Understanding weather Chapter 7 Notes

Water in the Air Weather is the condition of the atmosphere at a certain time and place. The condition is affected by the amount of water in the air.

The Water Cycle Water is constantly being recycled through the water cycle.

Humidity As water evaporates from lakes oceans and plants it becomes water vapor. The amount of water vapor in the air is called humidity. As the temperature increases, the amount of water vapor the air can hold increases.

Relative Humidity Relative Humidity is the amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temperature

Factors that affect Relative Humidity Amount of Water Vapor Temperature The more water vapor in the air the higher the relative humidity. The water vapor drops the relative humidity drops. The relative humidity decreases as the temperature rises and increases as the temperature drops

Measuring Relative Humidity A psychrometer is an instrument that is used to measure relative humidity. It consists of two thermometers: one wet bulb thermometer and one dry bulb thermometer. The difference in the temperature readings between the two thermometers indicates the amount of humidity.

Measuring Relative Humidity

Condensation Condensation: the process by which a gas, such as water vapor, becomes a liquid. Condensation occurs when saturated air (air full of humidity) cools.

Dew Point Dew point: the temperature at which a gas condenses into a liquid. Air is saturated at dew point

Clouds A cloud is a collection of millions of tiny water droplets or ice crystals. Clouds form as warm air rises and cools

Clouds

Cumulus Clouds Puffy, white clouds that tend to have flat bottoms Form when warm air rises Indicate fair weather When they get larger they produce thunderstorms – cumulonimbus clouds Clouds with nimbus or nimbo will produce precipitation

Stratus Clouds Stratus Clouds form in layers Cover large areas and often block out the sun Caused by a gentle lifting of a large body of air. Nimbostratus clouds are dark stratus clouds that produce continuous rain. Fog is a stratus cloud found near the ground.

Cirrus Clouds Cirrus Clouds are thin feathery white clouds found at high altitudes Cirrus clouds form when the wind is strong They can indicate a change in weather.

Clouds and Altitude Clouds are classified by the altitude that they form

Cloud Classification Low clouds – have no prefix Ex. Cumulus, Stratus Middle clouds – Have the prefix alto Ex. Altocumulus, Altostratus High clouds – Have the prefix Cirro Ex. Cirrostratus, Cirrus

Precipitation Precipitation is water in solid or liquid form that falls from the air to the Earth There are four major forms Rain Snow Sleet Hail

Rain The most common form of precipitation. Rain is produced when the water droplets become a certain size.

Sleet and Snow Sleet forms when rain falls through a layer of freezing air. Snow forms when temperatures are so cold that water vapor changes directly to a solid.

Hail Hail are balls or lumps of ice that fall from clouds. Hail forms in cumulonimbus clouds due to updrafts freezing the water droplets.

Air Masses and Fronts Changes in weather are caused by the movement and interaction of air masses. An air mass is a large body of air where temperature and moisture content are similar throughout .

Air Masses Air Masses are characterized by their moisture content and temperature which is determined by the area over which the air mass forms (Source Region)

Types of air masses Maritime (m) Forms over water; wet Continental ( c ) Forms over land; dry Polar ( P) Forms over polar regions; cold Tropical (T) develops over the tropics: warm

Cold Air Masses Cold air masses are responsible for bringing extremely cold winters.

Warm Air Masses Warm air masses bring warm air weather systems into the United States. This brings milder temperatures This can also bring severe weather during the summer months.

Fronts Front: the boundary between air masses of different densities and usually different temperatures Four kinds of fronts Cold front Warm front Occluded front Stationary front

Cold Fronts A cold front forms where cold air moves under warm air Move quickly and bring thunderstorms, heavy rain, or snow Cooler weather follows a cold front

Warm Fronts A warm front forms where warm air moves over colder denser air Warm air replaces cold air Bring drizzly rain Followed by clear and warm weather

Occluded Front An occluded front forms when a warm air mass is caught between two colder air masses This produces cold temperatures and large amounts of rain and snow.

Stationary Fronts Stationary fronts form when a cold air mass meets a warm air mass Brings many days of cloudy wet weather.

Air Pressure and Weather Areas that have lower pressure than the surrounding areas do are called Cyclones. These areas the air masses come together and rise

Air Pressure and Weather Areas that have high pressure are called anticyclones. Anticyclones are areas where the air moves apart and sinks

Air Pressure and Weather By keeping track of the cyclones and anticyclones, meteorologists can predict the weather. Cyclones cause stormy weather Anticyclones bring dry clear weather

Severe Weather Thunderstorms Tornadoes Hurricanes Severe weather safety

Thunderstorms Thunderstorms are small intense weather systems that produce strong winds, heavy rain, lightning, and thunder. Two atmospheric conditions are needed to produce a thunderstorm: Warm and moist air near the Earth’s surface Unstable atmosphere

Thunderstorms Cumulonimbus Clouds

Lightning Lightning is an electric discharge that occurs between a positively charged area and a negatively charged area. Can occur Between two clouds Between Earth and a cloud Between two parts of the same cloud

Lightning When lightning strikes, energy is released. Energy is transferred to the air. Thunder is the sound that results from the rapid expansion of air along the lightning strikes.

Severe Thunderstorms Can produce: High winds Hail Flash floods Tornadoes Lightning that causes forest fires and burning down homes

Tornadoes Tornadoes occur in 1% of all thunderstorms. A tornado is a small spinning column of air that has high wind speeds and low central pressure and that touches the ground. It starts as a funnel cloud that pokes through a cumulonimbus cloud.

Tornadoes

Tornadoes 75% of the world’s tornadoes occur in the United States. Occur in spring and early summer They average 120-180 km/h wind speed

Tornado Pictures

Tornado Pictures

Tornado Pictures

Hurricanes Hurricanes are large rotating tropical weather systems that have wind speed of at least 120km/h They are the most powerful storms on Earth. Pacific Ocean hurricanes are called: typhoons Indian Ocean hurricanes are called: cyclones

Hurricane Hurricanes happen over warm tropical oceans. Hurricanes vary in size from 160 to 1500 km in diameter and can travel for thousands of kilometers.

How Hurricanes form Begins as a group of thunderstorms moving over tropical ocean waters Winds traveling in different directions cause the storm to spin It gets its fuel from the contact with the warm ocean waters The hurricane continues to grow as long as it is over the moist warm source.

Hurricane

Hurricane Damage Very destructive Average wind speed is 120-150 km/h Can knock down trees and destroy buildings Flooding is the most destructive part.( storm surge).

Weather Safety Thunderstorm Safety Tornado Safety Lightning is attracted to tall objects Crouch down in open areas Stay away from water Weather forecast: watch and warning system Watch- tornado may happen Warning- tornado has been spotted Find shelter: basement or cellar

Weather Safety

Weather Safety Flood Safety Hurricane Safety Weather forecast: watches and warning system Find a high place Always stay out of flood waters Watch weather updates Evacuate the area Have a disaster supply kit available with food and water Board up windows with plywood Stay indoors during the storm

Weather Safety

Forecasting the Weather A weather forecast is a prediction of weather conditions over the next 3 to 5 days A meteorologist is a person who observes and collects data on atmospheric conditions to make weather predictions

Weather-Forecasting Technology High in the sky Weather balloons Measure weather conditions as high as 30 km above Earth Measures temperature, air pressure, and relative humidity Meteorologists can also measure wind speed and direction by tracking the weather balloons.

Weather-Forecasting Technology

Weather-Forecasting Technology Measuring Air Temperature and Pressure Thermometer: a tool used to measure air temperature Barometer: an instrument used to measure air pressure The mercury inside a barometer rises as the air pressure increases

Weather-Forecasting Technology Measuring Wind Direction Can be measured by using a windsock or wind vane Windsock: is a cone shaped cloth bag open at both ends Wind vane: is shaped like an arrow with a large tail and is attached to a pole

Weather-Forecasting Technology Measuring wind speed Anemometer: an instrument used to measure wind speed Consists of four cups connected by spokes to a pole The cups move The motion sends an electric current that measures the wind speed

Weather-Forecasting Technology

Weather-Forecasting Technology Radar and Satellites Radar is used to find the location movement and amount of precipitation Doppler Radar systems on the weather station Weather Satellites provide the Earth images of weather systems you see on TV weather reports

Weather-Forecasting Technology

Weather Maps National Weather Service (NWS) and National Oceanic and Atmospheric Administration (NOAA) use information from about 1000 weather stations in the US This is information from one weather station.

Reading Weather Maps