USGS researchers making fire weather observations during a fall 2001 prescribed burn carried out by NPS crews in the JFSP Fire and Fire Surrogates plots, Sequoia & Kings Canyon National Parks. On the right, the person is using a hand-held anemometer to obtain wind speeds while the person in the center of the photo is using a sling psychrometer to measure relative humidity. The psychrometer has both wet and dry bulb thermometers and when twirled in the air provides measurements that can be converted into relative humidity values. Both wind speeds and relative humidity values are important in determining fire behavior and understanding fire effects on vegetation. The person on the left is recording the data. © USGS photo by Eric Knapp.