In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation. Humidity Humidity is the amount of water vapor in the air in a particular spot. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, it means that the air contains 80% of the total amount of water it can hold at that temperature. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation. Since warm air can hold more water vapor than cool air, raising or lowering temperature can change air's relative humidity (Figure below). The temperature at which air becomes saturated with water is called the air's dew point. This term makes sense, because water condenses from the air as dew, if the air cools down overnight and reaches 100% humidity.
Clouds Clouds have a big influence on weather:
Clouds are classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure below).
High Clouds High clouds (Figure below) form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds.
Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrus clouds may indicate that a storm is coming. Middle Clouds Middle clouds, including altocumulus and altostratus clouds, may be made of water droplets, ice crystals or both, depending on the air temperatures (Figure below).
Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation, is coming. Low Clouds Low clouds (Figure below) are nearly all water droplets. Stratus, stratocumulus and nimbostratus clouds are common low clouds.
Nimbostratus clouds are thick and dark. They bring steady rain or snow. Vertical Clouds Clouds with the prefix 'cumulo-' (Figure below) grow vertically instead of horizontally and have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward.
Fog Fog (Figure below) is a cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. The several types of fog that each form in a different way.
Precipitation Precipitation (Figure below) is an extremely important part of weather. Some precipitation forms in place .
The most common precipitation comes from clouds. Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets (Figure below). They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. One million cloud droplets will combine to make only one rain drop ! If temperatures are cold, the droplet will hit the ground as a snowflake.
Questions:
MOISTURE, CONDENSATION AND PRECIPITATION Physical States of Water 1. Solid state (e.g. ice) occurs at temperature below freezing point 00 C or 320 F. (Ice) 2. Gaseous state (e.g. water vapor) occurs at temperatures 1000 C or 2120 F 3. Liquid state (e.g. sea water) occurs between freezing and boiling point temperatures (320 F - 2120 F). Sources of Moisture in the Atmosphere i) Evaporation from oceans, lakes, rivers & soil ii) Transpiration from plants and other vegetation iii) Perspiration from people and animals. iv) Sublimation: change from ice to water vapor HUMIDITY: Absolute Humidity is the maximum amount of water vapor that a given column of air can hold. Relative Humidity expressed as a percentage (%) is the amount of water vapor in a column of air compared to the total amount of moisture that the same column of air can hold. When water vapor becomes saturated, it reaches its capacity for holding water so further cooling results in condensation and possibly rainfall. CONDENSATION AND PRECIPITATION Condensation is a process by which a gas such as water vapor is changed into liquid water. When moisture cools and reaches saturation point, the tiny particles of water condenses into larger drops of water. Forms of Condensation: Dew: Tiny drops of water formed when condensation of water vapor occur at or near the surface of the earth. Frost: It is a frozen condensation that occurs when air at ground level is super cooled below the freezing point. Fog: A mass of tiny drops of water that form when water vapor condenses on a nuclei near the earth's surface. Clouds: A cloud is a mass of tiny drops of water that results from condensation which takes place high up in the atmosphere. Adiabatic Cooling: When air rises, it moves from a zone of dense air on the surface to areas of less dense air in the atmosphere. The rising air thus has less weight above it and the lower pressure allows the air to expand and cool down. The decrease in air temperature that result from expansion of rising air is called Adiabatic Cooling.Adiabatic Warming It is the warming of air that results from the compression of the air as it falls from the sky to the earth's surface. When air falls from higher up in the atmosphere it moves into a region of denser air on the earth's surface. The air is compressed in the process resulting in it being warmed.PRECIPITATION: Precipitation occurs when tiny droplets of water, ice, or frozen water vapor join together into masses too big to be held above the earth. They then fall to the ground as precipitation.Forms of Precipitation: a) Snow: When water vapor is frozen directly into a solid without first forming a liquid, it forms tiny ice crystals called snow b) Sleet: is a frozen rain that forms when rain droplets encounter a cold air and freezes into ice before falling from the sky. c) Hail: Rounded lumps of ice that falls from the sky. d) Rain: Consists of droplets of liquid water that falls from the sky. Types of Rainfall: There are three main causes of rainfall. These are: 1. Convectional Rainfall: The rainfall results when a heated air expands, rises and in the process condenses to form drops of rain. Convectional rainfall is more common in the Humid Tropical Regions that receive much of the sun's energy. It usually occurs in the afternoons after surface air has been heated. 2. Orographic (Relief) Rainfall: The rainfall results from uplift of air above some highland. Mountain ranges force air blowing over them to rise, cool and then condense to fall as rain. The side of the mountain that faces the wind and receives the rainfall is the Windward side. The opposite side that receives dry winds is the Leeward side. 3. Frontal (Cyclonic) rainfall: This type of rainfall occurs along the zone of contact between a warm and cool air mass. When two large air masses of different temperature meet, the warmer and hence lighter air is lifted above the cooler air. The warm air then rises, cools and condenses to form rain. The boundary that separates the cold air and the warm air is called a Front. |