Weather and Climate notes: Lasford Flackson CLOUDS - Clouds are collections of minute droplets of liquid water or tiny crystals of ice , suspended in air. - Cloud particles do not form in empty space. They need a tiny center of solid matter to grow around. This speck of matter is called a condensation nucleus - The surface of the sea is an important source of condensation nuclei. Droplets of spray from the summit of the waves are carried upward by turbulent (unstable) air. - When these droplets evaporate, they leave behind a tiny residue of crystalline salt suspended in the air. This aerosol strongly attracts water molecules, helping to initiate cloud formation. - Nuclei are also thrown into the atmosphere as dust in polluted air over cities, aiding condensation and the formation of clouds and fog. Classifying Clouds - Although clouds occur in an almost infinite variety of shapes and sizes, certain general forms recur commonly. - Clouds are classsified on basis on form and altitude Cloud Form: The international classification scheme for clouds recognizes three forms 1. Cirriform clouds (Latin cirrus, “a lock of hair”) are thin and wispy and composed of ice crystals rather than water droplets. 2. Stratiform clouds (Latin stratus, “spread out”) appear as grayish sheets that cover most or all of the sky, rarely broken up into individual cloud units. 3. Cumuliform clouds (Latin cumulus, “mass” or “pile”) are massive and rounded, usually with a flat base and limited horizontal extent but often billowing upward to great heights Weather and Climate notes: Lasford Flackson - Precipitation comes from clouds that have “nimb” in their name, specifically nimbostratus or cumulonimbus. Normally these types develop from other types; that is, cumulonimbus clouds develop from cumulus clouds, and nimbostratus clouds develop from stratus clouds. Cloud types based on altitude 1. High clouds are generally found above 6 kilometers . Because of the small amount of water vapor and low temperature at such altitudes, these clouds are thin, white, and composed of ice crystals. Included in this family are cirrus, cirrocumulus, and cirrostratus. These high clouds often are arbingers of an approaching weather system or storm. 2. Middle clouds normally occur between about 2 and 6 kilometers. They may be either stratiform or cumuliform and are composed of liquid water. Included types are altocumulus and altostratus. - The puffy altocumulus clouds usually indicate settled weather conditions, whereas the lengthy altostratus are often associated with changing weather. 3. Low clouds usually are below 2 kilometers . They sometimes occur as individual cloudsbut more often appear as a general overcast. Low cloud types include stratus, stratocumulus, and nimbostratus. These low clouds often are widespread and are associated with somber skies and drizzly rain. 4. A fourth family, clouds of vertical development, grows upward from low bases to heights of as much as 15 kilometers . Their horizontal spread is usually very restricted. -They indicate very active vertical movements in the air. The relevant types are cumulus, which usually indicate fair weather, and cumulonimbus, which are storm clouds. -Clouds are grouped into families based on height.Individual cloud types are named according to their form. Weather and Climate notes: Lasford Flackson Air masses - An air mass is any large body of the lower atmosphere that has fairly uniform conditions of temperature and moisture. Characteristics To be recognized as a distinct air mass, a parcel of air must meet three requirements: 1. It must be large. A typical air mass is more than 1600 kilometers (1000 miles) across and several kilometers deep (from Earth’s surface to the top of the air mass). 2. It must have uniform properties in the horizontal dimension. This means that at any given altitude in the air mass, its physical characteristics—primarily temperature, humidity, and stability—are relatively homogeneous. 3. It must travel as a unit. It must be distinct from the surrounding air, and when it moves it must retain its original characteristics and not be torn apart by differences in airflow. Origin - An air mass develops its characteristics when it stagnates or remains over a uniform land or sea surface long enough to acquire the temperature/humidity/stability characteristics of the surface below. - This stagnation needs to last for only a few days if the underlying surface has wellknown temperature and moisture characteristics. - Stable air is more likely to remain stagnant for a few days than unstable air, so regions with anticyclonic (high pressure) conditions commonly form air masses. Classification - Air masses are classified on the basis of source region. The latitude of the source region correlates directly with the temperature of the air mass, and the nature of the surface strongly influences the humidity content of the air mass. - Thus, a low-latitude air mass is warm or hot; a high-latitude one is cool or cold. If the air mass develops over a continental surface, it is likely to be dry; if it originates over an ocean, it is usually moist. - The source region of an air mass is any large body of land or water from which the air derives these characteristics. - In a source region, air moves slowly or not at all, which allows the air to acquire temperature and moisture characteristics from the region’s land or ocean surface .For example, a warm, moist air mass develops over warm equatorial oceans. In contrast, a hot, dry air mass forms over a large subtropical desert. - Over cold, snow-covered land surfaces in the arctic zone in winter, a very cold air mass with very low water vapor content is found. - Pressure gradients and upper-level wind patterns drive air masses from one region to another. When an air mass moves to a new area, it can retain its initial temperature and moisture characteristics for weeks before it comes to fully reflect the new surrounding environment. Air masses are categorized by a combination of letter designations. The first letter is lowercase and designates the moisture source of the air mass: c = continental (dry) m = maritime (moist) - Weather and Climate notes: Lasford Flackson - The second letter is uppercase and designates the latitude position of the source region: A = Arctic P = Polar—from between 50° and 60° N or S latitude T = Tropical—from between 20° and 35° N or S latitude Learning check Describe and explain the temperature and moisture characteristics of a maritime polar (mP) air mass.