How Vesper Decides What You Should Wear: Inside the Logic Behind the What to Wear Silhouettes

You stood in front of the closet this morning with a forecast open on your phone, and the forecast gave you a number. The number did not tell you whether the wool coat was too much or the linen shirt was too little.

The What to Wear silhouettes exist to close that gap between the reading and the decision. Visitors have asked our concierge how a weather app turns conditions into outfit advice, so this piece walks through the logic one input at a time.

What Is A What To Wear Silhouette?

A silhouette is the illustrated outfit that sits under the daily brief — a single figure dressed for the hours you will actually spend outside. It shows layers, fabric weight, coverage and anything you should carry, drawn as a picture rather than written as a list.

We chose a figure over a checklist on purpose. A list of items invites you to argue with each one, while a silhouette shows a complete decision you can accept, adjust or ignore at a glance.

Keep in mind that the silhouette sits at the end of a chain, and the chain starts with data. The same reasoning drives the written brief, which we describe in how Vesper writes a daily brief.

The Five Inputs, In Order

Order matters because each input answers a different question about the outfit. Here is how the five inputs divide the work:

• Feels-like temperature. Sets how many layers the figure wears and how heavy they are. It is the foundation, so everything after it adjusts it rather than replacing it.
• Dew point. Picks the fabric. Moisture in the air decides whether a layer breathes or traps heat against you.
• Wind. Adds or removes the outer shell. A steady breeze can change what a layer is worth more than a few degrees of temperature can.
• UV index. Sets coverage — sleeves, brim and sunglasses. It follows the angle of the sun, which is why it can run high on a cool day.
• Rain chance. Decides what you carry and whether the shell needs to be waterproof. It acts last because it changes the kit around the outfit more than the outfit itself.

All of these inputs come from the same hourly forecast. What separates the silhouette from a raw reading is that each number is assigned a job before it touches the outfit.

Temperature Sets The Starting Layer

The logic starts with apparent temperature, the feels-like figure, rather than the thermometer reading. Air temperature describes the air, while apparent temperature describes your body in that air, and your body is the thing you are dressing.

That is why the silhouette reads wind chill on cold days and the heat index on hot ones. The National Weather Service defines wind chill only at or below 50°F with wind above 3 mph, and the heat index applies from about 80°F — both explained in our guide to feels-like temperature.

From there, the figure moves through layer bands. Broadly, a cold morning earns a base, mid and shell; a cool one drops the mid layer; a mild one keeps a single light layer; and a hot one strips down to the most breathable option you own.

Note that these bands are only the starting point. Each of the next four inputs has the authority to move the figure up or down a band.

Dew Point Decides The Fabric

Two afternoons can share the same 82 degrees and still demand completely different shirts. The difference is almost always dew point — the temperature at which the air would saturate — and it is the most honest measure of how muggy a day will feel.

Relative humidity is a poor guide for this job because it swings with temperature through the day. A 90% reading at dawn can feel crisp, while a 50% reading at 4 p.m. on a hot day can feel like a wet towel.

Accordingly, the silhouette leans on dew point bands that most forecasters recognize:

• Below about 55°F. Dry and comfortable. Cotton and denim behave as expected, and sweat evaporates quickly.
• About 55°F to 65°F. Noticeable but manageable. The figure favors lighter weaves and looser fits.
• Above about 65°F. Sticky to oppressive. Linen, technical wicking fabric and open cuts take over, and heavy denim leaves the figure entirely.

Our full explainer on dew point versus humidity covers the physics. For the silhouette, the rule is simple: the wetter the air, the more the fabric has to breathe.

Wind Decides Whether You Need A Shell

Wind is easy to underestimate, and it explains why a silhouette will sometimes add a jacket on a 70-degree day. A steady breeze strips away the thin envelope of warm air your body holds against your skin, and a sweater alone cannot hold it in place.

In practice, a 15 mph wind on a 50-degree morning makes a lone sweater feel thin. Add a windproof shell over that same sweater, and the walk becomes comfortable without any extra insulation.

The logic weighs sustained wind and gusts separately. Sustained wind drives the shell decision, while strong gusts change the details — a hat that stays on, an umbrella that will not invert, a scarf that will not fight you on the walk.

Direction matters too, particularly in coastal cities, where an onshore breeze off cold water is why our San Francisco weather style guide treats a light shell as nearly permanent. For the forecast side of the equation, see how to read a wind forecast.

UV Index Decides Coverage

UV is the one input that largely ignores air temperature. It follows the angle of the sun, cloud cover, altitude and reflective surfaces such as snow, sand and water.

The silhouette uses the World Health Organization's standard UV index bands, and each band changes the figure in a specific way:

• 0 to 2 (low). No change to coverage.
• 3 to 5 (moderate). Sunglasses appear, and the figure favors a top that covers the shoulders for long stretches outside.
• 6 to 7 (high). A brim and a sunscreen reminder join the outfit.
• 8 to 10 (very high). Light long sleeves become the default during midday hours, even in warm weather.
• 11 and above (extreme). The silhouette treats midday exposure as something to plan around, with full coverage for anyone who has to be out.

This is where the silhouette earns its place on cool days. A clear spring afternoon at 5,280 feet can read in the 50s while UV runs high, which is why our Denver weather style guide pairs a light jacket with sunglasses and a brim.

Rain Chance Decides What You Carry

Rain chance acts last because it changes the kit around the outfit more than the outfit itself. A 30% chance of rain means a 30% chance that measurable rain falls at any given point in the forecast area — a probability, with no promise of drizzle for a third of the day.

We unpack that definition in what chance of rain actually means. For the silhouette, the logic pairs probability with timing and expected amount before it adds anything.

In practice, the decisions climb a ladder:

• Low chance, short window. Nothing changes, or a compact umbrella appears as an optional carry.
• Moderate chance during your hours outside. The shell becomes water-resistant, and footwear shifts away from suede and canvas.
• High chance with steady rain. A hooded waterproof shell, water-tolerant shoes and a full umbrella take over. The figure also loses anything that will not survive getting wet.

Remember that a thunderstorm risk and a drizzle risk at the same percentage call for different gear. A storm that may pass in twenty minutes argues for an umbrella and patience, while an all-afternoon soaker argues for the shell.

How The Inputs Combine

None of these inputs works alone, and the most instructive days are the ones where they disagree. The illustrative days below show how the same logic resolves four very different afternoons.

Notice that no single input decides any row. The Denver row stays light because dry air and strong sun outweigh the cool reading, while the San Francisco row adds a shell the thermometer alone would never suggest.

The Day Has A Shape

A daily high describes a single moment, and you will not spend the whole day in it. The silhouette reads the hourly arc across the window when you are likely to be outside, then dresses the figure for the coldest and wettest stretch with pieces that come off for the warmest one.

That is where the canonical translation sentence comes from — a trench coat morning and a shirtsleeves afternoon. When the arc spans a wide range, the figure carries a removable layer instead of compromising on one medium-weight piece that suits neither end of the day.

Fog mornings show this most clearly. A marine layer that holds until late morning, described in our piece on morning fog formation, can make 9 a.m. feel ten degrees colder than noon in the same spot.

Same Number, Different City

The five inputs also explain why a single temperature means different things in different places. 68 in Denver in March is a celebration and 68 in Miami in November is a cold snap.

Part of that gap is physical — dry air, altitude and strong sun on one side, humid air and a cooler breeze on the other — and part is seasonal, because a body that has spent months in heat reads a mild day as cool. That is why we write city guides alongside the silhouettes, from Miami weather style to Seattle weather style, where drizzle and a shell are the season's uniform.

Why We Built It This Way

Most weather apps already show every number this piece mentions. Apple Weather surfaces feels-like, UV, wind and precipitation in clean cards, and Carrot wraps the same data in personality — both handle the data well, and both leave the closet decision to you.

We think the decision is the product. A forecast that reports dew point without telling you to leave the denim at home has done the measuring and skipped the meaning, and our reasoning for that stance lives in why Vesper exists.

That said, the silhouette is a recommendation for a typical body on a typical walk. If you run cold, commute by bike or will stand still at an outdoor game, adjust a band in the direction you already know and let the figure handle the rest.

Common Questions