These may be infrequent, frequent, or continuous. However, ETTH is further sub-divided into frequent and infrequent TTH. Subjects were undergraduate students divided into frequent and infrequent blushers according to self-report. These are relatively infrequent and poorly understood. Because of the infrequent occurrence of rainfall the landscape is arid. These delays are typically caused by congestion, breakdowns or infrequent service. Infrequent adverse effects include erythema or itch. Earth tremors are infrequent and usually slight. Winters usually feature measurable, though infrequent, snow. Rainfall during the winter months not infrequent but is usually light. These energy economies are seldom practical with infrequent bottom blowdown. Summers are mild and pleasant, with hot and humid conditions very infrequent. Trogons are generally inactive outside of infrequent feeding flights. Infrequent or irregular ovulation is called "oligoovulation". Yet, it is a rarity mostly confined to infrequent appearances at certain weddings.