WEATHER

Hydrologic Cycle

Saturation and Dew Point• Saturated verses

unsaturated air• At the dew-point

temperature, air reaches saturation– temperature to

which air must be cooled to (100% RH)• water on outside of

drinking glass• ice on your car

window• dew and fog

After Saturation Occurs the AirMust Release Extra Water as FluidWater forms on the outside of a cold glass as the cold

Air surrounding the glass chills the air to the Dew Point Temperature

The resulting water is not from the glass, the water is from condensation of moisture in the air around the glass

Lifting Condensation Level (LCL)The altitude where the dew point is reached

and clouds form

• LCL / Cloud base = dew point altitude

In Nature Extra Moisture isTransformed to Water Droplets

Cold air next to the rain-soaked cliff is chilled To The Dew Point Temperature & creates a Misty Cloud along a RockyMountain slope

Air near theSlope is 100%Saturated

Measuring Relative Humidity

Sling psychrometer

Hair hygrometer

There are a number of ways in which clouds can form

ConvergenceConvergence occurs when large air masses meet & are forced to rise vertically by crowding of molecules.

This process is best seen at the ITCZ where the Trades Winds meet & rise to form towering clouds & heavy precipitation

Convectional LiftingAnywhere air is warmer than its surrounding air, it will rise.

In this example an island heats more than the surrounding water and causes a massive cumulus cloud to form.

Convectional Lifting Over FloridaWarmer temperatures over the peninsula of

Florida, which is land, cause air to rise compared to the cooler oceans nearby

Rising air in thisShuttle Picture isShown by a Cloud patternwhich generallyfollows the shape of the southern Florida peninsula

Convectional Lifting in the DesertExtremely high afternoon temperatures in late summer often leads to thunderstorms throughout the world’s arid regions.

The Grand Canyon in August

Mojave Desert

Orographic Lifting of AirWhen air movingHorizontally Encounters aMountain it mustRise over the crest

As it rises, it coolsTo create clouds,And most oftenprecipitation

MoistAir

MoistureLost

DryAir

Run off NO Run off

WHY SO MUCH SEVERE WEATHER OCCURS IN THE MIDWEST

CHANGES IN LATITUDE: WESTERLY WINDS WHERE WE LIVE; FLORIDA AND SOUTH TOEQUATOR ARE THE TRADE WINDS FROM THE EAST. THE TRADE WINDS FROM NORTH AND SOUTH OF THE EQUATOR MEET AND CAUSE UPLIFT. THE WEATHER NEAR THEEQUATOR: LOTS OF RAIN AND NO WINDS.

• Air Mass – an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given latitude.

• Source Regions – are regions where air masses originate. In order for a huge air mass to develop uniform characteristics, its source region should be generally flat and of uniform composition with light surface winds.

Classification

• Air masses are grouped into four general categories according to their source region– Polar (P) are air masses that originate in

polar latitudes– Tropical (T) are air masses that form in warm

tropical regions– Maritime (m) are air masses that originate

over water (moist in the lower layers)– Continental (c) are air masses with a source

region over land (dry)

Air mass source regions and their paths.

Fronts

• A front is a transition zone between two air masses of different densities. Since density differences are most often caused by temperature differences, fronts usually separate air masses with contrasting temperatures. Often they will also have contrasting humidities as well.

Cold Front

• Represents a zone where cold, dry stable polar air is replacing warm moist unstable tropical air.

• Drawn as solid blue line with the triangles along the front showing its direction of movement.

In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.

COLD FRONT

Cold front

Warm Fronts

• A warm front is a front that moves in such a way that warm air replaces cold air

In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front.

WARM FRONT

Warm Front

Frontal Lifting of AirAlthough not a mountain range, masses of moving airCreate the same effect – Unlike mountains air massesCan provide lifting in many different locations

Fronts can lift air Which is stable,Creating clouds& large amountsOf precipitationAs rain, snow,Sleet or hail