In situ measurements of cirrus ice water content (IWC) by the Harvard water vapor and total water instruments during Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment are compared with remote sensing data made by the Cloud Radar System instrument in order to derive and validate an empirical IWC-radar relflectivity Ze relationship. The comparisons show that for measurements of in situ IWC and remotely measured radar reflectivity, collocated within 2 km of each other, a single IWC-Ze relationship can be found that fits the data with an uncertainty of ±20–30%. A cloud resolving model shows this level of uncertainty to be consistent with sampling errors associated with comparing two measurements that are not collocated. Uncertainties are quantified in the use of in situ data to validate the retrieval algorithms used to derive the IWC of clouds from remote sensing observations, such as radar reflectivity Ze. Uncertainties are classified into instrumental uncertainties, uncertainties related to sampling errors, and uncertainties in using a single IWC-Ze relationship to describe a cloud.