Weather

Heating the Earth

• Weather is the daily conditions of the atmosphere

• The factors that interact to cause weather are heat energy, air pressure, winds, and moisture

Moisture in the Air• Water vapor or moisture in the air is called

humidity• Humidity can vary from place to place and

time to time• Latent heat is the amount of energy that is

absorbed or released during a phase change.

Absolute humidity• The mass of humidity contained in a

certain amount of air absolute humidity = mass of water vapor (g)

volume of air m³

Relative Humidity• The actual amount of water vapor in the air relative

to the amount of water vapor needed to reach saturation

relative hum = amt. of H2O vapor in air X 100 amt. of H2O vapor to saturationIf the temperature does not change and more

moisture enters the air, RH goes up. If the temperature decreases RH goes up. If temperature goes down RH decreases.

Dew Point• When the air temperature and the dew point is

the same 100% humidity occurs. This causes dew on cool objects. When the temperature is below freezing frost occurs.

Measuring Relative Humidity• Psychrometer has two thermometers, one dry

and one wet. The difference in the temperature of the two will determine

the humidity using the table.

Water Drop Formation• Water needs to condense on a condensation

nuclei

Clouds: collection of water drops

Stratus

• Low altitude sheet like clouds may produce rain (nimbostratus)

Cumulus

• Fair weather clouds, can form cumulonimbus if troposphere is unstable

Cirrus

• Ice crystals high in atmosphere, usually rain within 24 hours

Fog: Surface Clouds

• Radiation fog forms with nightly cooling

• Advection fog forms when warm, moist air moves across a cold surface

Some Unusual Clouds

http://www.webdesignerdepot.com/2009/10/the-weirdest-clouds-that-youll-ever-see/

Precipitation• When water vapor in the clouds becomes too heavy, it can fall as

rain, sleet, snow, or hail• Cloud droplets must first increase in size until gravity pulls them

down• If they pass through extremely cold layers of air, they can freeze and

produce sleet• Snow is formed when water vapor changes directly into a solid• Hail is formed when water droplets hit ice pellets

Causes of Precipitation

• Coalescence is the formation of a large droplet by the combination of smaller droplets

Measuring Rainfall• A rain gauge is used to measure precipitation• Snow usually is measured as 10” of snow = 1” of rain

Doppler Radar

• The intensity of precipitation can be measured using Doppler Radar which is the bouncing of radio waves off rain or snow.

Weather Patterns: Air Masses

• Changes in the weather are caused by very large areas of air with specific temperature and humidity called air masses

• There are four main air masses that affect the weather in the United States

• They are named for the regions they originate in

Maritime masses• Maritime tropical air mass originates near the

equator over the ocean, it brings warm moist air onto the continent

• Maritime polar air mass forms over the Pacific Ocean in both winter and summer, it forms over the North Atlantic and brings cold winter weather in the winter and cooler temperatures during the summer to the east coast

Continental masses

• The continental tropical forms over land in the Southwestern US it brings dry hot air

• The continental polar mass forms over the land in Canada and brings very cold dry air into the US

Fronts• Fronts form when large air masses meet

that have different temperatures and humidity

• The weather at a front is can be very unstable and stormy

• There are four different types of fronts: cold front, warm front, occluded front, and stationary front

Cold Front• A cold front forms when a mass of cold

air pushes over a mass of warm air• The cold air pushes its way below the

warm air• Violent storms, even tornadoes can be

associated with these front• Fair, cooler, and drier air usually follows

Cold Front

Warm Front• A warm front forms when a mass of

warm air overrides a mass of cooler air• Rain showers can accompany a

warm front• Hot and humid weather follows

Occluded Front• When a very fast moving

cold air mass overtakes a slower moving warmer air mass, an occluded front forms

• It produces less severe weather than a warm or cold front can

Stationary Front• A stationary front is formed when a cold air

mass and a warm air mass meet and do not move

• This produces several days of rain

High and Low Pressure Systems

High PressureSome high pressure systems are

associated with cold air systems that move in and change the weather

Subtropical high pressure systems are slow moving, usually bring very warm conditions

Low Pressure Systems• Low pressure systems are associated

with precipitation and a change to the weather for an extended time

• Low pressure systems can bring rain or snow

• Low pressure systems can bring a sudden dramatic drop in temperatures

Measuring Air Pressure

• A barometer measures the pressure of the air

Winds• Both local winds and global winds are caused by

differences in air pressure due to unequal heating of the atmosphere

• Isobars that are closer together indicate stronger winds

• Winds travel from areas of high pressure to areas of low pressure

Measuring Wind

• Wind is measured by an anemometer for speed and a weather vane for direction

Storms

• A storm is a violent disturbance in the atmosphere• It is marked by a sudden drop in

pressure• Some may cover a very large area

Rainstorms and Snowstorms

• Caused when two different fronts collide • When a warm front overrides a cold

front• Rain or snow occurs• During the winter, if the rain freezes it

can cause an ice storm

Thunderstorms• Occurs when a cold front

meets a warm front• Thunder and lightning form

within the clouds• Dangerous down drafts and

wind shears are often present

Lightning• Occurs because areas

of positive and negative charges build up inside the storm, then a sudden discharge between two clouds or the clouds and the ground

Thunderstorm Safety

Cyclones and Anticyclones

• Areas of low pressure that contain rising warm air are cyclones

• Winds spin in a counterclockwise inward movement with rain and storms

• Areas of high pressure, cool and dry winds are anticyclones

• Winds spin in a clockwise movement from the center outward

Hurricanes• Powerful cyclone• Warm moist air begins to rise rapidly• Cooler air moves in and the air begins to spin• The pressure in the center drops and more air

rushes in causing the winds to spin faster• Air forms a doughnut shape wall of strong winds

with a calm center• These storms are accompanied with strong winds

and a high storm surge• These are the most powerful storms on Earth

Tornadoes• Whirling funnel shaped cloud• Develops in heavy cumulonimbus clouds• Air pressure at the bottom is low so the funnel

acts like a vacuum cleaning• Occur in the spring and summer in the area

called Tornado Alley

Forecasting the Weather

• Just think the end is near!!!!

Predicting the WeatherMeteorologist is a person who

studies and predicts the weatherThey interpret weather

information from local weather observers, balloons, satellites, and weather stations around the world

Weather MapsData from stations around the country is

gathered and put on weather mapsThis includes temperature, air pressure,

precipitation, and the wind speed and direction

Information about cloud cover, air masses, and fronts may be included

The information is recorded by a number of symbols and numbers

Recording Weather Data

Isotherms and Isobars• Isotherms

connect points of equal temperatures

• Isobars connect points of equal pressure

Symbols

Controlling the Weather

• At the present time, weather control is limited to seeding of clouds

• This is done by placing dry ice or silver iodine crystals into the super cooled layers of the stratosphere

• These cause large ice crystals to form that eventually fall as rain

I know how you feel!!!!I know how you feel!!!!