A new cloud-detection scheme has been developed that utilizes 1.38-¿m reflectance in combination with 8.6--11 ¿m brightness temperature difference (BTD8.6--11) to detect thin cirrus clouds. The 1.38-¿m channel of the moderate resolution imaging spectroradiometer (MODIS) is useful in detecting thin cirrus due to its high sensitivity to upper tropospheric clouds and a nearly negligible sensitivity to low-level reflectance. Dependent upon neighboring cloud type, water vapor concentration, and the viewing geometry, specific 1.38-¿m reflectance threshold levels can be utilized to detect thin cirrus that has previously been undetectable by downward looking satellite imagery. BTD8.6--11 is also sensitive to ice clouds and is used as a second, independent, cirrus cloud test. Each test can either support or negate results from the other. Final cloud type results are produced by using cirrus detected by either the 1.38-¿m reflectance test or the BTD8.6--11 test or by using only that detected by both tests depending on whether a sizable amount of the neighboring cloud is opaque or not as determined by a simple visible reflectance test. It is found that 1.38-¿m reflectance can often detect a greater amount of thin cirrus than the BTD8.6--11. Satellite data from 10 MODIS cases over the atmospheric radiation measurement-tropical western Pacific and southern Great Plains sites were chosen because they provide land and ocean surface cases, variation in cloud type to test the algorithms reliability, and ground truth in the form of millimeter-wave radar data. Two-dimensional horizontal cloud type detection results are shown to correlate well with the 1-hour cloud radar reflectivity time series centered at the MODIS overpass time. Statistics indicate that the new algorithm more accurately identifies thin cirrus in cases involving only single-layer cirrus and where thin cirrus overrides low cloud.