High School Earth Science/Weather and Atmospheric Water
If someone across country asks you what the weather is like today, you need to consider several factors. Air temperature, humidity, wind speed, the amount and types of clouds and precipitation are all part of a thorough weather report. In this chapter, you will learn about these many of these features in more detail.
Lesson Objectives 
- Discuss the difference between weather and climate.
- Describe the relationship between air temperature and humidity, including the concept of dew point.
- List the basics of the different cloud types and what they indicate about current and future weather.
- Explain how the different types of precipitation form.
What is Weather? 
Weather is what is going on in the atmosphere at a particular place at a particular time. Weather may be cold or hot, or wet or dry, and it changes rapidly. A warm sunny day may rapidly turn into a cold and stormy one, making you wish you had brought your jacket. There are many factors that influence the weather; a few examples include the air temperature over a region, whether there is a second air mass nearby, and how close high and low pressure cells are. A location's weather depends on air temperature, air pressure, humidity, cloud cover, precipitation, and wind speed and direction, which are all directly related to the amount of energy that is in the system and where that energy is. The ultimate source of this energy is the sun.
Climate is the average of a region's weather over time. The climate for a particular place is steady, and changes only very slowly. Portland, Oregon has a mild, moist climate and Fairbanks, Alaska has a frigid, dry one. Portland or Fairbanks may experience a warm sunny day in February, but that doesn’t change their climate. Climate is determined by many factors, which are related to the amount of energy that is found in that location over time. Factors that determine the amount of energy include the angle of the sun, the likelihood of cloud cover, the air pressure, and many others.
Humidity is the amount of water vapor in the air in a particular spot. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, that does not mean that 80% of the molecules in the air are water vapor. It means that the air contains 80% of the total amount of water it can hold at that temperature. If the humidity increases to more than 100%, the excess water will condense from the air and form precipitation.
Humidity affects weather a great deal and is important for weather forecasting. When humidity is high, precipitation is more likely. The combination of high humidity and high temperatures can threaten people's health. People are more uncomfortable when both temperature and humidity are high. As people and some other animals sweat to cool themselves off; they lose heat as the sweat evaporates. But if the air is already saturated with water vapor, the sweat will not evaporate and the person will not cool. They will simply be hot and sweaty, and uncomfortable.
The National Weather Service has developed a heat index (HI). On an HI chart (Table 16.1), people can see what the temperature feels like, when the air temperature and humidity are known. For example, if the temperature is 85°F, but humidity is only 40%, the temperature feels like a pleasant 84°F. But if the temperature is 85°F, but the humidity is 90%, the air temperature feels like a very hot and sticky 101°F. This information is useful for people who are interested in outdoor activities. High humidity causes health problems, such as sunstroke or heatstroke, to occur more quickly.
Since warm air can hold more water vapor than cool air, raising or lowering temperature can change air's relative humidity (Figure 16.1). The temperature at which air becomes saturated with water is called the air's dew point. This term makes sense, because water will condense from the air as dew, if the air cools down overnight and reaches 100% humidity.
Sometimes there are lots of clouds in the sky and sometimes you can't see a cloud anywhere. Either way, clouds have a big influence on weather. Clouds affect weather in three ways: (1) preventing solar radiation from reaching the ground, (2) absorbing warmth that is re-emitted from the ground, and (3) as the source of precipitation. When there are no clouds, there is less insulation. As a result, cloudless days can be extremely hot, and cloudless nights can be very cold. For this reason, cloudy days tend to have a lower range of temperatures than clear days.
Clouds form when air reaches its dew point, the temperature when the air is saturated with water vapor. This can happen in two ways. First, the air temperature can stay the same while the humidity increases. This is common in locations that are warm and humid. Second, the humidity can remain the same, but the temperature decreases. When this happens, the air will eventually cool enough so that it reaches 100% humidity, and water droplets form. Air cools when it comes into contact with a cold surface or when it rises. There are three ways that rising air can create clouds: (1) It can be warmed at or near the ground level, (2) It can be pushed up over a mountain or mountain range, or (3) It can be thrust over a mass of cold, dense air.
Water vapor in the atmosphere is not visible, unless it condenses to become a cloud. Water vapor condenses around a nucleus, such as dust, smoke, or a salt crystal. This forms a tiny liquid droplet. Billions of these water droplets together make a cloud. If the atmosphere is very cold, the droplets freeze into ice. Most clouds appear white because sunlight reflects off the water droplets. If the clouds are thick, the droplets scatter or absorb the light and less solar radiation can travel through them. This is why storm clouds are dark black or gray.
Clouds have been classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure 16.2). High clouds, which have the prefix 'cirro-', are found above 6,000 m (20,000 feet) in altitude. Middle clouds, which have the prefix 'alto-', are between 2,000 to 7,000 m (6,500 to 23,000 feet). Low clouds, which have the word 'stratus' in their names, occur beneath 2,000 m (6,500 feet). Each of the clouds that occur in these groups is layered, and they grow horizontally.
|High clouds:||Cirrocumulus (Cc)||Cirrostratus (Cs)||Cirrus (Ci)|
|Middle clouds:||Altostratus (As)||Altocumulus (Ac)|
|Low clouds:||Nimbostratus (Ns)||Stratocumulus (Sc)||Stratus (St)|
|Vertical clouds:||Cumulus (Cu)||Cumulonimbus (Cb)|
Another group of clouds, which have the prefix 'cumulo-', describes clouds that grow vertically instead of horizontally. These impressive clouds have their bases at low altitude and their tops at high or middle altitude.
High clouds form where the air is extremely cold and can hold little water vapor. The ice crystals that form create thin, wispy cirrus clouds (Figure 16.3). Cirrus clouds may indicate an oncoming storm.
Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrostratus clouds are thin, white sheets of clouds, made up of ice crystals like cirrus clouds (Figure 16.4). Cirrostratus clouds are sometimes so thin that they cannot be seen, unless illuminated by the sun or moon.
Middle clouds may be made of water droplets, ice crystals or both, depending on the air temperatures. Altocumulus clouds appear as white to gray, puffy stripes rolling across the sky (Figure 16.5). These clouds often occur before thunderstorms. Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation is coming.
Low clouds usually hold droplets of liquid water, although they may also contain ice when temperatures are very cold. Stratus clouds are gray sheets that cover the entire sky (Figure 16.6). These clouds may produce a steady drizzle or mist, but do not carry hard rain. Nimbostratus clouds are thick and dark. They bring steady rain or snow. Stratocumulus clouds are rows of large, low puffs that may be white or gray. These clouds rarely bring precipitation.
Clouds grow vertically when strong air currents are rising upward. Cumulus clouds resemble white or light gray cotton and have towering tops (Figure 16.7). On fair days, cumulus clouds may grow upward but produce no precipitation. On hot summer afternoons, though, cumulus clouds may mushroom into a form that looks like a head of cauliflower. These clouds may produce light showers.
If the vertical air currents are strong, a cumulus cloud will grow upward until it develops into a cumulonimbus cloud (Figure 16.8). Tall, dark, and ominous cumulonimbus clouds are associated with lightning and intense thunderstorms.
Fogs are clouds located at or near the ground. When humid air near the ground cools below its dew point, fog is formed. Fogs develop differently from the way clouds form. There are several types of fog, each of which forms in a different way.
Radiation fogs form at night when skies are clear and the relative humidity is high. As the ground cools, the bottom layer of air cools also. Eventually the air temperature may be lowered below its dew point. If there is a light breeze, the fog will be carried upward. Radiation fog can grow to 30 meters (100 feet) thick. One to three hours after sunrise, radiation fog burns off as the ground warms. The Central Valley of California frequently experiences radiation fog, which is called tule fog in this area. Tule fog can be so thick that drivers cannot see the car in front of them and their headlights just reflect back off the sheet of water droplets.
San Francisco, California is famous for its summertime advection fog (Figure 16.9). Warm, moist air from over the Pacific Ocean blows over the cold California current just offshore. This cools the eastward moving air below its dew point and thereby creates fog. Advection fog is brought onshore by sea breezes. If the fog is accompanied by light wind, a thicker layer of air cools and the fog can grow to be up to 600 m (2,000 feet) thick.
Steam fog appears in autumn or early winter and can make a pond or lake appear to be steaming. The "steam" forms when cool air moves over a lake that still holds some of its summer heat. Water evaporates from the lake surface and condenses as it cools in the overlying air. Steam fog is rarely very thick.
When warm humid air travels up a hillside and cools below its dew point it creates an upslope fog (Figure 16.10).
As you know from your daily life, precipitation is an extremely important part of weather. Precipitation most commonly falls as rain or snow, but can also be sleet, hail, dew or frost. Sleet is a mixture of rain and snow, and often forms when snow partially melts as it falls. Dew forms when moist air comes into contact with a cold surface, like the ground or a car windshield, and cools below its dew point. Frost forms under similar conditions, but when the air cools to below freezing (Figure 16.11).
The other types of precipitation come from clouds. Rain or snow droplets fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. The most common way for rain or snow to droplets to grow, occurs in cold clouds, where the temperature is -10°C (14°F) or less (Figure 16.12). Here the water vapor freezes directly into ice crystals, which continue to grow as more water vapor freezes onto them. When the ice crystals become heavy enough, they fall. Even as they fall, the ice crystals collect more moisture. If temperatures are cold, the ice will hit the ground as a snowflake. If temperatures near the ground are greater than 4°C (39°F), the ice crystal may melt and become a raindrop. One million cloud droplets will combine to make only one rain drop!
Water may also precipitate from warm clouds. Here too, water droplets get trapped in rising and falling air currents. As the droplet travels around the convection cell, it collides with other small droplets. At some point, the droplet is large enough to escape the convecting air currents and it falls to the ground as rain. If the air currents are very strong, the droplets must be very large before they fall.
If a raindrop falls through warm air but hits a layer of freezing air near the ground, it becomes frozen into a small clear ice pellet known as sleet. Sleet usually is mixed with liquid water drops that did not freeze as they descended from the cloud. If the layer of frigid air near the ground is not thick enough for the raindrop to freeze before it reaches the ground, the drop may freeze on the ground, forming glaze. The weight of glaze covering a tree branch can make the branch fall.
Hail forms in cumulonimbus clouds with strong updrafts. An ice particle falling through a cloud is captured by an updraft and continues to grow as it travels around the convection cell. When it finally becomes too heavy, it drops to the ground. Although hail is usually less than 1 cm (about one-half inch), it's not uncommon to find hail that is 5 to 10 cm (2 to 4 inches) in diameter (Figure 16.13). The largest hailstone ever measured, 14 cm (5.5 inches) in diameter and weighing 766 grams (27 ounces), was collected in Coffeyville, Kansas in 1970.
Lesson Summary 
- Air temperature causes differences in pressure so that convection cells form.
- Air rising in a convection cell may cool enough to reach its dew point and form clouds or precipitation if the humidity is high enough.
- Clouds or fog may form if warmer air meets a colder ground surface. Air temperature and humidity also determine what sorts of clouds and precipitation form.
- These factors play a role in creating a pleasant or uncomfortable day, such as when it might be warm and dry or hot and humid.
Review Questions 
- What factors need to be included in a thorough weather report?
- If Phoenix, Arizona experiences a cool, wet day in June (when the weather is usually hot and dry), does that mean the region's climate is changing?
- Look back at the table that shows heat index. Which day would most people find more pleasant: An 85°F day with 90% humidity or a 90°F day with 40% humidity?
- What happens when a batch of air reaches its dew point? At what temperature does this occur?
- What effect do clouds have on weather?
- You are standing in a location which is clear in the morning, but in the afternoon there are thunderstorms. There is no wind during the day, so the thunderstorms build directly above you. Describe how this happens.
- In what three ways can air rise to create clouds?
- What are the four different cloud groups and how are they classified?
- How does sleet form? How does glaze form?
- What circumstances must be present for enormous balls of hail to grow and then fall to the ground?
- Gray puffy stripes of globular clouds arranged in lines across the sky.
- Thicker clouds than cirrostratus; like a gray veil, may completely hide Sun or Moon.
- High clouds that are small, white, and puffy, arranged in groups or lines.
- Thin, whitish, veil-like clouds that produce a halo around the Sun or Moon, but do not blur their outline.
- High, wispy clouds made of ice crystals.
- Tiny water or ice particles that are grouped together in the atmosphere.
- Tall, dark clouds that produce thunderstorms.
- dew point
- The temperature at which air is saturated with water vapor, or where the air has reached 100% humidity.
- A layer of smooth, transparent ice that forms when freezing rain or drizzle hit a cold surface.
- Pellets of ice or ice and snow that form only in cumulonimbus clouds.
- heat index
- A measurement that combine the effects of temperature and humidity; the heat index more accurately describes what weather will actually feel like.
- The amount of water vapor in the air, sometimes used synonymously with relative humidity.
- Thick, dark, continuous, low clouds that brings continuous rain or snow.
- radiation fog
- Fog caused by the radiation of heat on a cold, windless night.
- relative humidity
- The amount of water vapor in the air relative to the maximum amount of water vapor that the air could contain at that temperature.
- Partly frozen rain or partly melted snow and ice.
- Soft, globular, low clouds in groups or lines that rarely bring precipitation.
- Low clouds that are continuous and may produce drizzle but no hard rain.
- upslope fog
- Fog that forms from winds that blow up a slope and cool.
Points to Consider 
- When thinking about the weather, what factors do you consider important in the air that surrounds you?
- How do air temperature, humidity, and pressure differences create different sorts of weather?
- Think about the types of weather described in this lesson. Imagine types of weather that you have not experienced, look at photos, and ask friends and relatives who've lived in other places what their weather is like.