Poverty measurement with data whose reference period is one year masks family exposure to poverty that only lasts for part of the year. We use quarterly expenditure data and decomposable severity of poverty indexes to quantify consumption-based intra-annual poverty, determine its causes and its response to federal food assistance. Results show that twice as many households are poor for at least one quarter then would be classified as poor with annual consumption data. Severity indexes indicate that intra-annual poverty accounts for over one third of the total annual severity of poverty. The common determinants of intra-annual and annual poverty include low human capital, unemployment and minority status. Changes in family size during the year affect intra-annual but not annual poverty. We also find evidence that food stamp program use reduces intra-annual poverty.