Surface Condensation

• Water vapor condensing on large surfaces is called dew.

• Dew Point is the temperature that saturation occurs and condensation begins. – Measured as the amount of water vapor in the air.

• Saturation - When condensation equals evaporation.

– The warmer the air the more water vapor it can hold.

Dew Point Temperature

= temperature we get dew.

– Depends on the amount of moisture in the air

– the more moisture in the air, the higher the dew point temperature.

• Dew or Frost forms when moist air contacts a colder surface.– Air temps above 0oC dew forms– Air temps below 0oC frost forms

• Fog forms when air cools through contact and mixing– Droplets are so small they fall slowly so that the slightest air

movement keeps them suspended.– Can sometimes be ice crystals

Cooling and Condensation

Predicting Condensation Level• As air rises = it’s temperature decrease

= it’s dew point decreases• Knowing ground temperature and dew point we can predict the

condensation level. • Knowing the condensation level allows for predicting at what

level clouds will form and how high they will grow. This allows for the severity of a possible storm to be determined.

Kinds of Precipitation• There are several forms of precipitation:

– Drizzle– Rain– Snow– Sleet: rain freezes as it falls– Freezing rain: rain freezes as it hits a cold surface– Hail: balls or irregular clumps of ice.

HAIL!– Hailstones begin as a frozen raindrop

or small, dense clump of ice crystals

– Hailstones grow due to being kept aloft by strong updrafts until it is too heavy and falls.

– The stronger the updrafts the larger the hailstones.

Where does precipitation occur

• The major cause of precipitation is the rising and cooling of moist air.

• Warm air = raises higher and holds more moisture.

• Areas that receive the most precipitation are those where warm moist air rises high in large quantities.– Low pressure areas which cause air to rise and

cool to produce precipitation

Windward and Leeward When moist air blows across a mountain range, the windward side receives precipitations as the air rises and cools.

On the leeward side the colder and dryer air is sinking and warming so there is rare rainfall

(desert regions occur on the leeward side of a mountain range).

Humidity

• Water vapor in the air

• The actual amount of water vapor is specific humidity.

• Your weatherman reports the relative humidity in weather forecasts.– Relative humidity is how near the air is to being full

of water vapor.– Relative humidity

is a percentage.

Humidity

Measuring Humidity• Humidity is measured by using a psychrometer,

which works on the principle that evaporation causes cooling and a relative humidity chart.

• A pyschrometer consists of two thermometers, one dry bulb and one wet bulb.

Air Pressure and Wind

• Wind - horizontal movement of air

• Differences in air pressures are what set the air in motion.

Air Pressure• Air pressure is the weight of the atmosphere as

it pushes down upon Earth• Note air pressure decreases as elevation increase• Air pressure is most noticeable to people when

they experience a change in the air pressure. – A rapid ascent, the air pressure outside of your body

rapidly decreases causing your eardrums to hurt or pop because of the greater internal pressure.

– A rapid descent causes an increase in external pressure causing greater pressure on the outside of your eardrum.

Air Pressure• Air pressure is measured with an instrument

called a barometer. There are two types– Mercury – Aneroid

Sea and Land Breezes

• During the day the land warms so the air rises causing air to come in from the sea.(sea breeze)

• At night, the water is warmer than the land, so air rises, causing air to flow in from the land. (land breeze)

Air Pressure and Wind• Pressure differences and winds are caused by

the unequal heating of Earth’s surface.– The Earth’s land absorbs more heat than the

surrounding water. – The hot air rises and the air over the ocean blows in

to replace the warm air. – Thus air flows from high

pressure to low pressure

Air Pressure• We show air pressures by plotting isobars on

weather maps.– Isobar is a line that joins points of equal air pressure.– When air pressure is greater in the center, its called a

high-pressure area or high.

Air Pressure

– When air pressure is lower in the center, its called a low-pressure area or low.

– Closely spaced isobar lines indicate an area where the air pressure changes quickly.

Air Pressure and Wind

• The wind blows due to the air moving from a high pressure are to a low pressure area.– The greater the difference, the stronger the wind

– Closely spaced isobars = strong pressure gradient and strong winds.

– Widely space isobars = weaker pressure gradient and weaker winds.

Air Pressure and Wind

• Meteorologist use two instruments to measure wind speed and direction.– The wind vane is use to determine the direction

the wind is blowing from.

– The anemometer is used to measure the speed of the wind 10 meters above the ground.

Jet Streams• Super powerful winds• At the top of the troposphere• Boundary between Polar and tropical air

masses

• The Coriolis Effect: is when a flying object curves away from its path of travel, caused by

• the rotation of the Earth.

• North Hemisphere- – Curves right

• South Hemisphere– Curves Left

Factors Affecting Winds

• Coriolis effect on wind in the Northern Hemisphere:

Air Masses

Named according to WHERE they FORM.

• First name depends if it

forms over land or water. – Land = Continental– Water = Maritime

• Second name depends on

Latitude.– High Latitude = Polar– Low Latitude = Tropical

Fronts

• Are BOUNDARIES between two air masses

Cold Front• Cold air moves in and replaces warm air• BLUE triangles• Vertical, fast uprising• Usually come with thunderstorms

Warm Front

• Warm air moves in and replaces cold air.• RED CIRCLES• Slower, diagonal uprising

Stationary Front

• Neither air mass is strong enough to replace the other; warm and cold meet and stay

• Prolonged precipitation

Occluded Front

• When cold front catches up to warm front.

• Usually dry weather

• Station model:– Wind speed/direction– Temperature– Dew Point– Air pressure– Cloud Cover

Forecasting Weather