Weather Patterns & Forecasting

 Weather refers to the conditions of the atmosphere at a certain place and time.  Weather is often associated with pressure systems  Air pressure is the weight of the molecules in the air Pressure Systems

 High-pressure system- a large body of circulating air that has high pressure at it’s center and lower pressure on the outside.  Occurs when air masses move apart.  High-pressure air at the center sinks and moves toward low pressure areas.  Brings clear skies and “nice” weather  Winds rotate clockwise around high pressure  Symbol: High Pressure Systems

 Low pressure system- a large body of circulating air that has low pressure at it’s center and higher pressure on the outside.  Occurs when air masses come together.  Air in the center of the system rises, and the water vapor in it condenses and forms clouds.  Bring cloudy skies and showers  Winds rotate counter-clockwise around low pressure  Symbol: Low Pressure Systems

 Air masses- large bodies of air that have distinct temperature and moisture characteristics.  An air mass forms when a high pressure system lingers in one area for a few days.  Changes in weather are caused by the movement and interaction of air masses. Air Masses

 4 kinds of Air masses:  Maritime (m)- originates over a body of water  Continental (c) -originates over land  Polar (P) -originates over a cold area  Tropical (T) -originates over a warm area  The characteristics of an air mass are determined by where it is formed. Air Masses

Air Mass Combinations  cP = continental polar = cool and dry  cT = continental tropical = warm and dry  mP = maritime polar = cool and wet  mT = maritime tropical = warm and wet

 A weather front is the boundary between two air masses.  They occur where air masses come together  4 kinds of weather fronts:  Cold front  Warm front  Stationary front  Occluded front Weather Fronts

 Cold air mass meets a warm air mass  Pushes the warm air up, lifting the clouds  Results in heavy rains or thunderstorms and cooler temperatures  Symbol: Cold Fronts

 Warm air mass meets a cold air mass  Warm air rises above cold air and gradually replaces it  Results in clear weather and warmer temperatures  Symbol: Warm Fronts

Cold & Warm Fronts Cold FrontWarm Front

 Cold air mass meets a warm air mass, but there is not enough force for the warm air to be lifted  Results in days of cloudiness and constant rain  Symbol: Stationary Fronts

 Warm air mass gets caught above two cold air masses that converge  Results in large amounts of rain and cooler temperatures  Symbol: Occluded Fronts

Stationary & Occluded Fronts Stationary Front Occluded Front

 Meteorology – the study of weather  Meteorologists( scientists who predict weather) measure weather variables before making a forecast.  A surface report describes a set of these weather measurements made on Earth’s surface.  These measurements include:  Temperature  Air pressure  Humidity  Precipitation  Wind speed and direction Weather Forecasting

 Temperature is the measure of the average kinetic energy of molecules in the air.  Molecules in warm move faster than molecules in cooler air.  Thermometers can measure heat by expanding and contracting metal coils (left) or liquids (right) Temperature

 Air pressure is the measure of the force exerted by air molecules as they push down on objects—including you—with a force that we call the “barometric pressure.”  Air pressure decreases as altitude increases.  Metal coils in barometers, as in thermometers, respond to differences in air pressure by expanding or contracting, causing a dial to move or change the LED display Air Pressure

 Humidity is the amount of water vapor in the air  When air is saturated, it holds the maximum amount of water vapor possible at that temperature.  Relative Humidity (RH) is the amount of water vapor in the air compared to the maximum amount of water vapor the air can hold at that temperature.  Relative humidity is reported as a percent  Ex: at 100% humidity, the air is saturated Humidity

 Sling Psychrometers and Hygrometers are used to measure relative humidity. Measuring RH

 When air near the ground becomes saturated, the water vapor condenses into a liquid and forms dew.  It the temperature is below 0 C, ice crystals referred to as frost form.  The temperature at which saturation occurs is called the DEW POINT. Dew Point

 Rain is easy to measure—all you need is an “open tube with a ruler”—a RAIN GAUGE.  “Tipping bucket” rain gauges can measure rainfall outside and connect to show a display inside. Measuring Precipitation

Measuring Snowfall-  Measuring how much snow falls is trickier than measuring rain, because wind can carry it away or pile it up in drifts.  Meter sticks or rulers in carefully selected locations are used to record how deep the snow lies.  Meteorologists may also melt snow to find its “water equivalent”—what it would have been had it fallen as rain. Measuring Precipitation

 When air moves—”wind”—we feel both its direction and speed (force.)  Wind direction is measured with a “vane”—an arrow or other shape that points into the wind.  Speed is measured with an anemometer.  The faster the cups spin, the faster the wind. Wind Direction and Speed

 An upper –air report describes wind, temperature, and humidity conditions above Earth’s surface.  Weather balloons with an attached instrument package called a radiosonde are released into the atmosphere to measure weather conditions above the surface  Radiosondes contain:  Temperature sensor  Relative humidity sensor  Barometer  Radio transmitter  Battery  Parachute  String to attach to balloon Weather Conditions above the Surface

 Weather satellites provide even larger views of weather systems  They provide weather information by measuring the radiation given off by Earth and by taking photographs.  Infrared Satellite images provide information about cloud temperature & height  Doppler radar- a special form of radar that can be used to detect precipitation and approximate wind speed. Weather Satellites & Radar

This color-enhanced image shows a storm passing Des Moines, Iowa. Heavier precipitation is shown by yellows and greens.

Here is a satellite image for the same time. ”X” shows the area where the storm in the radar image was located X

 A station model displays many weather measurements for a specific location  It appears on weather maps.  The next set of slides will explain how to read one. Weather Maps

 Air temperature is shown in Fahrenheit degrees by the number to the upper left of the circle. Temperature 75

 Pressure—measured in mb (millibars)—is given by the number to the upper right of the circle  For example, suppose it’s mb  Only the 10s/1s/0.1s are shown—so it is represented as 194  Pressures below 1000 mb would start with high numbers, such as 964 for mb Pressure 194

 The dew point temperature in Fahrenheit degrees is shown to the lower left of the circle. Dew Point Temperature 57

 Wind direction is show by an “arrow” going into the circle. The example shows wind blowing from the lower right, so this is a southeast wind.  Each long “feather” represents 10 mph and each short “feather 5, so the wind speed is 25 mph. Wind Direction and Speed

 The amount of the circle that is filled in indicates how much of the sky is covered by clouds.  It may range from “clear” (left) to “overcast” (right). Cloud Cover

no clouds about 10% about 25% about 40% about 50% about 60% about 75% about 80% completely overcast sky obscured

 When rain, snow, or other forms of precipitation, lightning, and special weather conditions exist, these are shown by symbols to the left of the circle between air temperature and dew point temperature Present Weather thunderstorm

Samples of present weather symbols. fog haze thunderstorm, drizzle lightning rain * snow slight rain showers. ice or snow pellets

Try to interpret this station model 1. Temperature: 45 o F 2. Pressure: mb 3. Dew Point: 29 o F 4. Clouds: overcast 5. Wind: from southeast at 15 mph 6. Precipitation: light rain

Weather maps have isobars which are symbols made up of lines that connect places that have equal air pressure.

Isotherms – are lines that connect places that have the same temperature.

Fronts- are represented as lines with symbols on them.

 Modern weather forecasts are made with the help of computers.  Computer Models are detailed computer programs that solve a set of complex mathematical formulas. The formulas predict temperature, winds, precipitation, and types of clouds. Predicting The Weather