Weather Forecasting: Part 2

Satellite Map Satellites give bird's-eye view of clouds Satellites orbiting high above the earth allow meteorologists to observe clouds at all levels of the atmosphere over both land and the oceans. In addition to taking photos of clouds, satellites use radiometers to measure infrared radiation emitted by clouds. Lower clouds are warmer and emit more radiation than higher clouds, which are much colder. Low clouds appear gray on an infrared satellite picture while high clouds appear white. By closely analyzing satellite images, meteorologists determine cloud heights and thickness.

Satellite Map: Example

Radar Map Radar maps show precipitation and cloud movement. Radar works by sending out a beam of energy then measuring how much of that beam is reflected back and the time needed for the beam to return. Objects that reflect the beam back to the radar include rain, snow, sleet and even insects. If more of the beam is sent back, the object is said to have a high reflectivity and is indicated by a bright color. Objects which return a small part of the beam have a low reflectivity and are indicated by darker colors.

Radar Map continued… Radar images are color-coded to indicate precipitation intensity. The scale below is used on the USATODAY.com images. The light blue color is the lightest precipitation and the purple and white are the heaviest. Sometimes radar images indicate virga, or precipitation that isn't reaching the ground. Reflectivity not only depends on precipitation intensity, but also the type of precipitation. Hail and sleet are made of ice and their surfaces easily reflect radio energy. This can cause light sleet to appear heavy. Snow, on the other hand, can scatter the beam, causing moderate to heavy snow to appear light.

Radar Map: Example

Precipitation Maps Precipitation maps show precipitation Types of Precipitation

Precipitation Maps continued… When cloud particles become too heavy to remain suspended in the air, they fall to the earth as precipitation. Precipitation occurs in a variety of forms; hail, rain, freezing rain, sleet or snow. This portion of the Clouds and Precipitation module focuses on precipitation and has been organized into the following sections. Rain and Hail Atmospheric conditions that lead to the development of rain and hail. Freezing Rain A detailed look at freezing rain, associated dangers and the conditions that lead to its development. Sleet Atmospheric conditions that lead to the development of sleet. Snow Atmospheric conditions that lead to the development of snow.

Precipitation Map: Example

Temperature Maps Temperature maps show current or high/low temperatures A thermometer actually measures the average kinetic energy of the various gas molecules that make up the air around it - let's call them "air molecules." Cold air moves slower. This means that colder air has less kinetic energy than warmer air. When air molecules collide with a thermometer, kinetic energy is transferred from the air molecules to the glass and then to the mercury molecules inside the thermometer. As the mercury molecules begin moving faster they move farther apart, pushing the mercury up in the thermometer. In colder air, the energy from the air molecules colliding with the thermometer transferring to the mercury molecules is less than the energy from warmer air. As a result, the mercury molecules move slower in the colder air and the mercury inside the thermometer does not expand as far up the tube as it does in the warmer air.

Temperature Map: Example

Wind Map Wind maps show wind speed and direction Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated.

Wind Map: Example

Pressure Fronts Front maps show high/low pressure and warm/cold fronts When forecasters say a low pressure area or storm is moving toward your region, this usually means cloudy weather and precipitation are on the way. The centers of all storms are areas of low air pressure. Air rises near low pressure areas. As air rises, it cools and often condenses into clouds and precipitation. Often, you hear a weather forecaster say that an area of high pressure will dominate the weather. This usually means your region has several partly to mostly sunny days in store with little or no precipitation. Air tends to sink near high-pressure centers, which inhibits precipitation and cloud formation. This is why high-pressure systems tend to bring bright, sunny days with calm weather.

Temperature Fronts The term "warm front" sounds like something you'd like to have coming your way on a cold winter's day. Think again. A warm front is the boundary between warm and cool, or cold, air when the warm air is replacing the cold air. Warm fronts often bring days of inclement weather. Warm fronts often form to the east of low pressure centers, where southerly winds push warm air northward. A cold front is a warm-cold air boundary with the colder air replacing the warmer. While a winter cold front can bring frigid air, summer cold fronts often can more accurately be called "dry" fronts. A cold front is the boundary between cool and warm air when the cool air is replacing the warm air. A warm front is the boundary when the warm air is winning the battle. When the pushing is a standoff, the boundary is known as a stationary front. Stationary fronts often bring several days of cloudy, wet weather that can last a week or more.

Front Map: Example