Weather

Forecasting

Elisabeth (Lis) Cohen

Motivation

Foxtrot Comic Strip

What will the weather be tonight?

1 – Cloud and Rain Observations

2 – Pressure Systems

3 – Current Observations

4 – Computer Models

5 – Predict & Verify

Our Forecasting Steps

Why focus on clouds?

• Rain/Snow producers• Heating and Cooling• Indicator of weather• Come in interesting

shapes

Common Cloud Types

Cloud Levels

• High Cumulus– Size of top of thumb

at arms length

• Low Cumulus– Size of fist at arms length

Photo: WBZ-Mos

Cloud Levels

• High Stratus– Sun and Moon easily seen

• Low Stratus– Sun and Moon difficult to see

Sign of moisture high up in the atmosphere.

(Storms possible in 6-24 hours).

Three Cloud LevelsHigh Clouds

Possible marker of afternoon thunderstorm when clouds seen in the morning are growing or are

very tall.

Three Cloud LevelsMiddle Clouds

Cloud LevelsLow Clouds

Can form drizzle like precipitation

Often indicative of thunderstorms and heavy rain.

Cloud LevelsLarge Vertical Development

Cold and Warm Front Clouds

Cold Front Warm Front

Frontal Movement

Cloud Levels

Satellites

• Monitor one area constantly, as it remains stationary with respect to an observer on earth’s surface

Scan from north to south

Click here for:

Satellite loop of the United States

Are the clouds moving in your direction?

When will they reach you?

RadioMicrowavesInfraredVisible UltravioletX RaysGamma Rays

Wavelengths of LightW

avel

engt

h In

crea

ses

Earth Surface

Infrared Energy

Infrared Energy

High CloudHigh Cloud

Low CloudLow Cloud

Cold

Satellite

Infrared Image

Warm

Appears Gray Appears White

High and Low Clouds in Infrared Image

Visible image Infrared image

How RADAR works

USA Today Weather Book Jack Williams

Click here for:

Infrared Satellite loop of the United States

Are the clouds moving in your direction?

When will they reach you?

Click here for the :Radar Map of the United States

Where is the rain moving?Is it getting heavier or lighter?

Look at the scale

Clear During the Day

Cooler Air

Relatively Warmer Air

All Solar Energy Received = Warmer

Low Clouds During the Day

Some Solar Energy Received = Cooler

Clear at Night

Clear at Night

Ground radiatively cools at night and the energy is lost to space. The ground can get very cool on these nights.

Cloudy at Night

Cloudy at Night

The ground radiately cools at night. Some of that radiation is absorbed by the clouds and reradiated in all directions. Some of that radiation reaches the ground and keeps warms the ground. This keeps it warmer at night.

What clouds are outside now? Click on the image to check the clouds in Colorado.

Ingredients for Cloud Formation

1. Condensation Nuclei

2. Moisture

3. Cooling / Lift

Surface Weather Map

General Rules• Low-

– cloudy skies

– Sucks air

– Lousy

• High-– mostly clear

– blows air

– Happy

Click here to see the: United States Surface Weather Map

Where are the highs and the lows?Are their lows close by that might

be associated with clouds?

Winds Moving Air Masses

Warm Air Cold Air

Temperature

During the Day…

Temperatures decrease with height, in this layer of the atmosphere.

At Night….

Air at the ground is often cooler than the

air above it.

Temperature

Winds at Night

Cooler Air

Relatively Warmer Air

Local Observations

• Past and Current

– Wind Speed

– Wind Direction

– Temperature

– Humidity

Click here for: Surface Observations in Denver (Mesowest data)find your city on the map

Click here for:Computer model forecast for United States

Weather 6 hours from now

Climate Normals for Boulder

Helpful Hints

Observation Forecast Reason

NightCooler

than day

Ground Cools (Outgoing Longwave Radiation> Incoming Shortwave

Radiation +Incoming Longwave Radiation)

Clear SkyCool

EveningThe atmosphere is mostly transparent to the outgoing

longwave radiation.

Calm WindsCool

EveningLittle mixing of air from aloft keeps the air close to the

ground cooler.

Low Humidity

Cool Evening

The lack of moisture will mean less heat will be reradiated back to earth.

Your Forecast

Observation Forecast Reason

Clouds-satellite

-sky cover-highs/lows

Temp -clouds-winds

Rain-high/lows

-radar

Other

Thank you!

Questions?

Lis Cohen

eac26@cornell.edu or liscohen@met.utah.edu