What they are, why scientists use them, and what they tell you about climate
When you hear "the temperature is 72°F," that's an absolute temperature—the actual number on a thermometer. Simple enough.
But here's the problem: 72°F in Phoenix is freezing. 72°F in Miami is a chilly day. The same number means completely different things in different places.
That's why climate scientists don't talk about absolute temperatures when comparing locations or trends. They talk about anomalies—how much a temperature differs from what's normal for that place.
An anomaly is simply the difference between a temperature and a baseline.
Think of the baseline as the "normal" for your location. In climate science, baselines are usually calculated from a 30-year period (called a "climate normal"). For example:
Phoenix's baseline is around 120°F in July. London's baseline is around 68°F in July. These are different places with different climates. So comparing them by absolute temperature is useless.
But comparing their anomalies is meaningful. "Phoenix is 2°F warmer than normal" and "London is 1.5°F warmer than normal" tells you something real: both places are experiencing above-normal heat, relative to their own patterns.
Raw temperature measurements from different decades can have small errors or measurement changes. By using anomalies—deviations from a baseline—scientists can smooth out these measurement artifacts and focus on the actual signal: is the temperature trend upward or downward?
Absolute temperatures range wildly (from -50°F to 120°F depending on location and season). Anomalies cluster around zero. That makes anomalies much clearer to visualize: positive = above normal, negative = below normal.
| Year | Actual July Avg (°F) | Baseline (1991-2020) | Anomaly | Interpretation |
|---|---|---|---|---|
| 2010 | 74°F | 72°F | +2°F | 2°F warmer than normal |
| 2015 | 75°F | 72°F | +3°F | 3°F warmer than normal |
| 2020 | 76°F | 72°F | +4°F | 4°F warmer than normal |
| 2024 | 77°F | 72°F | +5°F | 5°F warmer than normal |
See what's happening? The absolute temperatures are just getting higher (74, 75, 76, 77). But when you look at the anomalies (+2, +3, +4, +5), you see a clear trend: each decade, July is further above the baseline than it was before.
A baseline is an average. For a 30-year period, it's the mean temperature (or mean of a specific season, like summer). It's not magic—it's just math.
In tempo, you see multiple baseline periods:
If today's temperature is +3°F from the 30-year average, it's warmer than normal. A one-day positive anomaly doesn't mean much (weather happens). But a consistent pattern of positive anomalies over years? That's climate signal.
A negative anomaly means the temperature is colder than the baseline. This is normal and expected—it's part of natural variability. The question is whether negative anomalies are becoming more or less common.
If you plot anomalies over eighty years and see them trending upward, that's strong evidence that the baseline is shifting—that the "normal" is getting warmer. That's what global warming looks like in data.
When you open tempo and look at your local temperature history, you're seeing anomalies visualized in two ways:
This lets you see at a glance: Is my location's climate trending warmer? By how much? How consistent is that trend?
No. Anomalies are calculated from real temperature measurements. The baseline is an average of real data. The anomaly is just the difference. It's simple arithmetic.
Because absolute temperatures vary wildly by location. An absolute temperature tells you "it's 72°F today." An anomaly tells you "it's 5°F warmer than normal"—which is meaningful information about whether something unusual is happening.
Baselines are calculated from publicly available data and the math is simple. Anyone can recalculate them. So no—they can't be hidden or manipulated without evidence.