WEATHER

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Hydrologic Cycle 6

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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 8

After Saturation Occurs the Air Must Release Extra Water as Fluid Water 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 9

Lifting Condensation Level (LCL) The altitude where the dew point is reached and clouds form LCL / Cloud base = dew point altitude 10

In Nature Extra Moisture is Transformed to Water Droplets Cold air next to the rain-soaked cliff is chilled To The Dew Point Temperature & creates a Misty Cloud along a Rocky Mountain slope Air near the Slope is 100% Saturated 11

Measuring Relative Humidity Sling psychrometer Hair hygrometer 12

There are a number of ways in which clouds can form 13

Convergence Convergence 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 14

Convectional Lifting Anywhere 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. 15

Convectional Lifting Over Florida Warmer temperatures over the peninsula of Florida, which is land, cause air to rise compared to the cooler oceans nearby Rising air in this Shuttle Picture is Shown by a Cloud pattern which generally follows the shape of the southern Florida peninsula 16

Convectional Lifting in the Desert Extremely high afternoon temperatures in late summer often leads to thunderstorms throughout the world’s arid regions. Mojave Desert The Grand Canyon in August 17

Orographic Lifting of Air When air moving Horizontally Encounters a Mountain it must Rise over the crest As it rises, it cools To create clouds, And most often precipitation Moisture Lost Dry Air Moist Air Run off NO Run off 18

WHY SO MUCH SEVERE WEATHER OCCURS IN THE MIDWEST

CHANGES IN LATITUDE: WESTERLY WINDS WHERE WE LIVE; FLORIDA AND SOUTH TO EQUATOR ARE THE TRADE WINDS FROM THE EAST. THE TRADE WINDS FROM NORTH AND SOUTH OF THE EQUATOR MEET AND CAUSE UPLIFT. THE WEATHER NEAR THE EQUATOR: 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.

COLD FRONT 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

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

WARM FRONT 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

Although not a mountain range, masses of moving air Create the same effect – Unlike mountains air masses Can provide lifting in many different locations Fronts can lift air Which is stable, Creating clouds & large amounts Of precipitation As rain, snow, Sleet or hail Frontal Lifting of Air 31