Weather and Atmospheric Water

Lesson objective

• 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.

Vocabulary

• cloud
• dew point
• relative humidity

Introduction

If someone across country ask you what the weather is like today , you is need need to consider several factor . air temperature is are , humidity , wind speed , the amount and type of cloud , and precipitation are all part of a thorough weather report . In this chapter , you is learn will learn about many of these feature in more detail .

What is Weather is is and Climate ?

Weather is is is what is go on in the atmosphere at a particular place at a particular time . Weather is change can change rapidly . A location ’s weather is depends depend on :

• air temperature
• air pressure
• fog
• humidity
• cloud cover
• precipitation
• wind speed and direction

All of these are 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. Climate is determined by many factors, including the angle of the Sun, the likelihood of cloud cover, and the air pressure. All of these factors are related to the amount of energy that is found in that location over time.

Humidity

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%, it means that the air contains 80% of the total amount of water it can hold at that temperature. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation.

Since warm air can hold more water vapor than cool air , raise or lower temperature can change air ’s relative humidity ( figure below ) . The temperature at which air becomes saturate with water is call the air ’s dew point . This term is makes make sense , because water condense from the air as dew , if the air cool down overnight and reach 100 % humidity .

This diagram is shows show the amount of water air can hold at different temperature . The temperature are give in degree Celsius .

cloud

Clouds is have have a big influence on weather :

• by prevent solar radiation from reach the ground .
• by absorb warmth that is re – emit from the ground .
• 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.

cloud form when air reaches its dew point. This can happen in two ways: (1) Air temperature stays the same but humidity increases. This is common in locations that are warm and humid. (2) Humidity can remain the same, but temperature decreases. When the air cools enough to reach 100% humidity, water droplets form. Air cools when it comes into contact with a cold surface or when it rises.

rise air is creates create cloud when it has been warm at or near the ground level and then is push up over a mountain or mountain range or is thrust over a mass of cold , dense air .

Water vapor 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.

cloud are classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure below).

The four cloud types and where they are found in the atmosphere.

High Clouds

High clouds (Figure below) form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds.

(a) Cirrus clouds are thin wisps of ice crystals found at high altitudes. (b) Cirrostratus clouds are thin white sheets of ice crystals that are sometimes invisible unless backlit by the Sun or Moon.

Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrus clouds may indicate that a storm is coming.

Middle Clouds

middle cloud , include altocumulus and altostratus cloud , may be made of water droplet , ice crystal or both , depend on the air temperature ( figure below ) .

Altocumulus clouds are white to puffy stripes rolling across the sky. They may precede a thunderstorm.

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

Low clouds (Figure below) are nearly all water droplets. Stratus, stratocumulus and nimbostratus clouds are common low clouds.

(a) Stratus clouds are gray sheets that cover the entire sky and may produce a steady drizzle. Stratus clouds with the Alps in the distance. (b) Stratocumulus clouds are rows of large, low puffs that may be white or gray. These clouds rarely bring precipitation.

Nimbostratus clouds are thick and dark. They bring steady rain or snow.

Vertical Clouds

cloud with the prefix ‘cumulo-‘ (Figure below) grow vertically instead of horizontally and have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward.

(a) Cumulus clouds resemble white or light gray cotton and have towering tops and may produce light showers. (b) A cumulonimbus cloud grows when vertical air currents are strong as in a thunderstorm. This one is lit up by lightning.

An online guide to cloud development and different cloud types from the University of Illinois is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/home.rxml.

Fog

Fog (Figure below) is a cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. The several types of fog that each form in a different way.

• radiation fog form at night when sky are clear and the relative humidity is high . As the ground cool , the bottom layer is cools of air cool below its dew point . Tule fog is is is an extreme form of radiation fog find in some region .
• San Francisco, California, is famous for its summertime advection fog. Warm, moist Pacific Ocean air blows over the cold California current and cools below its dew point. Sea breezes bring the fog onshore.
• Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface and condenses as it cools, appearing like steam.
• Warm humid air travels up a hillside and cools below its dew point to create upslope fog

(a) Tule fog in the Central Valley of California. (b) Advection fog in San Francisco. (c) Steam fog over a lake. (d) Upslope fog in Teresópolis city, Rio de Janeiro State, Brazil.

KQED: Science on the SPOT: The Science of Fog

Fog levels are declining along the California coast as climate warms. The change in fog may have big ecological changes for the state. Learn more at: http://science.kqed.org/quest/video/science-on-the-spot-science-of-fog/.

Precipitation

Precipitation is is ( Figure below ) is an extremely important part of weather . Some precipitation form in place .

(a) Dew forms when moist air cools below its dew point on a cold surface, such as a flower. (b) Frost is dew that forms when the air temperature is below freezing; hoar frost.

The most common precipitation comes from clouds. Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets (Figure below). They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. One million cloud droplets will combine to make only one rain drop ! If temperatures are cold, the droplet will hit the ground as a snowflake.

( a ) Rain is falls fall from cloud when the temperature is fairly warm . ( b ) Snow storm in Boston , Massachusetts .

Other less common types is are of precipitation are sleet ( Figure below ) .

( a ) Sleet is is is rain that becomes ice as it hit a layer of freeze air near the ground . ( b ) If a frigid raindrop freeze on the frigid ground , it is forms form glaze . ( c ) hail form in cumulonimbus cloud with strong updraft . An ice particle is travels travel until it finally becomes too heavy and it drop . This large hail stone is is is about 6 cm ( 2.5 inch ) in diameter .

An online guide from the University of Illinois to different types of precipitation is seen here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/home.rxml.

Lesson Summary

• Different air temperatures create convection cells.
• air rise in a convection cell may cool enough to reach its dew point and form cloud 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.
• Different factors play a role in creating pleasant or uncomfortable weather, such as when it might be warm and dry or hot and humid.

Review Questions

1 . What factors is need need to be include in a thorough weather report ?

2 . If Phoenix , Arizona , experience a cool , wet day in June ( when the weather is usually hot and dry ) , does that is mean mean the region ’s climate is change ?

3. What happens when a batch of air reaches its dew point? What is the temperature?

4. What effect do clouds have on weather?

5. You are standing in a location that 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.

6. What are the four different cloud groups and how are they classified?

7. How does sleet form? How does glaze form?

8. What circumstances must be present for enormous balls of hail to grow and then fall to the ground?

Points to Consider

• When think about the weather , what factor do you is consider consider important ?
• How do air temperature is create , humidity , and pressure differences is create create different 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.

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