Opposing Pitcher. Bullpen. Spot in the Batting Order. Ballpark. Everyone knows to consider these when we're talking about "matchups." If you're streaming a hitter, you'd rather he be facing Bruce Zimmermann than Jacob deGrom. But there's one massively important matchup factor that goes strangely overlooked: weather.
Consider this your "Everything I Ever Needed to Know About Weather and Probably a Whole Lot I Really Didn't Need to Know but Derek Explained It All Anyway to Make Sure It Makes Logical Sense to Me" Guide.
Is Weather Really That Important?
Yes! I wouldn't have written this article if it wasn't!
It's my belief that most fantasy players don't care about weather because they can't see it. It's all about the air... and air is invisible, after all! They can see deGrom's 100 mph fastball or the way Tyler Glasnow's curveball moves. They can see the short porch in Yankee Stadium. But you can't really see the way a ball slices through the air quicker in 80 degree heat than it otherwise would have in 50. And because of how variance-riddled baseball is, there are plenty of examples of high-offense games in cold weather and low-offense games in hot weather for the casual observer to write the whole thing off as irrelevant.
But the great things about statistics is we can quantify each and every game, and when you look at the impact as a whole, it is actually quite large. The difference in home run rates between a 90 degree day and a 40 degree day is roughly equivalent to the difference between hitting in Citizens Bank Park versus Citi Field. You might not see the difference, but it's there, and it's pretty darn huge.
Wind
The easiest weather concept to explain is wind, because we've all felt wind, even if we can't see it. It pushes with or against you. When wind is blowing in, offense decreases. When wind is blowing out, offense increases as balls have a great chance of flying over the fence. Easy peasy.
The addendum to this is that wind is actually super complicated and available wind data is super basic (ideally we'd measure wind at several locations throughout each ballpark since wind can interact with different elements of each park and behave differently as a result), but it's better than nothing. And wind matters the most in Wrigley Field -- by a lot. Even 10 mph winds in Wrigley are like 40 mph winds anywhere else.
Air Density
We're going to get a bit science-y here for a minute, so bear with me; I'll try to make this as painless as possible. Air Density refers to how tightly packed the air molecules are. The closer together they are, the more of them collide with a baseball in the air, slowing the ball down. Think back to fifth grade science class where you learned about the states of matter. Solids are more dense than liquids, which are more dense than gases. It's much easier to walk down the street surrounded only by air (gas) than it is to walk through a pool (liquid), which is easier still than walking through wet sand (solid).
The less dense the material you're walking through, the less pressure is being applied to your body and the easier it is to move. The same goes for a baseball, just to a smaller, less noticeable degree. Luckily there are no baseball stadiums in the Sahara Desert with sand whipping through the air, so we're only dealing with gases, but there are varying degrees of density for gas.
Temperature
So when we talk about temperature, what we really want to know is how it impacts air density. Air Density is a combination of temperature, humidity, and sea-level pressure, but the most important is temperature. If you knew nothing else except temperature, you'd get yourself almost all the way there. The R-squared between temperature and air density (looking at all baseball games from 2018 to 2020) is .94. That's the statistical way of saying that 94% of differences in air density can be explained by temperature. Humidity is just 6%, and sea level pressure is 17%. They all matter, and ideally we'd combine all three to get a final air density measure, but if all you have is temperature, you'll do just fine.
The hotter the temperature, the better it is for hitting. When matter gets hot, it expands as the molecules spread themselves further and further apart. (When matter gets colder, it contracts as molecules pull closer together). So when the ball comes off the bat in hot air, the air is less dense, there are fewer molecules colliding with the baseball, there is less friction applied to the ball to slow it down, and it winds up traveling farther than it would have in denser air.
Humidity
When it comes to humidity, most people assume that more humid = more dense. Humid air is often characterized as "heavy" or "sticky" against our skin. But humidity is essentially a measure of how much water is in the air and, in fact, humid air is actually less dense. After all, our air is primarily made up of oxygen (O2) and nitrogen (N2). If you consult your periodic table (what, you don't have one in your pocket at all times?), you'll see that Oxygen's atomic weight is roughly 16. So two oxygens have a combined weight of 32. (Nitrogen is 14; two are 28). Water (H2O) has two hydrogens (each with a weight of 1) and one oxygen, for a total weight of 18 -- much lower than the Oxygen (32) or Nitrogen (28) it displaces. Despite how it feels, humid air is actually lighter than dry air, and therefore better for offense.
Sea-Level Pressure
That's quite enough science for one article, so I won't bore you with details on the third component: sea-level pressure. All you need to know you can learn by the name. More pressure = denser air = less offense (and visa-versa). Easy as that.
Altitude
The one component I've neglected to mention is altitude, which actually has the biggest impact on air density. The higher the altitude, the lower the air density. However, while temperature changes day-to-day, altitude is constant. Until the day the Oakland Coliseum is put on stilts so the A's can raise the park to the height of a mountain on a whim and lower it back down to sea-level the next day, we don't have to worry about altitude as it relates to weather.
The entire impact of altitude will be caught in a good park factor-that's why Coors Field is such a great hitters park! It has nothing to do with the park itself -- Coors actually has the deepest fences in baseball!-but rather with how thin the air is a mile above sea level. So when I talk about air density as it relates to baseball games, I'm talking about it independent of altitude.
Don't Double Count Weather with Your Park Factors!
And, of course, that means we need to consider park independent of weather. Let's say that Oakland is the coldest park on average (it is), but we don't adjust our park factor for it. Well, today when it's 60 degrees there and it's the coldest game on the slate, if we try to account for that 60-degree weather we'll wind up double-counting since the park factor is already reflecting the average coldness of Oakland. Considering each separately is the key. As far as I know, my park factors for THE BAT are the only ones publicly available that are weather-neutral, so while I'm biased, I would also highly recommend them.
What About Open and Closed Roofs?
You'll see some analysts say things like "Chase Field is so much better with the roof open!" That's kind of true, but not always. It's too simplistic to think of it in such a concrete way. The only thing that ever matters is what the temperature is like relative to other games. A closed roof generally means air-conditioning and 70-ish degree temperatures. So in April and May when most outdoor games are in the 40s and 50s, a closed roof will often mean the best hitting "weather" on the slate. In July when everywhere is 80+, a closed roof will often mean the pitching "weather" on the slate.
Relative Impact is Everything
If everywhere is hot, nowhere is hot. Even though the Blue Jays left Dunedin before it got too hot in an absolute sense, it was still elite hitting weather because 70 degrees in Florida was way better than 40-50 everywhere else (same as our closed-roof example). Now that they'll be in Buffalo, one of the coldest parks, Jays hitters could see a high relative drop-off in performance.
And it's not just the Jays because they're moving cities. As we get deeper into the season, everywhere starts to heat up, but not at the same rate. Take a look at this chart:
What you're looking at is a comparison between early- and late-season temperatures by park. In Colorado, Chicago, and most of the East Coast, temperatures are cool early on and then spike the most later in the season. This is VERY actionable for fantasy players because hitters on those teams stand to get the biggest rest-of-season boost over their early season performance. Meanwhile, players on teams at the bottom of the list (West Coast and roof teams!) get more consistent temperatures throughout the season, so the weather advantage they were getting early in the year that boosted their numbers will completely disappear as it gets hotter everywhere else. This could make for some interesting sell high/buy low opportunities.
Factoring in Weather On The Daily Level
And, of course, on the daily level (whether we're talking DFS, sports betting, or streaming in season-long leagues), factoring in weather can be huge. You don't want to just look at one game and say, "It's hot in Philly, niiiiiice!", you'll want to compare it to every other game and target hitters from the hottest and pitchers from the coldest (even if the coldest is just 75 degrees when everywhere else is 85).
Weather is a big component in my projection system, THE BAT, and in my recommendations every Tuesday in the Daily Notes. All else equal with a matchup, I'll take the dude in hot weather every time.