1 Climate and Climate Patterns

Weather and climate

Weather describes atmospheric conditions at a specific time and place. Temperature, precipitation, humidity, air pressure, and wind are among the variables used to describe it. Conditions can change over minutes, hours, or days.

Climate describes long-term statistical patterns of weather in a place or region. It includes what conditions are typical, how much they vary, and how often unusual conditions occur. A cold day alone does not show that a place has a cold climate, just as a hot day alone does not establish a long-term warming trend.

Climate normals and averages

Climate scientists use many years of observations to describe typical conditions. A climate normal is a standard reference calculated for a period of $30$ years. Normals can describe daily, monthly, seasonal, or annual conditions, such as the average January low temperature or usual annual precipitation at a weather station.

A normal provides a baseline for comparing a particular day, season, or year with what is typical for that location and time of year. Averages are useful, but they do not describe every day or year. Two places can have the same average annual temperature while having different seasonal ranges, and an average can hide unusually wet or dry periods.

Climate descriptions may therefore include more than a single mean. They can report minimums, maximums, ranges, variability, and how frequently certain conditions occur.

Anomalies, records, and variability

An anomaly shows how far an observation differs from a chosen reference average. For example, if a city's usual high temperature for a particular date is $20\, ^\circ\mathrm{C}$, a high of $24\, ^\circ\mathrm{C}$ is $4\, ^\circ\mathrm{C}$ above its normal for that date. The anomaly alone does not explain why the difference occurred or establish a long-term trend.

A climate record is a measured value—such as the highest temperature, greatest daily rainfall, or longest dry spell—compared with observations in a defined record. Records help show the range of conditions experienced, but they depend on the location, the length and quality of observations, and the variable measured. A record from one station or one year should not be treated as a description of an entire region's usual climate.

Weather varies naturally from day to day and year to year. A single unusual event can set a record while the long-term average changes little. Conversely, long-term averages may shift without a new record being set every year. Comparing observations with climate normals and longer records gives a fuller picture.

Ways to represent climate patterns

Different summaries reveal different aspects of climate patterns:

• Monthly or seasonal averages show how temperature or precipitation typically changes through the year.

• Annual averages and totals summarize a year, such as mean temperature or total rainfall.

• Anomalies show how much a value differs from a selected baseline.

• Ranges and extremes describe variability, including record highs and lows or unusually wet and dry periods.

• Graphs and maps make patterns easier to compare. A climograph commonly plots average monthly temperature and precipitation for a location, revealing seasonal changes at a glance.

Interpreting climate descriptions

A clear climate description identifies where and when the observations apply, which variables are being described, and, when relevant, the reference period. Climate statistics summarize what has been observed: they describe typical conditions and variation, not the exact weather on a future day.

Using averages, anomalies, ranges, and records together gives a more complete picture than relying on a single average or event.