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Cold-core low
A cold-core low, also known as a upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere. It is a low pressure system which strengthens with height in accordance with the thermal wind relationship. These systems can be referred to as upper level lows. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day. Severe weather, such as tornadoes, can occur near the center of cold-core lows. Cold lows can help spawn cyclones with significant weather impacts, such as polar lows, and Von Karman Vortices. In regard to tropical cyclones, cold lows can lead directly to their development due to their associated cold pool of air aloft or by acting as additional outflow channels to aid in further development.
==Characteristics==
Cold cyclones are stronger aloft than at the Earth's surface, or stronger in areas of the troposphere with lower pressures, per the thermal wind relationship and the hypsometric equation. The hypsometric equation dictates that colder atmospheres have less room between pressure surfaces, which corresponds to the concept of lower atmospheric thickness, and the thermal wind relation indicates that in this situation, the wind increases with height. It also means that an isolated pool of cold air aloft is associated with the feature. Because of its cold center, the center shows an area of thickness minimum. Since the temperature at any given radius is similar and parallel to the orientation of the pressure gradient, and therefore the wind, it is barotropic in nature. The movement of cold-core cyclones can be erratic since they are separated from the main belt of the Westerlies, which would otherwise steer them progressively to the east.
Most cloud cover and precipitation in association with cold lows occurs during the daylight hours as sunlight warms the Earth's surface, destabilizing the atmosphere and causing upward vertical motion. The development of severe weather, particularly tornadoes, can occur near the center of these systems over land during any season of the year. During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach move over open waters, deep convection forms which allows polar low development to become possible.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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