The fact that we have weather at all is dependent on the existence and nature of Earth’s atmosphere. A unique feature of the blanket of air that envelopes our planet is the presence of water vapor, and a temperature profile that makes it possible for this water to exist in three forms: gas, liquid, and solid.

The term weather technically refers to the daily variation of conditions in our atmosphere. What we experience on the ground can be measured by recording temperature, humidity, windspeed, precipitation type and amount, and other aspects.

The sun is the source of these variable atmospheric conditions. The sun affects our atmosphere in different ways depending on global location. Land areas heat up (and cool down) more rapidly than oceans. Regions located at the equator get more intense solar radiation than areas at Earth’s poles.

Earth’s atmosphere is a huge complex system, and like most complex systems, it tries to maintain its own equilibrium. Warm air from the equator is funneled outward towards the cold polar regions. But air movements are also affected by friction with land and the Earth’s rotation. The whole system is confined to a relatively narrow space by Earth’s gravity.

The combination of irregular heating, Earth’s attempt to balance out the irregularities, the effect of geography, and the forces of rotation and gravity produce incredibly complex patterns of high and low air pressure. These areas of high and low pressure interact with each other and with the ground to produce ever-changing and sometimes unpredictable weather conditions.

Countries almost the world invest in weather research, thinking that the dividends of accurate forecasts will far outweigh the costs. The more industrialized nations of North America, Europe, Asia, and Australasia have put together highly advanced weather services. Other nations with a big stake in weather include South Africa and China. Saudi Arabia has also invested heavily in weather forecasting research in hopes of increasing the amount of arable land.

The Earth’s weather is determined by a huge, interconnected, interrelated system. Therefore, accurate observation and prediction demands international cooperation. As weather systems charge (or meander) around the world, forecasting in 1 region is dependent on knowing current conditions in adjacent regions.

In recognition of the need to work together, the World Meteorological Organization (WMO) was created. The WMO has been one of the world’s most successful and fruitful international organizations. Member nations have been maintaining contact and sharing weather information since the early 1950s. This level of cooperation has given forecasters daily access to a huge global weather database.

Other factors that are improving daily forecasts include an improved understanding of the physics involved, real-time information gathered from weather satellites, and computing power that has grown to handle mathematical simulations of complex weather system processes. The computerized study of weather systems has also been enhanced by a virtual tsunami of weather observations from reporting stations, satellites, aircraft and ships.

The amount of detail contained in a weather forecast depends on the range of the forecast–that is, how far into the future forecasters are attempting to gaze. Short-range forecasts cover 24 to 48 hours and are highly detailed. Medium-range forecasts look at conditions from 48 hours to 10 days ahead. They are more general, but still include temperature and precipitation estimates. Long-range forecasts covering from 10 days to up to 3 months usually confine themselves to predicting departures from average expectations.

Forecasts have become increasingly accurate, but there is an element of chaos in global weather. Current short-term forecasts have achieved accuracy rates up to 85 percent. As technology and knowledge continue to advance, we may begin to approach these accuracy standards in medium-term forecasts. But without any measure of weather control, long-term forecast accuracy may never be achieved.

It has been stated elsewhere in this series of articles on weather that if the Earth had no atmosphere, then we would have no weather. Just for the sake of argument, imagine an Earth with an atmosphere like we have today, but without a sun. If there were no sun, there would be no weather (and not just because the atmosphere would likely be frozen solid). It is the interaction of the sun’s heat and our atmosphere that forms air masses, causes circulation patterns to appear. Air movements create pressure differentials which ultimately result in wind.

The air in Earth’s atmosphere, like all matter, is composed of molecules that move about constantly. Where the concentration of air molecules is denser, the air pressure is greater. Air molecules are subject to gravity, so the air pressure is best nearest the Earth’s surface, and decreases with altitude. At sea level, the atmosphere exerts about 14 and a half pounds of pressure on every square inch, but we are generally unaware of it.

Air pressure is measured in hectopascals or millibars. Typical air pressure measurements range from 980 to 1040 hectopascals, depending on local conditions.

These constantly moving air molecules move at different speeds depending on temperature (warmer results in faster movement, cooler in slower). When a mass of air is heated and the molecules speed up, the air mass expands. Expansion causes the density of the molecules to decrease, which makes the air mass lighter. The now lighter air mass begins to rise. This entire process is called convection.

Convection is taking place constantly in the atmosphere almost the world. But it doesn’t happen uniformly or regularly. Many local factors affect whether convection occurs, and if so, how much.

Air cools as it rises. So rising warm air masses eventually cool off and start to sink back toward the ground. Where air is rising, low pressure occurs. Where it is sinking, high pressure occurs. Since the atmosphere “prefers” equilibrium, air from high pressure areas move toward low pressure areas. This movement of air is wind. The greater the difference between the high and low pressure areas, the stronger the winds it will create.

As air rises and creates a low pressure area, water vapor usually condenses to form clouds. But falling air in high pressure areas usually does not enable condensation. This all means that low pressure areas usually have clouds and wet weather, while high pressure areas usually have clear skies with sun.

Weather is what results when a huge complex system (the Earth’s atmosphere) reacts to the effects of an outside source (the sun), while encountering random factors (friction with geography) and limiting factors (gravity).

Climate supplies a long-term view of average weather conditions. What is the weather likely to be in a sure place? Describing climate involves observation of average conditions over a long period of time (30 years or more) while also taking into account the extremes that can be experienced.

Describing the climate of an area includes information about temperature, rainfall, humidity, cloud cover, hours of sunshine, wind speed and direction, and more.

In the same way that we desire to predict the weather, we also are seeking to learn how to predict possible changes in climate. It appears that climates can and do change, but the time scales are much longer than changes in weather.

Weather records in some countries go back as far as 2 centuries, and many countries have reliable weather observations covering more than 100 years. Although this historical information helps us begin to understand climates and climate change, its value is limited when we begin to consider changes that may take area over thousands of years.

Recently developed techniques have given us new insight into historical weather conditions. Natural features like tree rings, ice cores, and coral can be examined to read natural recordings of weather conditions. It is extra-long-term information like this that suggests climate is not unchanging.

As human technology as progressed, we have also introduced another variable into the atmospheric weather system–ourselves. Statistics show that our own activities have an impact on weather and climate. The release of pollutants–both gases and particles–changes the atmosphere and alters the weather. But can these human activities also alter climate? Studies are ongoing. Perhaps only time will tell.

Without the atmosphere that envelopes the Earth, we would not have weather. Indeed, we would roughly certainly not exist. The atmosphere contains oxygen that is necessary for life. It also contains water vapor, which is a crucial component of weather.

Earth’s atmosphere is a protective layer that also happens to be surprisingly thin. The atmosphere is made up of distinct layers. The existence of the first atmospheric layer was discovered in 1899. Research since then has identified five distinct layers that exist between the ground & outer space. All five atmospheric layers have an effect on weather and climate, but 99% of our weather occurs in the first layer, the troposphere.

The troposhere begins at ground level and extends from 5 to ten miles high. It is lowest at the poles and highest at the equator. As you rise through the troposphere, the air temperature falls about 4 degrees Fahrenheit for every 1,000 feet of altitude. The temperature stops decreasing at the boundary to the next atmospheric level. The boundary is called the tropopause. Temperatures at the tropopause can be as low as -70 degrees Fahrenheit.

The next atmospheric layer, the stratosphere, begins at the tropopause and reaches to about 30 miles above ground level. Air temperatures in the stratosphere increase to about 40 degrees Fahrenheit. The ozone layer lies about 15 miles above the Earth’s surface, which puts it inside the stratosphere.

The mesosphere is the next atmospheric level above the stratosphere. Temperatures fall again in the mesosphere, dropping to as low as -130 degrees Fahrenheit. The mesosphere extends to about 50 miles above ground level.

Above the mesosphere lies the thermosphere (sometimes called the ionosphere). Temperatures in the thermosphere skyrocket to as high as two,700 degrees Fahrenheit. The thermosphere provides much of our protection from space debris. Meteors, satellites, and any other objects falling from space rarely survive the high temperatures of the thermosphere.

The highest atmospheric level is known as the exosphere. The exosphere contains several different kinds of gases. However, gravity is so weak at this level that molecules can escape into space, so the quantities are very small.

To sum up, most weather occurs in the troposphere, but the entire system of atmospheric layers–troposphere, stratosphere, mesosphere, thermosphere and exosphere–functions together to make life possible on earth.

Clouds are produced when condensation occurs above ground level. If the surrounding air temperature is above freezing, water vapor condenses into droplets of water. If the air temperature is below freezing, water vapor may change directly (sublimate) into ice crystals. In some cases, water vapor can remain as a liquid in a supercooled state below freezing.

Because temperature normally decreases with altitude, high level clouds tend to be ice crystal clouds, low level clouds tend to be water droplet clouds, & middle level clouds are often a combination. Clouds are made up of hundreds of millions of droplet and/or ice crystals. The type of cloud that forms depends on a variety of factors.

An air mass can hold less and less water as it rises and cools. At some altitude, the air mass will eventually reach its dewpoint and condensation will begin. The more moisture there is in an air mass to begin with, the lower the altitude at which the dewpoint will be reached.

There are three processes that cause air masses to rise. The first of these is convection. When surfaces are heated by the sun, the air above these surfaces is also heated and begins to rise. Some surfaces like soil, sand and pavement are more efficient and heat up faster. Wherever these warm parcels of air begin to rise, they eventually reach their dewpoint and form clouds.

Air also rises when fronts collide. Warm air is forced over the cold air front. If there is enough moisture in the warm air, clouds will form.

Air is also lifted when it encounters geographic features like mountain ranges. This is called orographic lifting. Again, if the lifted air contains enough moisture, clouds will form. Orographic lifting often produces unusual cloud shapes.

An air mass rises as long as it is warmer than surrounding air. If it keeps rising, then the conditions are unstable. Stable conditions occur with equilibrium is reached quickly. Generally, if cold air exists in the upper atmosphere over warm air, conditions are unstable. Stable conditions usually result when warm air is over cold air.

Technically speaking, clouds are the evidence of rising air masses and condensing water vapor. Weather forecasters and observers can deduce current and future weather events based on cloud shapes. Aesthetically speaking, the infinite variety and beauty of clouds have delighted us since the beginning of man. Surprising, a system for classifying clouds did was not developed until the early 19th century.

In 1803, English scientist Luke Howard presented a classification scheme to his local scientific society. Using the language of science–Latin–Howard classified the basic cloud shapes. His system allowed for words to be combined which enabled further classification. His system was immediately accepted and universally admired.

Today’s classification system begins with two basic types: cumuliform and stratiform. Cumuliform is taken from the Latin word cumulus, which means heap. Cumuliform clouds are puffy clouds. This type is usually formed from convection or orographic lifting.

Stratiform is taken from the Latin word stratus, which mean layer. Stratiform clouds have a flat, layered shape. Stratiform clouds result from uniform lifting usually associated with frontal systems. Stratiform clouds are generally accompanied by stable weather.

These two basic types can be further classified by the height at which they form. High-level clouds (above 16,500 feet) are called cirrus clouds. Sometimes the prefix cirro- is added to a basic cloud type. Clouds forming between six,500 and 16,500 feet take the prefix alto-. Clouds forming below Six,500 feet have no prefix, so the basic types (stratus and cumulus) refer to low level clouds by default. By combining height prefixes with basic types, you can describe several types of clouds. For example, altostratus refers to a layer cloud at mid-level altitudes.

The familiar cumulonimbus cloud is in a category by itself, because it extends from low to high altitudes. The word nimbus means rain-bearing.

Cloud classification is not an exact science. But since clouds are infinitely and constantly variable, a flexible cloud classification system may actually work most outstanding.

Weather results from the complex and never-ending interaction of the sun’s heat and the Earth’s atmosphere. Weather systems and storm features can often produce astounding colors in the sky. Yet, the atmosphere itself is colorless. How are these astonishingly varied colors produced?

The colors we see in the atmosphere result from particles suspended in the atmosphere that scatter, refract and diffract the sunlight. The visible light of the sun contains a mixture of all the colors–red, orange, yellow, green, blue, indigo and violet. It travels throughout the solar system in straight waves.

When sunlight enters our atmosphere, dust particles and air molecules scatter the light waves in different directions. Violet and blue waves have shorter wavelengths and are therefore scattered more effectively than the longer orange and red waves. This results in a mixture of violet, blue and green waves scattered across the sky along with only tiny amounts of the other colors. This mixture combines to form blue. The exact shade of blue depends on how much water vapor and dust particles are in the air. More droplets and particles yield a paler blue sky.

Clouds are white because they are made up of water droplets that scatter all the colors of the spectrum. This means that the original white light of the sun is reconstituted. However, if clouds are thick and don’t allow all the light to pass through, then the clouds appear gray or black.

The sky frequently changes color during sunrises and sunsets. This is because as the sun is lower in the sky, light travels farther through atmosphere allowing more red and orange waves to be scattered. Pollution, dust and ash can enhance the red and orange hues of setting and rising suns.

Rainbows are produced when light is refracted (bent) by airborne water droplets. Each color exits the droplets at a slightly different angle. But the angles are precisely the same for each of the millions of droplets. Therefore, viewers of rainbows see distinct color bands.

The heat of the sun & the Earth’s atmosphere interact on a continuous basis to produce weather. Uneven heating causes a complex system of wind flows. There are three major circulations of air that result from solar heating of the atmosphere.

Hadley cells (named after scientist George Hadley) refer to circulations of air taking place in and near the tropics. Air in the tropics (between 23.5 degrees latitude north and south) is heated and rises. It spreads out when it encounters the tropopause and much of this air falls back toward the ground at about 30 degrees north and south. It displaces air when it sinks, and the displaced air flows back to the equatorial regions to complete the cycle.

Similar circulations occur between 30 and 60 degrees north and south (Ferrel cells) and in the polar regions (polar Hadley cells).

Interestingly, these air flows don’t move in a straight north-south direction. The rotation of the Earth results in what is called the Coriolis effect. Any freely moving object or fluid appears to turn right to the direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect is named for Gustave-Gaspard de Coriolis who first identified it in 1835.

Air tends to flow toward an place of low pressure, but the Coriolis effect deflects the air. It ends up establishing an equilibrium by creating a circular movement, or a cyclonic flow. Air flows roughly low pressure areas in a counter-clockwise direction in the Northern Hemisphere, and a clockwise direction in the Southern Hemisphere. Air flow around high pressure systems is the opposite of this.

If Earth did not rotate, then air would quickly flow along straight lines and air pressure would be quickly equalized. The Coriolis effect is exactly zero at the equator. Therefore, cyclones rarely form on the equator and rarely travel towards the equator. The Coriolis effect is best at the poles.

Heat from the sun is constantly interacting with the Earth’s atmosphere. The irregular heating of the atmosphere coupled with the atmosphere’s “desire” to maintain equilibrium results in the movement of large air masses, leading to different types of weather.

When an air mass moves into a region, it encounters the air mass that is already there. The boundary between these different air masses is called a front. When cold air is replacing warm air, it is known as a cold front. A warm front occurs when warm air meets cold air.

When a warm front encounters a cold air mass, it rides up over the top of the colder air. As it rises, it cools, & clouds are formed from condensation of water vapor. High cirrus clouds appear first, followed by mid-level clouds. Thick stratus clouds come next, and they may produce wind and precipitation.

Weather produced by cold fronts is often more volatile. When a cold front meets a warm air mass, it forces the warm air sharply upward. This strong upward movement produces instability and convection. Large cumulus clouds form and storms are triggered along the front. The quickly rising air also creates an location of low pressure, strengthening the winds. Heavy rain and strong winds will accompany the actual front, with showers persisting after frontal passage.

Sometimes a normally slower moving warm front is overtaken by a cold front. When this happens, the warm front is pushed aloft. The two fronts continue to move together and the line between them is called an occluded front. Occluded fronts are usually accompanied by stratus clouds and light precipitation.

If two different air masses meet, but neither is strong enough to replace the other, a stationary front is formed. Weather near stationary fronts is usually cloudy with long periods of precipitation. Stationary fronts may dissipate after several days or may begin moving as a warm or cold front. Stationary fronts are more likely in the summer.