national weather service hazardous weather and flooding preparedness weather overview

National Weather Service

Hazardous Weather and Flooding Preparedness

Weather Overview


Elements of Weather

Temperature

Pressure

Moisture

Wind


TemperatureTemperature

Represents heat energy or the speed of motion of molecules.

Absorption of heat and temperature change vary by substance. The sun heats land more quickly than water.

The sun supplies most of the energy that keeps our earth warm.

The atmosphere is transparent to the sun’s heat. Most of the sun’s energy passes through the air and heats the land and water.


Temperature (Continued)Temperature (Continued)

Most of the heat supplied to the air comes from the earth radiating heat, from heat released by water, and from air movement.

The amount of solar energy striking the earth depends on the earth’s rotation and orbit.

Molecules will move to equalize temperatures where possible.


Temperature (Continued)Temperature (Continued)

Why is it colder at thenorth and south poles?

The sun must pass through a larger section of the atmosphere to warm the earth surface.

The weather on our planet is created by sun heating areas of the planet unequally. This occurs because:1. The sun's radiation reaches only

half the planet at any one time. 2. The amount of radiation reaching

the surface varies at different places.

3. The planet tilts as it revolves around the sun.


SeasonsSeasons

The earth revolves around the sun every 365 days.

Different parts of the planet tilt toward the sun and receive more radiation.

June 21 (the summer solstice), the North Pole is tilted toward the sun and has continuous day.

December 21, the North Pole is tilted away from the sun.

In April, the planet has relatively little tilt, meaning that day and night are about equal length in most places.


SeasonsSeasons

Why is it warmer in Why is it warmer in July then January?July then January?

The earth is actually closerto the sun in January (3%)

It is the tilt of the earth!!!!!!!!!!!!!The earth is tilted toward the sun in the Summer (North Hemisphere). This is a larger factor then proximity to sun.


DAY - Landwarms fasterthan water

NIGHT - Landcools fasterthan water

The varioussurfaces absorb,retain, andradiate heat anddetermine thedaily surface airtemperataure to4000 ft.

Practically nodaily temperaturevariance occurs inthe free air above4000 ft.

SOLARRADIATION

Daily Temperature Variations


Hadley CellsHadley Cells


Hadley CellHadley Cell


Climate RegionsClimate Regions


PressurePressure

A measure of the weight of the atmosphere on top of us.

Pressure decreases with height. As you go higher in the atmosphere, there are fewer molecules to exert pressure.

Molecules in the atmosphere move to equalize pressure, moving from high pressure to low pressure areas.

The flow of molecules from areas of high pressure to low pressure produces wind.


Pressure (Continued)Pressure (Continued)

Low pressure is usually associated with rising (warm) air and stormy weather.

Air moves counter clockwise around Lows (Coriolis Effect due to earth’s rotation).


Pressure (Continued)Pressure (Continued)

High pressure is usually associated with sinking (cool) air and nice weather.

Air moves clockwise around Highs (Coriolis Effect).

Pressure lines on weather maps are called isobars.


Some Definitions

Isobars – lines on a weather map which connect points of equal pressure

High Pressure – areas of sinking air

Low Pressure – areas of rising air


Surface Pressure MapSurface Pressure Map

HIGHHIGH

HIGHHIGH

HIGHHIGH

LOWLOW

LOWLOW

10241024

10241024

10241024

10241024

10201020

10201020

10201020

10161016

10161016

10161016

10161016

10121012

10081008

10081008

10041004

10121012

ISOBARISOBAR

What is the wind direction over Ohio?


Elements of Weather

Moisture

Dew Point – Temperature to which air must be cooled to be saturated

Used by Meteorologists – gives a more accurate picture of amount of moisture in atmosphere

Relative Humidity – Percentage of water vapor in the air


GreatLakes

Atlantic

GulfInflow

Pacific

How do you thinkthe Great Lakesimpact weather inwarm conditions?

Moisture Source Regions


ContinentalPolar

MaritimePolar

ContinentalTropical

MaritimeTropical

MaritimePolar

MaritimeTropical

Air Masses


WindWind

Wind is the horizontal flow of air caused by differences in Temperature and Pressure.

Winds tend to flow around areas of high and low pressure rather than directly from high to low pressure.

Winds tend to flow parallel to isobars. This is caused by the earth’s rotation which turns winds to the right (Coriolis Effect).

The more isobars, the stronger the wind.


Wind (Continued)Wind (Continued)

Winds are a primary contributor to severe weather. Why?

They move moisture, as well as warm and cool air.

They provide shear for tornadoes to form.

Strong winds not only increase the strength of thunderstorms, but contribute to damage from downbursts.


What’s Happening?What’s Happening?

Temperature PressureVisibility/Weather Pressure Change Dewpoint Wind Precipitation

Blizzard of ‘78 Jan 26, 1978


Ohio Surface MapOhio Surface Map


Basic Surface Station ModelBasic Surface Station Model

Temperature (F)

Dewpoint (F)

• Simple Station Model•Common on Many Maps•Major Weather Elements Covered

•Temperature and Dew point•Left Hand Side of Model•Temperatures in Fahrenheit



Basic Weather Symbols



Interpreting Pressure Reports: (Millibars)

If reported value greater than 500: Initial 9 is missing. Place it on left, then divide by 10. For example: 827 becomes 982.7 mb.

If reported value less than 500: Initial 10 is missing. Place it on left, then divide by 10. For example (as in above diagram): 027 becomes 1002.7 mb



Wind Speed (Knots) and Direction:



Cloud Cover (Percent):



Exercise 1



Exercise 2



Exercise 3


Front Types and CharacteristicsElements of Weather

Cold Front Thunderstorms, severeweather

Stationary Front

Warm FrontSteady precipitation

Potential for long term precipitation ranging from benign to severe weather and flooding


Surface FeaturesSurface Features


How Low Pressure Systems Develop


LOW PRESSURE DEVELOPMENT

In the area of the front, temperature, moisture, pressure and wind are out of balance.


Low Pressure Development (Continued)

The front begins to move as winds change from along the front to across the front.



A frontal wave forms. One portion of the front has now changed to a warm front as winds push warm air northward.

Western parts of the front become a cold front and cold air begins to move southward. Rotation has begun.



Winds are increasing and warmer air is now helping to intensify the system.

Precipitation has probably already begun ahead of the warm front.



The system strengthens. A closed low forms. Winds are even stronger.

Rain falls east of the warm front. Thunderstorms will probably form out ahead of the cold front.



The system has mixed the cold and warm air, into a cool mixture. Most of the energy is now gone.



Finally, the low pressure system has nearly disappeared, and a new stationary front is ready to begin the process again.



Now, lets put it all together and into motion.


Severe Weather OutbreakSevere Weather Outbreak


Jet Streams and Storm

Tracks


Jet StreamJet Stream


Prevailing Westerlies


Jet Streams


How do you think this airflow patterncould affect the weather in the U.S.?

Prevailing Westerlies Affected by the Jet Stream


What kind of weather might youexpect from this airflow pattern?

Shift in the Jet Stream


Jet Streams (Continued)Jet Streams (Continued)

A Winter Storm forms in Oklahoma, who will get heavy snow?

Kansas, Iowa into Wisconsin and Michigan.

What type of weather might be expected across Ohio and PA?

Warm weather, with a chance of rain.


Jet Streams (Continued)Jet Streams (Continued)

Low pressure forms off the South Carolina Coast the 15th of January.

A coastal (Nor’easter) snowstorm?


Where would a low likely form? Where would a high likely form?


Level Height

1000 mb 400 ft 850 mb 4,800 ft 700 mb 9,800 ft

500 mb 18,400 ft 300 mb 30,100 ft 200 mb 38,700 ft

Pressure Levels versus Height


End of Weather Overview

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