It's not about two instruments, but rather two key categories of measurements that are most important for understanding climate:

1. Temperature: Measuring temperature is crucial for tracking changes in the Earth's heat balance. This involves various instruments like thermometers, satellites, and weather balloons.

2. Precipitation: Precipitation is essential for understanding the water cycle and its impact on climate. Measuring this involves rain gauges, radar systems, and satellites.

While other factors like wind speed, humidity, and solar radiation are also important, temperature and precipitation are the foundational elements for defining and understanding climate.