1. Water in the atmosphere
By science Supernova © 2018

2. Humidity and Condensation
The molecules of liquid water are always in motion. Even in a glass of water that looks still – the molecules are moving in random directions.
Water molecules are also moving in the air around you. This invisible water vapor strongly affects weather.

3. Characteristics of water
Water is unique because it naturally exists in all three states of matter on Earth.
Water is in a solid state at temperatures of 0ºC or below, appearing as ice, snow, hail, and ice crystals.
Water is in a liquid state between 0ºC and 100ºC, present as rain, and cloud droplets.
At 100ºC or above, water evaporates and enters the atmosphere as water vapor. The bubbles in boiling water are an example of water vapor.
Clouds and steam are liquid droplets, not gas.

4. Characteristics of water
Although you cannot see water vapor, sometimes you can feel it. The more water vapor the air contains, the more humid the air feels.
When water changes from one state to another, energy is either given off or absorbed. The change from water vapor to liquid water is called condensation. Products of condensation include dew, fog, and clouds.
The change from liquid water to water vapor is called evaporation. While condensation releases heat, evaporation absorbs heat.

5. Characteristics of water
Frost forms by deposition when water vapor condenses as a solid.
Snowbanks can become smaller not only through melting but also through sublimation, which occurs when water changes directly from a solid to a gas without first becoming a liquid.

6. Humidity
The amount of water vapor in the air varies widely. The actual amount of water vapor in the air at a given time and place is the specific humidity. It is expressed as the number of grams of water vapor per kilogram of air.
There is a limit to the amount of water vapor that can be present in the air. When there is so much water vapor in the air that the rate of condensation equals the rate of evaporation, the air is saturated.
If any additional water vapor evaporates into saturated air, an equal amount will condense.

7. humidity
The amount of water vapor present in the air depends on the temperature of the air.
The warmer the air, the more water vapor it can contain. The water vapor capacity of air roughly doubles for every rise in temperature of about 11º C.

8. Relative humidity
When meteorologists refer to the relative humidity, they are reporting how near the air is to its maximum capacity for holding water vapor.
Relative humidity compares the actual amount of water vapor in the air with the maximum amount of water vapor that can be present in the air at a given temperature and pressure.
Relative humidity is usually stated as a percentage. Saturated air has a relative humidity of 100%. Air that contains no water vapor has a relative humidity of 0%.

10. Measuring humidity
Humidity is measured using a psychrometer, an instrument that works on the principle that evaporation causes cooling. A psychrometer consist of 2 thermometers. One is a dry bulb and one is a wet bulb.
The wet bulb usually shows a lower temperature than the dry bulb because water is evaporating from the wick and cools the wet bulb. The drier the air the greater the cooling from the wet bulb. If both thermometers read the same it means that the air is saturated and evaporation cannot occur. The temperature difference between the wet and dry bulbs can be evaluated using a chart.

12. Condensation
On a warm sunny day the air is not saturated with water vapor. As the air cools it loses the capacity to contain water vapor. The air becomes saturated. If the air continues to cool beyond the saturation point, condensation occurs.
Water vapor may condense into droplets forming a cloud, or fog.
If the water vapor condenses on a surface, it is called dew.
The temperature at which saturation occurs and condensation begins is called the dew point. The dew point is a measure of the water vapor in the air.

14. Cooling and Condensation
Two conditions are necessary for water vapor to condense: 1) there must be material for water vapor to condense onto, and 2) air must cool to or below its dew point.
When fog, or clouds form, the water vapor is condensing onto tiny particles called condensation nuclei. A cubic centimeter of air can contain as many as 10,000 condensation nuclei.
Air cooled below its dew point is said to supersaturated.

15. Cooling and condensation
Condensation nuclei are usually substances such as salt, sulfate particles, or nitrate particles. Condensation nuclei are so small that one puff of smoke contains millions of them. Some types of bacteria and clay particles contaminated with organic material make good nuclei.
Dew and frost form when moist air contacts a colder surface. Fog forms when air cools through contact and mixing.
Air may cool or lose heat, through the following ways: 1) contact with a colder surface, 2) radiation of heat, 3) mixing with colder air, or 4) expansion as it rises.

16. Formation of dew and frost
When air cools to its dew point through contact with a cooler surface, water vapor condenses directly onto that surface. Dew may form on the ground, leaves, grasses or other surfaces. At night these surfaces become cooler than the air, because they lose heat more rapidly than air does.
If the air temperature is below 0ºC the water vapor becomes frost through deposition. This atmospheric frost is not a killing-frost. Killing-frost occurs when the temperature near the ground drops to -2ºC for several hours causing the liquid in the plants to freeze.

17. Formation of fog
Fog forms when a cold surface cools the warmer moist air above it. As water vapor condenses in the air, tiny droplets fill the air and form fog.
Each droplet is centered around a condensation nucleus. Droplets are so small that they fall slowly. The slightest air movement keeps them suspended in the air. At very cold temperatures, the fog may consist of tiny ice crystals.
Radiation fog forms when the night sky is clear and the ground loses heat rapidly through radiation. As the ground cools, light winds mix the cooled bottom air with the warmer air a short distance above it. Eventually the whole layer cools to its dew point. The resulting fog at ground level is colder than the layer of air above it – a temperature inversion.

18. Formation of fog
Advection fog, may form when warm, moist air blows over a cool surface. In the northern US and southern Canada, advection fog forms when warm, moist southerly winds blow over snow-covered ground. Summer fogs in coastal California form when warm ocean air moves over cold coastal waters. Winter fogs form along parts of the Gulf Coast when cold Mississippi River waters chill the gulf air at the river’s mouth.

19. Clouds
Clouds form when the air cools to its dew point. Clouds can form at any altitude in the troposphere. At temperatures above freezing clouds are made of water droplets. Below, freezing clouds are usually a mixture of snow crystals and supercooled water.
Supercooled water is water that is at 0ºC without freezing. When supper cooled droplets contact ice nuclei, they form snow and ice crystals. At temperatures below -20ºC, clouds are usually made up of snow and ice crystals.

20. Types of clouds
There are 4 types of clouds: low clouds, middle clouds, high clouds, and clouds of vertical development.
Clouds are classified according to their altitude – low, middle or high – and their shape.
If air movement is mainly horizontal, clouds form in layers; these are stratiform clouds. If air movement is mainly vertical, clouds grow upward in great puffs; these are cumuliform clouds.

21. Types of Clouds
When stratiform and cumuliform clouds appear at altitudes between 2000 and 7000 meters, they are called altostratus and altocumulus clouds.
When stratiform and cumuliform clouds appear at altitudes above 7000 meters, they are called cirrostratus and cirrocumulus clouds.
Among clouds found below 2000 meters are stratocumulus and nimbostratus clouds. Stratocumulus clouds are layers of puffy clouds; they often cover the whole sky especially in winter.
Cirrostratus clouds are thin, smooth, or fibrous sheets that sometimes cause halos around the sun or moon and may indicate snow or rain.

22. Types of clouds
Nimbostratus clouds are dark gray layers that produce steady rain.
Cumulonimbus clouds are another kind of rain cloud. They grow to great heights and produce heavy rain with thunder lightening and sometimes hail.

23. Types of clouds
Cloud heights are measured in distance above the ground, not distance above sea level. The height ranges mentioned are average heights in the troposphere at middle latitudes.

24. Types of clouds
Cloud names are formed from one or more of the same five words or word parts.
Stratus and strato – describe clouds that form in layers.
Stratus clouds are layered, low clouds.
Cumulus and cumulo – describe clouds that grow upwards. (cumulus is the Latin word for heap.)
Cumulus clouds are fluffy clouds with flat bases.
Cirrus and cirro – describe feathery clouds. (Cirrus is the Latin word for curl of hair)
6) Cirrus clouds are high, feathery ice clouds.
7) Alto – is used in describing clouds located between 2000 and 7000 meters.
8) Nimbus and nimbo – refer to dark rain clouds.

25. Cloud formation
The shape of a cloud shows how the air moves through it. Cumulus clouds and other clouds with vertical development form when rising air currents are buoyant, or lighter that the surrounding air.
On a sunny day, darker areas of the ground absorb more of the sun’s heat. The warmer ground heats the air above it, making that air less dense than the surrounding air. As it rises it cools. If the air cools to its dew point, a cumulus cloud forms. The atmospheric level at which condensation occurs is the condensation level.

26. Cloud formation
Without a steady influx of warm, moist, air, a cloud will soon evaporate. If there is a continuous supply of moist air and if the air in the cloud stays warmer than the surrounding air it will continue to rise and grow in height.
A cumulus cloud ceases to grow when it loses its buoyancy, which happens when the air in the cloud reaches the same temperature as the air surrounding the cloud at the same altitude.

27. Dry- and moist-adiabatic lapse rates
The adiabatic lapse rate is the rate at which air cools and rises. Unsaturated air cools at a rate of about 10ºC for every kilometer it rises.
This rate is called the dry-adiabatic lapse rate. The cooling is caused only by the air’s expanding. The air expands as it rises because it is surrounded by areas of low pressure.
The moist-adiabatic lapse rate is the rate at which saturated air cools as it rises; this rate varies from about 5ºC per km to about 9ºC per km.
AD-ee-uh-BAT-ihk
When water vapor condenses it releases heat. This heat is important in the formation of clouds and storms.

28. Cumulonimbus clouds
Cumulonimbus clouds begin forming when moist air rises and cools to its dew point. Cumulonimbus clouds have a base at condensation level. These clouds can grow to heights taller than mountains. The heat from condensation keeps the air rising inside the clouds warmer and less dense than the air outside.
If a cumuliform cloud receives a continuous supply of moist air, it will grow until it enters a layer of stable air. Air is said to be stable if the temperature of rising air inside a cloud decreases more quickly than the temperature outside of the cloud.
Meteorologists can predict cloud height based on air temperatures. As the clouds become increasingly less dense than the air outside of the cloud, it is said that air is unstable.

29. Layer Clouds
Stratiform clouds form in stable air. In stable air, air cannot easily move up or down, so it tends to spread out horizontally in layers.
Clouds can form in stable air in two ways:
Air can be forced slowly upward to its condensation level. This happens with moving up a mountainside or over a layer of colder denser air.
A layer of air can cool to its dew point by radiating heat or mixing with cooler air. The water vapor in the air will then condense to form clouds.

30. Predicting the condensation level
As warm air rises and expands not only does its temperature decrease, but its dew point also decreases. Given the dew point at ground level meteorologists can predict the condensation level.
For dry air the rate of cooling by expansion is about 10ºC for every kilometer. As the air rises, its dew point falls at a rate of about 2ºC for every kilometer. When temperature and dew point are the same condensation occurs.
Knowing the condensation level is important for forecasting the weather. For example, by knowing the height at which clouds will form and how high they will grow, meteorologists can predict the severity of a resulting storm.

31. Precipitation
When water droplets or ice crystals grow heavy enough to fall, precipitation occurs. Precipitation is any form of water that falls from a cloud to Earth’s surface. Rain, snow, sleet, and hail are all examples of precipitation.

32. Growth of water droplets
In a cloud, tiny droplets that form by condensation grow by bumping into and combining with other droplets.
Bigger droplets fall faster than smaller ones, so big droplets catch up with smaller droplets, collide with them and “capture” them. Other droplets are not incorporated; instead they just bounce off the bigger droplets or are pushed aside by them.
Droplets that have been in the cloud longer have had more time to grow. Some droplets are larger because they formed around bigger condensation nuclei.
The mixing of air from different parts of the cloud and the falling of larger drops from higher up bringing droplets of different sizes together.

33. Growth of ice crystals
Except in the shallowest clouds in the warm tropics, temperatures in the upper layers of clouds are usually below freezing. Both ice crystals and supercooled droplets are present in these clouds.
Some supercooled water evaporates, and the resulting water vapor is usually deposited on the ice crystals.
When ice crystals get heavy enough they start to fall. The falling crystals then grow by capturing both smaller ice crystals and water droplets in their path.
If the temperature in the lower layers of the cloud is above freezing, the crystals may melt to form water droplets, which grow through collision.

34. Kinds of precipitation
Precipitation comes in many forms, including drizzle, rain, snow, sleet, freezing rain and hail.
Drizzle consist of very fine water drops that fall slowly and close together. Raindrops are larger and fall faster and farther apart. Snow forms when ice crystals in a cloud collide and clump together. When snowflakes fall into warm air, they partially melt into sticky, wet clusters. If the snowflakes melt completely they fall as rain.
Sometimes rain falls into a cold layer of air. There the raindrops become supercooled. If the raindrops freeze, they fall as sleet.

35. Kinds of precipitation
If the layer of cold air is thin, the supercooled raindrops might not freeze until they hit solid surfaces. Then the drops freeze instantly. This kind of rain is called freezing rain, causes sheet ice, or glaze on sidewalks, trees, roofs, and power lines. If the ice becomes heavy enough, trees and power lines may break under its weight.
In the summer frozen precipitation usually melts before hitting the ground. The exception is hail, precipitation in the form of balls or irregular clumps of ice.

36. Kinds of precipitation
A hailstone begins as a frozen raindrop or small, dense clump of ice crystals. It grows by collecting smaller ice particles, cloud droplets and supercooled raindrops that freeze onto it. The growing hailstone is kept aloft by strong updrafts until it becomes too heavy and falls. The stronger the updrafts, the larger the hailstones can become.
A hailstone has a layered structure that is like the layers of an onion. The layers form as the hailstone encounters different forms of moisture and different temperatures – which cause melting and refreezing – on its journey through the cloud. A hailstone may be as large as 14 centimeters across.

37. Measuring precipitation
The National Weather Service reports rainfall in hundredths of an inch. Rainfall is measured using an instrument called a rain gauge. The recorded, and reported measurement is the depth the water would be if the rain did not soak into the ground, flow away or evaporate.
Snowfall is measured using a stick. Because a dry snow is deeper than an equal weight of wet snow, the depth of the snow is not an accurate measure of how much water it contains. The rain equivalent of snowfall is determined by melting the snow.

38. Where does precipitation occur?
Precipitation occurs in every part of the world. In some places there may be no precipitation for years at a time, while other places it may rain almost every day. One of the main causes of precipitation is the rising and cooling of moist air.

39. Where does precipitation occur?
The higher air rises the more precipitation it can release. Therefore, areas that receive the most precipitation are those where warm, moist air rises high in large quantities.
Here are descriptions of where these conditions occur: 1) Heat from the sun produces high land temperatures near the equator, and in turn cause the air near the surface to be very warm and rise. The result is almost daily thunderstorms. The heavy rain allows the land around the equator to be home to dense tropical rain forests. 2) In storm areas of all kinds, including hurricanes and many low-pressure areas and fronts, air rises and cools to produce precipitations. 3) In areas where moist air often blows across a mountain range, the windward side of the mountain may receive large amounts of precipitation, while the leeward side of the mountain gets little rain.

40. Where does precipitation occur?
Precipitation occurs when moist air rises and cools. There is no precipitation in areas where air sinks and warms. Where sinking air persists, the dry conditions produce deserts.
Areas of persistent high pressures occur at about 30 degrees north or south latitude. Deserts occur in these areas. The polar areas are dry not only because air sinks but also because the air is so cold it contains little water vapor.
Air sinks on the leeward side of mountains. The precipitation often falls on the windward side of the mountain as clouds rise to go over the mountain. Dry winds form on the leeward side as evidenced by the Santa Ana winds in CA.

41. Weather modification
Often it does not rain when or where rain is needed most. Scientists have developed various methods of producing rain.
One method of making rain is to “seed” a supercooled cloud by dropping pellets of frozen carbon dioxide (dry ice) into it. These pellets cool the cloud so much that tiny ice crystals form. The crystals grow by normal processes until they get heavy enough to fall.
Both methods require rain to already be present.
Another method of seeding includes putting artificial ice nuclei into clouds. These are often made of tiny silver-iodide crystals. Precipitation grown by normal means after attaching to the nuclei.