set to a cubemap) uses it both as the visible backdrop and as the light source, so the scene is lit by the actual captured sky - superb for photoreal stills and for matching a real site's light. The trade-offs: a single HDRI is a fixed moment (the sun is baked into the image, so you cannot freely re-pose it like the dynamic sky), and its resolution limits how sharp reflections and the backdrop can be. Overcast and heavily clouded skies are also where a good HDRI often beats the clear-sky atmosphere model. Rule of thumb: dynamic Sky Atmosphere when you want to move the sun and study daylight; HDRI when you want a specific, photoreal, fixed sky.

When you do commit to an HDRI, a few specifics keep it looking crisp. HDRIs ship as equirectangular (longitude-latitude) .hdr or .exr panoramas; for a backdrop that reads sharply behind glazing you generally want at least a 4K, and ideally 8K, source, since the image is stretched across the whole sky dome. You can drive it two ways: an HDRI Backdrop actor (which also projects the ground so reflections and contact read correctly), or a SkyLight set to a specified cubemap. Either way the SkyLight lights the scene from the image, so a warm sunset HDRI tints the whole interior warm for free. If you want the dynamic sky but with real captured ambient, a SkyLight set to Real Time Capture samples the live Sky Atmosphere every frame and feeds that back as fill - the best-of-both default for archviz, giving you a poseable sun and accurate sky-coloured bounce at once.

A common beginner result is a scene that is either blown-out white or full of harsh black shadows - and the cause is almost never the sun, it is the balance and the exposure. Bright sun with too little sky fill gives you inky shadows; the fix is the SkyLight (and, indoors, Lumen bounce) doing its job as soft ambient. Conversely, an overcast day is all soft fill and barely any direct sun - low contrast, even, shadowless - which is why overcast is forgiving for interiors and unflattering for dramatic exteriors. Learning to dial the ratio of hard sun to soft sky is a large part of lighting a scene with mood.

The other half is exposure, which we treat fully in Lesson 4.4, but the principle belongs here: the real world spans an enormous range of brightness between deep shade and direct sun, far more than a screen can show at once, so the camera must choose an exposure. If your daylight looks wrong, check the exposure before you touch the lights - a perfectly good sky can look bleached because auto-exposure has adapted to a dark interior, or muddy because it adapted to bright sky. Set a sane exposure, get the sun-to-sky balance right, add a little fog for depth, and daylight in Unreal becomes exactly what it is for a real building: the primary material of the design, one you can now pose to any hour and walk a client straight into. Cross-reference Module 5's Water, Sky and Atmosphere lesson for the wider environment, and Module 6 for how a camera frames all this light.