Meteorology: The Basics

ObjectiveStudents will understand the difference between weather and climate.Students will comprehend the different types of air masses and fronts that influence national and international weatherStudents will know how to use clouds to predict changing weather patterns.Students will learn how to interpret weather mapsStudents will examine how weather data is collected, transferred, and used to make forecasts

Weather Versus Climate

Meteorology: The study of the atmosphere.

Weather: The current state of the atmosphere in a given geographic area at a given time.

Climate: Long variations in weather for a particular area.Weather involves describing / predicting atmospheric conditions over a short period of time, whereas, climate involves the conditions over longer periods of time.

What is an air mass? IAir Mass: A large body of air that takes on the characteristics of the area which it is over.Air masses originate in

a source regionAir masses have unique:(1) Temperatures(2) Moisture contents(3) Air pressuresThere are five major

types of air masses

What is an air mass? II

What is an air mass? III

World wide source regions for air masses:

What is an air mass? IV

When air masses move from source regions into new areas, the following things happen:(1) The region where the air mass has moved undergoes a major change in temperature and humidity(2) The Air mass becomes more moderate

What are Fronts? I

Fronts are boundaries that separate air masses with differing temperature and other characteristics

Fronts are observable on surface weather maps and have symbols that represent the four different types

What are Fronts? II

Cold front: Occurs when a wedge of cold air advances toward warmer airWarm front: Represents the boundary of warm air moving toward cold airStationary front: Occurs when a cold front remains stationaryOccluded front: Occurs along the boundary between two polar air masses with slightly different temperatures.

What are Fronts? IIIOn surface weather maps, the four types of fronts are denoted by the following symbols:

Cross Section of Fronts I

Cold front cross section:

Cross Section of Fronts II

Warm front cross section:

Cross Section of Fronts III

Stationary front cross section:

Cross Section of Fronts IVOccluded front cross section:

Cross Section of Fronts V

Cold air is generally denser than warm airTherefore, when two air masses interact, the colder air usually wedges under the warmer airThe warmer air mass generally rises

Fronts and Clouds/Precipitation IFigure: Weather conditions associated with cross section A-B on Figure 10. Warm air (mT) lies between the cold front and warm front. The cold front advances more rapidly than the warm front, forcing warm air to rise, forming thunderclouds and heavy rains. Warm air is forced to rise above the more gently sloping warm front, resulting in the formation of a series of low to high clouds.

Fronts and Clouds/Precipitation IIFigure: Nimbostratus clouds generate precipitation along an occluded front.

Fronts and Clouds/Precipitation III

Conditions Before Warm Front Between Warm/Cold fronts After Cold Front

Pressure Decreasing Small decrease, then small increase

Increasing

Winds South, southeast Southwest West, northwest

Temperature Cool Warm ColdClouds Cirrus, cirrostratus,

altostratus, nimbostratus

Cumulus, cumulonimbus

Cumulus, altostratus

Precipitation Light-moderate, increasing

None, then heavy rains prior to cold front

Moderate-light, decreasing

Weather Conditions Associated with a Passing Frontal System

Using Surface Weather Maps ISurface data is reported hourly from places like airports and automated observing platforms. The types of plots using surface data include a composite surface map, weather depiction for aviation, regional plots for the US, Canada, Mexico and Alaska. Contoured analyses of various parameters including temperature and pressure. Finally, there are meteograms which are time cross-sections for individual cities. These data are updated hourly at around 30 minutes past the hour.

Using Surface Weather Maps IIA

Simple surfaceweather mapfrom Accuweather.This map showsfronts, relative pressure, and weather conditions

Using Surface Weather Maps IIBThe black lines are called isobars, which are lines of equal pressure (using the pressure unit called millibars)The winds are highest

where the isobars are closest together

The winds are lowestwhere isobars arefurthest apart

The contours show what is known as a “pressure gradient”

Using Surface Weather Maps IIISurface Observation Weather SymbolsSurface weather observation maps display current conditions such as wind speed, wind direction, cloud cover, dew point temperature, and pressure for a number of sites

Using Surface Weather Maps IV

Collecting Weather Data INWS sites, airports, and other locations have weather stationsAirplanes and ships collect weather data and report it to central locationsThere are weather stations located on buoys in ocean basinsThe National Weather Service (NWS) and other government / international organizations collect weather data in the atmosphere via weather balloons that are sent into the atmosphere Volunteer weather observers also report conditions

Collecting Weather Data IINOAA’s NWS and other organizations launch weather balloons which make an ascent into the Earth’s upper atmosphere (35 km up) to collect data about temperature, pressure, wind speed/direction, and other conditions Weather balloons carry a device called a radiosonde, which collects weather information and transmits it back to EarthThere are over 900 balloon launch sites, most of which are located in the Northern Hemisphere

Collecting Weather Data IIIWeather balloons rise until they burstThe weather balloon falls to the surface of the Earth and a parachute helps to slow it downAbout 20% of the radiosondes used in weather balloons are recovered by the NWS to be refurbished for future launchesWatch weather balloon videos

Developing a Forecast IThere are several methods used to develop a forecast:(1) Climatologic Forecast: what have the conditions been like on average over a long period of time?(2) Analog Forecast: A weather forecast that compares current weather conditions to those that have occurred in the past.(3) Persistence Forecast: A weather forecast that assumes the conditions at the time of the forecast will not change (if it was 90 degrees yesterday, it will be about 90 degrees tomorrow)

Developing a Forecast II(4) Numerical Weather Prediction (NWP) Forecast: uses the power of computers to make a forecast. Complex computer programs, also known as forecast models, run on supercomputers and provide predictions on many atmospheric variables such as temperature, pressure, wind, and rainfall. A forecaster examines how the features predicted by the computer will interact to produce the day's weather. Despite these flaws, the NWP method is probably the best of the five discussed here at forecasting the day-to-day weather changes

Weather Satellites

Meteorologists find it useful to look at weather on a very large scale. Because geostationary weather satellites can only make images of relatively small portions of the earth's surface, several images were combined to form this picture of North and South America.

Satellite Imagery Links

Other Important Terms I

Jet Streams: Narrow bands of fast, high-altitude, westerly winds. Moves at around 115 m.p.h.Doppler Effect: A change in wave frequency occurs as an object moves closer or farther from the source point.Station Model: A record of weather for a certain place at a certain time.

Other Important Terms IITrade Winds: Air sinks from high altitudes and moves towards the equator in a westerly manner. When the air is warm enough it will rise and move northward.Prevailing Westerlies: Wind progress towards the poles in an easterly manner. Once cooled the air mass returns south again. This is the major driving force of weather in the U.S.Air Mass Modification: The exchange of heat or moisture with the surface over which the air mass travels.Coriolis Effect: Particles in the northern hemisphere move to the right or clockwise. Particles in the southern hemisphere rotate to the left or counter clockwise. This is a result of the rotation of the Earth on its axis.

The intertropical convergence zone (ITCZ) is a dynamic weather area located near the equator. Sunlight at the

equator warms the air, making it rise and thereby creating low pressure at the equator, along with rain-producing

cumulonimbus clouds. The rising air eventually cools and sinks back to the surface at latitudes roughly 30° north

and south of the equator. The sinking cool air creates high-pressure systems that keep those latitudes relatively dry.

The sinking air is drawn back to the low pressure near the equator, creating a flow of air known as the trade winds.

Eventually, the trade winds converge near the equator and create the ITCZ.

The prevailing winds include the doldrums near the equator, the northeast (or southeast) trade winds somewhat above (or below) the

equator, the prevailing westerlies near the middle latitudes, and the polar easterlies near the poles. The winds are named with respect to the

direction from which they blow. For example, the northeast trade wind blows in the northern hemisphere from the east toward the west.

A rainbow forms when raindrops separate white sunlight into a spectrum. As sunlight passes through a drop of water, it is first bent and then reflected from the back surface of the drop toward the viewer’s eye. The amount of bending, known as refraction, differs for light of different colors. Red light bends the

least and violet light bends the most. Here, a primary rainbow arcs through the sky after a rainstorm. Primary rainbows have red on the outside arc; the colors

of secondary bows are reversed .

References IUnderstanding Weather and Climate,

Aguado and Burt, Prentice Hall

Scuba Diving NJ and NY Web Site, http://njscuba.net/index.html

West Virginia State University Weather Page, http://fozzy.wvstateu.edu/~weatherlink/http://fozzy.wvstateu.edu/~weatherlink/

Weather Systems, http://www.mhhe.com/earthsci/geology/mcconnell/wea/front.htm

References IIUnisys Weather, http://weather.unisys.com/

University of Illinois WW2010 Web Site, http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/home.rxml

NOAA Radisonde Web Site, http://www.erh.noaa.gov/gyx/weather_balloons.htm/

Nature of Severe Weather

At any given time there are more than 2000 storms

occurring around the Earth.

Important TermsThunderstorm: A storm caused by the collision of two different air masses.

Air-Mass Thunderstorm: Most common type of thunderstorm. Occurs because of air rising that has been heated unequally within a single air mass. Usually small and not severe.

Sea-Breeze Thunderstorms: Storms caused by an extreme temperature difference between sea and land.

Frontal Thunderstorms: Usually produced by advancing cold fronts that collide with warm fronts.

Important Terms

Supercells: A severe storm that can be self-sustaining. It is characterized by intense rotating updrafts. Can last for hours and have wind speeds of over 150 m.p.h.

A meteorologist tracks a tornado as part of ongoing weather observations to better understand the earth’s atmosphere. Since the 19th century, scientific forecasting has greatly improved. Weather radar can detect and track tornados, hurricanes, and other severe

storms.

Sidney Tornado, 2001

Doppler radar measures the speed and direction of the movement of clouds, in addition to cloud density. In this image of a thunderstorm over

Oklahoma, Doppler radar shows a mesocyclone, a rotating mass of air that may signal that the formation of a tornado is imminent.

A snow blizzard that struck the East Coast of the United States in January 1996, left the Boston area with about 46 cm (about 18 in) of snow. A snowstorm is called a blizzard

when visibility is less than 0.40 km (0.25 mi) and the snowfall is accompanied by strong winds.

Hurricanes cause most of their damage with strong winds, but they can also spawn tornadoes and cause sudden storm surges, or rises in water level.

Hurricanes are defined as tropical storms that have winds of 119 km/h (74 mph) or more and are given ratings from 1 (minimal) to 5 (catastrophic) based on wind

speed, height of storm surge, and barometric pressure.

Meteorologists use satellite images taken in infrared light to determine the heat of areas of the atmosphere. The data is translated into a visible

image, which is often enhanced, or colored with shades representing temperatures. In enhanced images, red usually represents the hottest temperatures and violet usually represents the coolest temperatures.

Tropical storm Sam hit Christmas Island and the Cocos Islands off the western coast of Australia in January 1990. Astronauts aboard the space shuttle Columbia took this picture of the storm after it had started to lose strength, but the circular motion of the clouds is still

apparent. Tropical storms have wind speeds from 63 to 119 km/h (39 to 74 mph), faster than those of a tropical depression and slower than those of a typhoon.

Most weather services use satellite information in their forecasts. Pictures such as this one of Hurricane Gloria reveal patterns and movement that provide clues about what the storm

might do next. The weather is continuously monitored and photographed by satellites worldwide, but it is still difficult to predict weather because of the many variables involved. Infrared sensors help meteorologists to interpret the pictures by determining the height and

temperature of the clouds.

Questions?