Weather Effects: Does Sunshine Lift Stock Returns?

Key Takeaways

• Weather effects on stock returns are small links between local weather, especially sunshine, and daily market moves.
• Hirshleifer and Shumway studied 26 cities from 1982 to 1997 and found sunshine strongly tied to higher returns.
• After controlling for sunshine, rain and snow showed no separate link to returns in that study.
• The pattern likely runs through mood, and trading costs erase the apparent profit, so it is not a strategy.

What It Is

A weather effect is a statistical relationship between local weather conditions and stock market returns. The most studied version is the sunshine effect: sunnier mornings in the city of a country's main exchange line up with higher returns that day.

The landmark paper is "Good Day Sunshine: Stock Returns and the Weather" by David Hirshleifer and Tyler Shumway, published in The Journal of Finance in 2003. It is one of the cleanest demonstrations that something with no link to fundamentals can still nudge prices.

The Intuition

Weather changes mood, and mood can change risk appetite. Bright sunshine tends to lift feelings, and a brighter feeling can make investors a touch more willing to buy and hold risk. The reverse is gloom on grey days.

The key word is small. Nobody claims sunshine sets the direction of a market. The claim is narrower: across many days and many cities, a sunny tilt in mood leaves a faint, repeatable footprint in returns. Because weather has nothing to do with company earnings, that footprint is hard to square with the idea that prices reflect only rational, fundamental information.

How It Works

Hirshleifer and Shumway examined morning sunshine in 26 cities with major stock exchanges over 1982 to 1997. They found sunshine strongly and significantly correlated with that day's market return. Sunnier mornings went with higher returns.

The detail that strengthens the mood story is what did not matter. After accounting for sunshine, rain and snow had no separate relationship with returns. If weather worked through inconvenience or disruption, you would expect rain and snow to bite. They did not. A pure mood channel, driven by light, fits better.

The authors also tested whether you could trade it. A weather-timing strategy could in principle pay off for a trader with near-zero costs, but the trades are frequent, and even modest transaction costs wiped out the gains. The effect is real and yet not exploitable in practice.

Worked Example

Imagine two otherwise identical trading days for a national index. On the first, the morning over the exchange's city is bright and clear. On the second, it is overcast.

Across thousands of such pairs in the study, the bright-morning days carried a small return edge over the grey-morning days. For any single day the difference is tiny and easily swamped by news. It only becomes visible by averaging over a long history and many cities.

Now suppose you tried to act on it: buy on forecast-sunny mornings, lighten up on grey ones. You would trade constantly, and the commissions and bid-ask spreads on all that turnover would eat the entire edge. The footprint shows up in the data but not in a tradable profit.

Common Mistakes

1. Treating the effect as a strategy. The edge exists in the averages but disappears once you pay to trade it. Frequent weather timing loses money after costs.

2. Overstating the size. Sunshine nudges returns slightly. It does not forecast rallies or crashes, and it is dwarfed by real news on any given day.

3. Confusing correlation with a money machine. A reliable statistical link is not the same as a profit opportunity, especially once frictions are included.

4. Ignoring the mood lesson. The useful takeaway is not the weather, it is that pure mood can move prices. Watch your own feelings, not the forecast.

5. Generalizing to your own city. The studies link the weather where the exchange sits to that market. Your local skies have no special bearing on your portfolio.

Frequently Asked Questions

What are weather effects on stock returns in simple terms? Weather effects on stock returns are small links between the weather near a stock exchange and that day's market return. Sunnier mornings tend to go with slightly higher returns, probably because sunshine lifts mood.

How do weather effects relate to investment decisions? They show that mood, not just fundamentals, can move prices, which is a reason to manage your own emotional state. They are not a basis for trading, because the edge vanishes after transaction costs.

What is a real-world example of the weather effect? Hirshleifer and Shumway found that across 26 cities from 1982 to 1997, sunny mornings in the exchange's city went with higher daily returns, while rain and snow showed no separate effect.

How can investors use the weather effect? They cannot trade it profitably, since the gains disappear after costs. The practical use is the underlying lesson: recognize that mood influences markets and your own decisions, and keep a rule-based process.

How is the weather effect different from the seasonal affective effect? The weather effect is about day-to-day conditions like sunshine. The seasonal affective effect is about the longer swing in daylight across fall and winter and its link to risk aversion over months.

Sources

1. Hirshleifer, D., & Shumway, T. (2003). "Good Day Sunshine: Stock Returns and the Weather." The Journal of Finance. https://www.jstor.org/stable/3094570
2. Hirshleifer, D., & Shumway, T. (2003). "Good Day Sunshine: Stock Returns and the Weather." Wiley Online Library. https://onlinelibrary.wiley.com/doi/abs/10.1111/1540-6261.00556
3. UC Irvine. "Good Day Sunshine: Stock Returns and the Weather" (abstract). https://sites.uci.edu/dhirshle/abstracts/good-day-sunshine-stock-returns-and-the-weather/
4. ScienceDirect. "Does mood affect trading behavior?" https://www.sciencedirect.com/science/article/abs/pii/S1386418115000488

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.