UNDERSTANDING WEATHER The condition of the atmosphere at a specific time in a specific place is known as the weather. All weather starts with the sun because weather is driven by energy from the sun. The weather includes and is affected by water, wind, temperature, and air pressure. Meteorologists are scientists who study and predict weather and weather patterns. The Sun’s Energy, Heat Transfer and Wind The sun’s energy comes to the Earth as electromagnetic radiation. Molecules on the Earth and in the atmosphere absorb this radiation and it causes them to vibrate. This vibration energy becomes heat. Wind is created by differences in heating air masses. Simply put, wind is air in motion. Winds can be local meaning that they formed and moved over a relatively short distance. There are also global winds that move over large portions of the Earth. The global winds in the Northern hemisphere curve to the right. The global winds in the Southern hemisphere curve to the left. One would expect them to travel in a straight line, but the Earth’s rotation on its axis causes them to curve. This is due to the Coriolis effect. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Differences in temperature between the poles and the equator create convection cells of wind. Belts of cold air sinking and warm air rising in well-defined regions create this circular motion. As the graphic shows, these cells occur between the poles and 60 degrees N and S latitude, between 60 degrees and 30 degrees N and S latitude and between 30 degrees N and S latitude and the equator. Air pressure and weather are closely related to the temperature of air masses that move into an area. Changes in the weather of any region are the result of the interaction of air masses. An air mass is a large body of air with similar temperature and water content throughout. When a warm air mass moves into a region, the air pressure is low because warm air rises away from the Earth’s surface. By contrast, air pressure is high when a cold air mass moves into a region because cold air is dense and pushes down toward the surface. Low-pressure systems are unstable and bring storms with them. High-pressure systems bring pleasant weather. The Sun’s Energy and the Water Cycle © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
The continuous movement of water from the Earth’s surface to the atmosphere and back is called the water cycle. Energy from the sun heats water causing it to evaporate. Plants and animals also give off water: this is called transpiration. The largest source of atmospheric water vapor is the oceans. Evaporation from lakes, rivers, streams and surface water runoff also contributes to water vapor. This water vapor accumulates in the atmosphere and when the conditions are right, clouds form. A cloud is simply water. Water eventually precipitates out of the clouds and falls back to Earth either as rain, snow, or sleet. The form of precipitation (liquid or solid) depends on the temperature. Lesson Checkpoint: What is an air mass and how does it affect us? Humidity As water evaporates from oceans, lakes and rivers, it is held in the air. The amount of water vapor in the air is called humidity. Meteorologists typically report relative humidity on their daily weather report. “Humidity” and “relative humidity” are not synonyms. Relative humidity is a percentage that compares the amount of water vapor in the air with the maximum amount of water vapor the air could hold at a specific temperature. Humidity is measured in grams per cubic meter (g/m3). Relative humidity is affected by temperature. It decreases as temperature increases and increases as temperature decreases. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
The dew point is the same as 100% relative humidity. It is the temperature at which the water in a saturated air mass comes out of the air mass as rain or dew or, if it is cold enough, snow or frost. Cloud Types Clouds form when minute droplets of water or ice crystals accumulate in the atmosphere. Different types of clouds form under different meteorological conditions. Clouds are categorized based on their form and composition. 1. Cirrus clouds are thin clouds that are composed of ice crystals and form at very high altitudes in the atmosphere. Cirrus clouds 2. Stratus clouds are clouds that form in layers and cover large areas of the sky. They can bring continuous rainstorms. Stratus clouds 3. Cumulus clouds are large, white, puffy clouds with significant vertical development. They are typical of fair weather. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Cu mulus clouds 4. Cumulonimbus clouds are extremely large cumulus clouds that contain significant amounts of energy. They are responsible for thunderstorms and other extreme weather. They form when the atmosphere has a lot of moisture in a mass of warm, unstable air and there is an energy source that pushes the air mass rapidly upward. The tops of cumulonimbus clouds can reach to over 50,000 feet into the atmosphere. 5. Types 1-3 occur at altitudes under 6,000 feet. When they occur at higher altitudes, between 6,000 and 20,000 feet, cloud types are identified by adding the prefix alto-. Therefore meteorologists refer to altocumulus clouds and altostratus clouds. 6. Cirrocumulus clouds are broken, cotton-ball like clouds that occur above 18,000 feet in the atmosphere. The look created by cirrocumulus clouds is often referred to as mackerel sky. Fog is a cloud formation. Fog is simply layers of stratus clouds that have formed very close to the ground. Lesson Checkpoint: Name the types of clouds. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Precipitation Precipitation is water that has condensed in the atmosphere and falls to Earth. Sleet, snow, rain and hail are different forms of precipitation. The type that falls at a given time depends on temperature. • Snow is crystallized water. Water crystallizes in the hexagonal crystal system. Minerals in the hexagonal system have six sides and snowflakes always have six-fold symmetry. • Sleet generally refers to precipitation that is a mixture of water and ice. In some cases rain has only partially frozen. In others snow has partially melted as it falls to the ground. • Hail is formed in cumulonimbus clouds associated with thunderstorms and severe weather. Water droplets are pushed upward by the energy in the storm clouds. The water droplets freeze and begin to fall but the currents within the cloud push the frozen drops back up into the cloud where another layer of water freezes onto the first frozen droplet. This process can continue until balls of ice the size of baseballs are formed, although typical hail is pea-sized to marble-sized. Hail eventually falls to the ground when the weight of the hail is greater than the energy in the cloud that holds it aloft. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.