One of the first things you probably do every morning is look out the window to see what the weather is like. Looking outside and listening to the day’s weather forecast helps you decide what clothes you will wear and maybe even what you will do throughout the day. This tells the importance of weather in our everyday lives. In this lesson, we will get a deeper understanding of the following:
People often confuse weather with climate, but they are not the same, though they share common components.
| Weather | Climate |
| It represents the daily changes in the atmosphere or the state of the atmosphere of any place for a short period of time with respect to one or more of its elements. | It represents the combination of multiple weather patterns of a specific location averaged over several years. For example, Greenland has a cold desert climate and the climate of Central Asia is temperate continental. |
| Two places even a short distance apart may have different kinds of weather at one and the same time. | The climate of a region is considered more or less permanent. |
| In some locations, weather changes daily or hourly. | Climate doesn’t change as rapidly as weather because it’s a compilation of several years of recorded weather conditions. |
Both weather and climate share common elements including wind speed and direction, type of rain and amounts, humidity levels, air pressure, cloud cover, and cloud types, and air temperature. Due to reckless human interventions both weather and climate are changing.
On any given day, the weather dictates what you wear. For example, you look outside and see it’s a bright and sunny day, so you wear something light; or if it is rainy you will take an umbrella before you step outside. Daily weather reports also play a crucial role in informing us about the incoming severe weather conditions, if any.
Weather can be sunny, rainy, cloudy, windy, snowy or clear. It is part of the natural phenomenon which maintains the equilibrium in the atmosphere.
Weather varies according to the altitudes, latitudes and region and pressure differences. When the atmospheric conditions are extreme or intense enough to cause property loss or life loss, such weather is termed as severe weather. Severe weather, such as tornadoes, hurricanes, and blizzards, can disrupt many people’s lives because of the destruction they cause.
There are six main elements or components of weather
Together, these elements form the weather of a place at any given time. The scientists who study weather are called 'meteorologists' - they forecast the weather based on the knowledge of atmospheric processes and the changing elements.
Let’s look at these six elements in greater detail.
Temperature measures how hot or cold the atmosphere is on a day-to-day basis. The temperature is dependent on the angle of the sun; hence it may change repeatedly in a day. Temperature is measured with a thermometer and is reported in two ways: Celsius and Fahrenheit. The coldest weather usually happens near the poles, while the warmest weather usually happens near the Equator.
Atmospheric pressure is the weight of the air in the atmosphere. The rise of warm air and the descent of cold air results in changes in atmospheric pressure. Atmospheric pressure occurs mostly in regions near water bodies. As coastal regions and islands are near to water bodies, they frequently experience severe storms.
Atmospheric pressure is expressed in a unit of measurement called an atmosphere and is measured in millibars or inches of mercury. The average atmospheric pressure at sea level is about one atmosphere (about 1013 millibars or 29.9 inches).
Atmospheric pressures changes with altitude. It is higher at lower altitudes and is lower at higher altitudes.
Wind is air in motion. It is produced by the uneven heating of the earth’s surface by the sun. Since the earth’s surface is made of various land and water formations, it absorbs the sun’s radiations unevenly. Two factors are necessary to specify wind: speed and direction.
| Wind direction is described by using the direction that the wind came from. For example, a southerly wind would blow from the south to the north. Wind direction is measured in a number of ways using weather vanes, flags, and windsocks. |
| Wind speed is measured in miles per hour or kilometers per hour. Anemometer is the tool used to measure the speed of the wind. |
As the sun warms the Earth’s surface, the atmosphere warms too. Some parts of the Earth receive direct rays from the sun all year and are always warm. Other places receive indirect rays, so the climate is colder. Warm air which weighs less than cold air rises. Then cool air moves in and replaces the rising warm air. This movement of air is what makes the wind blow.
Humidity refers to the amount of water vapor in the air. Water vapor makes up only a small fraction of the mass of the atmosphere. However, this small amount of water vapor has an important effect on weather and climate. When the sun’s energy heats up Earth’s surface, water in oceans and water bodies evaporates. Water vapor is a gas in the atmosphere that helps make clouds, rain, and snow.
The amount of water in the air is described using relative humidity. Warmer air holds more water vapor than cool air. If the amount of water vapor in the air stays the same, but the temperature goes down, the relative humidity will increase. This is because the colder air cannot hold as much water vapor. If the temperature gets cold enough, the air gets to the point that it is holding the most water vapor it can hold. The relative humidity for this temperature would be 100 percent. This is also known as the dew point temperature. Excess water falls down as precipitation.
On cooler nights, when the temperature gets down to the dew point, some of the water vapor turns back to liquid water (this is called condensation) and settles as ‘dew’ on the grass and glass windows.
A cloud is a group of millions of tiny water droplets or ice crystals. Clouds form as air rises and cools. When air cools below the dew point, water droplets or ice crystals form. Water droplets form when water condenses above 0°C. Ice crystals form when water condenses below 0ºC. Not all clouds produce precipitation. clouds usually signal mild weather.
The liquid and solid water particles that fall from clouds and reach the ground are known as precipitation. It is a very common phenomenon in the Earth’s atmosphere. Precipitation always comes from clouds but not all clouds form precipitation. This is because the water droplets and ice crystals found in most clouds are too small, and thus not heavy enough to fall to the surface of the Earth. A raindrop large enough to have the weight needed to fall to Earth is millions of times larger than the individual water droplets found inside most clouds.
There are four main kinds of precipitation – rain, snow, sleet, and hail. Rain and snow are the most common kinds of precipitation. Sleet and hail are less common.
| Rain |
Liquid water droplets that are 0.5 or larger and fall from the clouds in the sky are called rain. Rain often takes one of two main forms – showers and drizzles.
Small cloud particles strike and bind together creating bigger drops. As this process continues, the drops get bigger and bigger to an extent where they become too heavy to suspend in the air. As a result, gravity pulls them down to the earth. This is how raindrops fall. When high in the air, the raindrops start falling as ice crystals or snow but melt when they proceed down the earth through the warmer air. |
| Sleet | Sleet forms when rain falls through a layer of very cold air. If the air is cold enough, the rain freezes in the air and becomes falling ice. Sleet is also known as ice pellets, as it is composed of small and semitransparent balls of ice. |
| Hail | Hailstones are big and irregular lumps of ice that fall from large thunderstorms. It is solid precipitation. Hail forms in cumulonimbus clouds. As opposed to sleets that can form in any weather when there are thunderstorms, hailstones are predominantly experienced in the winter or cold weather. Hailstones are mostly made up of water ice and measure between 0.2 inches (5 millimeters) and 6 inches (15 centimeters) in diameter. They are highly damaging to crops. |
| Snow | Snow forms when temperatures are so low that water vapor turns directly into a solid. It occurs almost every time there is rain. However, snow often melts before it reaches the earth's surface. It is normally seen together with high, thin and weak cirrus clouds. Snow can fall as single ice crystals. In many cases, the crystals join together to form larger snowflakes. Snowflakes occur in sub-freezing temperatures. |
Air masses
An air mass is a very large volume of air that has a relatively steady temperature and moisture content. Air masses typically cover areas ranging from hundreds of thousands to millions of square miles.
Air masses form when a body of air comes to rest over an area that has consistent surface features. These are called source regions which are simply geographic areas with flat uniform surface composition with light surface winds where an air mass originates. For example, deserts, plains, and oceans typically cover very wide areas with relatively few topographical variations – these are source regions. These areas provide a stable atmosphere in which high winds are absent. In such areas, large masses of air can accumulate without being broken apart by mountains, land/water intersections, or other surface features.
The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
There are 4 general air masses classified according to the source region:
| Polar latitudes P | located poleward of 60 degrees north and south |
| Tropical latitudes T | located within about 25 degrees of the equator |
| Continental c | located over large landmasses – dry |
| Marine m | located over the oceans – moist |
We can then make combinations of the above to describe various types of air masses.
Cold air masses – Most of the cold winter weather in the United States comes from three polar air masses:
Warm air masses – Four warm air masses influence the weather in the United States.
On maps, meteorologists use two-letter symbols to represent different air masses. The first letter indicates the water content of the air mass. The second letter indicates its temperature.
Air masses can control the weather for a relatively long time period: from a period of days to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.
Front
The boundary at which two air masses of different temperature and moisture content meet is called a front. When air masses meet, the less dense air mass rises over the denser air mass. Warm air is less dense than cold air. Therefore, a warm air mass will generally rise above a cold air mass.
There are four main kinds of fronts:
| Cold fronts | A cold front forms when a cold air mass moves under a warm air mass. The cold air pushes the warm air mass up. The cold air mass replaced the warm air mass. Cold fronts can move quickly and bring heavy precipitation. When a cold front has passed, the weather is usually cooler. This is because a cold, dry air mass moves in behind the cold front. |
| Warm fronts | A warm front forms when a warm air mass moves in over a cold air mass that is leaving an area. The warm air replaces the cold air as the cold air moves away. Warm fronts can bring light rain. They are followed by clear, warm weather. |
| Occluded fronts | An occluded front forms when a warm air mass is trapped between two cold air masses. The cold air masses move together and push the warm air out of the way. Occluded fronts bring cool temperatures and large amounts of rain and snow. |
| Stationer fronts | A stationary front forms when a cold air mass and a warm air mass move toward each other. Neither air mass has enough energy to push the other out of the way. Therefore, the two air masses remain in the same place. Stationary fronts cause many days of cloudy, wet weather. |
Air produces pressure. However, air pressure is not always the same everywhere. Areas with different pressures can cause changes in the weather. These areas may have lower or higher air pressure than their surroundings.
| Cyclone | Anticyclone |
| A cyclone is a system of winds that rotates around a center of low atmospheric pressure. Cyclones are commonly known as lows. They are generally indicators of rain, clouds and other forms of bad weather. Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. | An anticyclone is a system of winds that rotates around a center of high atmospheric pressure. Anticyclones are commonly known as highs. Generally, they are predictors of fair weather. Winds in an anticyclone blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. |
Vertical air movements are associated with both cyclones and anticyclones. In cyclones, air close to the ground is forced inward toward the center of the cyclone, where the pressure is lowest. It then begins to rise upward, expanding and cooling in the process. This cooling increases the humidity of the rising air, which results in cloudiness and high humidity in the cyclone. In anticyclones, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there. That air is replaced in the center by a downward draft of air from higher altitudes. As this air moves downward, it is compressed and warmed. This warming reduces the humidity of the descending air, which results in a few clouds and low humidity in the anticyclone.
A thunderstorm is an intense storm with strong winds, heavy rain, lightning, and thunder. It is produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. The basic conditions necessary for a thunderstorm to form are - moisture, unstable air and lift. The atmosphere is unstable when a body of cold air is found above a body of warm air. The warm air rises and cools as it mixes with the cool air. When the warm air reaches its dew point, the water vapor condenses and forms cumulus clouds. If the warm air keeps rising, the clouds may become dark cumulonimbus clouds. Thunderstorms can occur year-round and at all hours. But they are most likely to happen in the spring and summer months and during the afternoon and evening hours.
Lightning is a bright flash of electricity produced by a thunderstorm. All thunderstorms produce lightning and are very dangerous. As a cloud grows bigger, parts of it begin to develop electrical charges. The upper parts of the cloud tend to become positively charged. The lower parts tend to become negatively charged. When the charges get big enough, electricity flows from one area to the other. Electricity may also flow between the clouds and the ground. These electrical currents are lightning. If you hear the sound of thunder, then you are in danger from lightning.
Lightning is most likely to hit tall objects, including trees, mountains, and people – anything that stands up from the ground.
Less than 1% of thunderstorms produce tornadoes. Tornadoes are violent columns of wind that spin very fast on touching the ground. The rapidly spinning column of air before touching the ground is called a funnel cloud. They extend from the bottom of thunderstorms to the ground and can have winds of up to 300 miles per hour. Tornadoes are smaller than hurricanes and form over land rather than the sea. They get their energy from large thunderstorms. Tornadoes that form over water are called waterspouts. The air in the center of a tornado has low pressure. When the area of low pressure touches the ground, material from the ground can be sucked up into the tornado.
Cyclones that form over warm tropical oceans are called tropical cyclones. They are also known as tropical storms or tropical depressions.
A tropical cyclone that drastically increases in intensity is known as a hurricane when it occurs in the Atlantic Ocean or adjacent seas. In the western Pacific Ocean and adjacent seas, a hurricane is known as a typhoon. To be classified as a hurricane, a tropical cyclone must produce winds over 74 miles per hour. Most hurricanes form between 5°N and 20°N latitude or between 5°S and 20°S latitude. They form over the warm, tropical oceans found at these latitudes. At higher latitudes, the water is too cold for hurricanes to form.
The rotation of the Earth causes an interesting phenomenon on free-moving objects on the Earth. Objects in the Northern Hemisphere are deflected to the right, while objects in the Southern Hemisphere are deflected to the left. The Coriolis effect thus tries to force winds to shift towards the right or left. A hurricane begins as a group of thunderstorms traveling over tropical ocean waters. Winds traveling in two different directions meet and cause the storm to spin. Because of the Coriolis effect, hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Hurricanes are powered by solar energy. The sun’s energy causes ocean water to evaporate. As the water vapor rises in the air, it cools and condenses.
At the center of the hurricane is the eye. The eye is a core of warm, relatively calm air with low pressure and light winds. There are updrafts and downdrafts in the eye. An updraft is a current of rising air. A downdraft is a current of sinking air.
Around the eye is a group of cumulonimbus clouds called the eyewall. These clouds produce heavy rains and strong winds. The winds can be up to 300 km/h. The eyewall is the strongest part of the hurricane. Outside the eyewall are the spiraling bands of clouds called rain bands. These bands also produce heavy rain and strong wind. They circle the center of the hurricane.
The hurricane will continue to grow as long as it is over warm ocean water. When the hurricane moves over colder waters or over land, the storm loses energy. This is why hurricanes are not common in the middle of continents. The storms lose their energy quickly when they move over land. Hurricanes bring high winds, heavy rain, floods, and a storm surge from the ocean that can cause terrible devastation.
The prediction of weather conditions over the next few days is known as a weather forecast. Meteorologists make weather forecasts using information on atmospheric conditions. They use a wide variety of different instruments to measure weather conditions.
There are different types of weather maps:
