Ch. 23 Water in the Atmosphere Three states or phases of water in the atmosphere. – ice (solid), water (liquid), water vapor (gas) Temperature is a measurement of the movement of molecules. - As you heat a substance, the molecules move faster. - As you cool a substance, the molecules move slower.

Phase changes (Changes in State) EVAPORATION – changes from liquid to gas CONDENSATION – changes from gas to liquid MELTING – changes from solid to liquid FREEZING – change from liquid to solid SUBLIMATION – changes from solid to gas DEPOSITION – changes from gas to solid

Temperature Water freezes at 32 o F or 0 o C Water boils at 212 o F or 100 o C Lines on a weather map that connect points of equal temperature are called Isotherms

Humidity - water vapor in the atmosphere “moisture” in the air. controlled by rates of condensation and evaporation When rate of evaporation equals rate of condensation, then the air is saturated (equilibrium reached). Saturated air holds 100 % of the water vapor that it could hold at that temperature. The temperature at which equilibrium occurs is called the dew point. At temperatures below the dew point, condensation occurs and liquid water droplets form. At this point, clouds form.

Condensation Nuclei Particles of dust, ash, salt, or other solids in the air on which water vapor can form. – At the poles there is very little precipitation because of the cold temps and lack of condensation nuclei. Coalescense

Absolute Humidity Relative Humidity A measure of the actual amount of water vapor in the air. A way to express the amount of moisture in the air. Absolute = mass of water vapor (g) Humidity volume of air (m 3 ) Compares the amount of water vapor in the air to the amount the air could hold at a certain temperature (%), A ratio of the actual water vapor content of the air to the amount of water vapor needed to reach saturation. It is a measure of how close the air is to reaching the dew point. A common way to express the amount of water vapor in the air. Example: at 25 o C, air is saturated when it contains 20g of water vapor (per 1 kg of air). If air at 25 o C contains 5 g of water vapor, the relative humidity is 5/20, or 25%. Relative = amount of water vapor in air X 100 Humidity amount of water vapor needed to reach condensation

The Relative Humidity – Saturation curve 1.If T = 50 2.If T = 30 3.If T = 0 4.If H 2 O = 10 5.If H 2 O = 40 6.If H 2 O = 70

Dew Point and Dew When the air is nearly saturated with a relative humidity of almost 100%, a small temperature drop will allow it to reach its dew point. At the dew point, condensation occurs. Dew occurs when air cools by conduction (contact with cooler surface features) – Dew forms on these cooler surfaces (condensation) – Most likely to form on cool, clear nights with little wind. – If the dew point falls below the freezing temperature of water, deposition can occur. – Frost is not frozen dew (frozen dew is rare and forms as clear beads of ice)

The heat index gives the temperature according to how it feels compared to the actual temperature. The higher the humidity, the higher the index.

Measuring Relative Humidity Psychrometer (sling psychrometer) Used with a table that lists relative humidity based on differences between wet-bulb and dry-bulb thermometer readings No difference  100% humidity (air is saturated) Hygrometer (hair hygrometer) Works on the principle that hair becomes longer as relative humidity increases and shorter has relative humidity decreases.

Comparing wet-bulb and dry-bulb to determine Relative Humidity Dry = 80 differ = 10 Dry =100 differ= 13 Dry = 60 differ = 4

Measuring Humidity at High Altitudes Uses an electronic hygrometer Carried into the atmosphere by a radiosonde The radiosonde is attached to a weather balloon

Isotherms

Collections of water droplets or ice crystals suspended in air which form when air is cooled and condensation occurs

Four major processes can cause the cooling that is necessary for clouds to form. Adiabatic Cooling – The process by which the temperature of an air mass decreases as the air mass rises and expands. – The altitude at which this condensation occurs is called the condensation level, and marks the base (bottom) of the clouds. Mixing -Occurs when two moist bodies of air mix, cooling the combined air below its dew point.

Lifting -occurs when air is forced upward and cools; common as air is forced upward to move over mountains Advective cooling – the process by which temperature of an air mass decreases as the air mass moves over a cold surface

Clouds need these factors to form: Water vapor must be able to condense. The dew point temperature must be reached (saturation). There must be condensation nuclei in the atmosphere for the water to form on.

Formation of a Water Droplet

Classification of Clouds (classified by altitude) Three altitudes High: (above 6,000 m) “cirro” Middle: (2000 m – 6000 m) “alto” Low: (0 m – 2000 m ) “strato”

Classification of Clouds (Classified by shape/form) Stratus Stratus means sheet- like or layered Flat, uniform, low altitude Cover large areas of the sky Produce little precipitation Nimbostratus - dark rain makers Altostratus – thinner at middle latitudes Stratocumulus

Classification of Clouds (Classified by shape/form) Cumulus Low altitude, billowy, puffy Cumulus means piled or heaped Hot humid days Cumulonimbus – high, dark, storm clouds, thunderheads Altocumulus – middle altitudes Stratocumulus – low forming combinations of stratus and cumulus

Classification of Clouds (Classified by shape/form) Cirrus Feathery clouds composed of ice crystals Highest altitude of any clouds; above 6,000 m Cirro and cirrus means curly Cirrocumulus – rare, high altitude, billowy Cirrostratus – high, transparent veil. Halo around moon or sun

Fog (a cloud that forms near the surface) Radiation fog Forms from radiation The layer of air in contact with the ground becomes chilled to below dew point. Thickest in valleys and low places. Also common around cities, where pollutants act as condensation nuclei. Other types of fog Advection fog – Forms when warm, moist air moves across a cold surface. – Common along coasts Upslope fog – Forms by the lifting and cooling of air as air rises along land slopes. Steam fog – A shallow layer of fog that forms when cool air moves over an inland warm body water such as a river.

STEAM FOG

Weather Modification Cloud seeding: – In areas of drought, scientists may attempt to induce precipitation. – Freezing nuclei or condensation nuclei are introduced into a cloud. – Silver iodide crystals are released from burners on the ground or from flares dropped by aircrafts. – Powdered dry ice is dropped from aircraft to cool cloud droplets and cause ice crystals to form which melt to form raindrops. – Works too well sometimes, not at all sometimes. – Could eventually be a way to reduce precipitation from clouds before a storm gets too large.

Cloud Cover Symbols

Cloud Type Symbols

-any moisture that falls from the air to Earth’s surface Forms in troposphere cumulonimbus and nimbostratus clouds

Forms of Precipitation Rain - Liquid, 0.5 – 5 mm in diameter (smaller  drizzle) Snow – Solid, ice particles Sleet – Ice pellets, occur when rain falls through a layer of freezing air near the ground. Sometimes it may not freeze until it hits the ground  glaze ice (ice storm). Freezing rain - rain falls and freezes when it touches cold surfaces such as highways and trees Hail – Solid, lumps of ice – Forms in cumulonimbus clouds that keep the particles aloft until they are too heavy and then fall as hail

Precipitation Formation

Hail

Causes of Precipitation (Most cloud droplets are 20 micrometers in diameter) Coalesence The formation of large droplets by the combination of smaller droplets. Supercooling A condition in which a substance is cooled to below its freezing point, condensation point, or sublimation point without changing state. Ice crystals do not form because there are to few freezing nuclei. Water vapor condenses on available ice crystals.

Measuring Precipitation Rain gauge – measures rainfall Measuring stick – snow depth Water content of snow – melt a measured volume of snow 10 cm of snow = 1 cm of liquid water

Doppler Radar Measures the intensity of rainfall. Works by bouncing radio waves off rain/snow. By timing the waves return rate, meteorologists can detect the location, direction of movement, and intensity of precipitation.