Weather is the day-to-day conditions of the atmosphere at a particular place. It includes temperature, precipitation, cloud cover and wind speed. Climate is the average weather conditions of a place taken over a long period of time, typically 30 years.

Weather

The term “weather” refers to the temporary conditions of the atmosphere, the layer of air that surrounds the Earth. We usually think of weather in terms of the state of the atmosphere in our own part of the world. But weather works like dropping a pebble in water—the ripples eventually affect water far away from where the pebble was dropped. The same happens with weather around the globe. Weather in your region will eventually affect the weather hundreds or thousands of kilometers away.

Weather doesn’t just stay in one place. It moves, and changes from hour to hour or day to day. Over many years, certain conditions become familiar weather in an area. The average weather in a specific region, as well as its variations and extremes over many years, is called climate.

Climate changes, just like weather. However, climate change can take hundreds or even thousands of years. Today, the Sahara Desert in northern Africa is the largest desert in the world. However, several thousand years ago, the climate in the Sahara was quite different. This “Green Sahara” experienced frequent rainy weather.

What Makes Weather

There are six main components, or parts, of weather. They are temperature, atmospheric pressure, wind, humidity, sunshine, precipitation, and cloudiness. Together, these components describe the weather at any given time. These changing components, along with the knowledge of atmospheric processes, help meteorologists scientists who study weather forecast what the weather will be in the near future.

Temperature is measured with a thermometer and refers to how hot or cold the atmosphere is. Meteorologists report temperature two ways: in Celsius (C) and Fahrenheit (F). The United States uses the Fahrenheit system; in other parts of the world, Celsius is used. Almost all scientists measure temperature using the Celsius scale.

Temperature is a relative measurement. An afternoon at 70 degrees Fahrenheit, for example, would seem cool after several days of 95 degrees Fahrenheit, but it would seem warm after temperatures around 32 degrees Fahrenheit. The coldest weather usually happens near the poles, while the warmest weather usually happens near the Equator.

When discussing the temperature of various areas, it is important to understand the following terms:

• Diurnal temperature range: This is the difference between the highest and lowest temperature in a day. It is also known as the daily temperature range.
• Annual temperature range: This is the difference between the average temperature of the hottest and coldest months in the year.

Sunshine: Sunshine is the term used to refer to direct sunlight. It is measured in hours. The instrument used to determine the amount of sunshine experienced in an area in a day is a Campbell Stokes Sunshine Recorder (seen below).

In the photo above, notice the card which is under the glass sphere. The sphere focuses the rays of the Sun onto a point on this card, burning a small hole at that point. As the Sun moves across the sky its rays are focused on different parts of the card. At the end of the day, the card can be examined to determine the amount of sunshine the area experienced on that day. An entire day of sunshine will create one long, unbroken burn on the card. A day with many cloudy periods will create several small burns with spaces in between them. The spaces represent periods when there was no direct sunlight.

Atmospheric pressure is the weight of the atmosphere overhead. Changes in atmospheric pressure signal shifts in the weather. A high-pressure system usually brings cool temperatures and clear skies. A low-pressure system can bring warmer weather, storms, and rain.

Meteorologists express atmospheric pressure in a unit of measurement called millibars (mb) and is measured using a barometer.

The pressure indicates  millibars or inches of mercury. Average atmospheric pressure at sea level is about one atmosphere (about 1,013 millibars, or 29.9 inches). An average low-pressure system, or cyclone, measures about 995 millibars (29.4 inches). A typical high-pressure system, or anticyclone, usually reaches 1,030 millibars (30.4 inches). The word “cyclone” refers to air that rotates in a circle, like a wheel.

Atmospheric pressure changes with <altitude. The atmospheric pressure is much lower at high altitudes. The air pressure on top of Mount Kilimanjaro, Tanzania—which is 5,895 meters (19,344 feet) tall—is 40 percent of the air pressure at sea level. The weather is much colder. The weather at the base of Mount Kilimanjaro is tropical, but the top of the mountain has ice and snow.

Wind is the movement of air. Wind forms because of differences in temperature and atmospheric pressure between nearby regions. Winds tend to blow from areas of high pressure, where it’s colder, to areas of low pressure, where it’s warmer. Wind speed is measured in km/hr. both wind speed and direction is measured using an anemometer.

In the upper atmosphere, strong, fast winds called jet streams occur at altitudes of 8 to 15 kilometers (5 to 9 miles) above the Earth. They usually blow from about 129 to 225 kilometers per hour (80 to 140 miles per hour), but they can reach more than 443 kilometers per hour (275 miles per hour). These upper-atmosphere winds help push weather systems around the globe.

Wind can be influenced by human activity. Chicago, Illinois, is nicknamed the “Windy City.” After the Great Chicago Fire of 1871 destroyed the city, city planners rebuilt it using a grid system. This created wind tunnels. Winds are forced into narrow channels, picking up speed and strength. The Windy City is a result of natural and manmade winds.

Humidity refers to the amount of water vapor in the air. Water vapor is a gas in the atmosphere that helps make clouds, rain, or snow. Humidity is usually expressed as relative humidity, or the percentage of the maximum amount of water air can hold at a given temperature. Humidity is measured using a hygrometer and is given as a percentage.

Cool air holds less water than warm air. At a relative humidity of 100 percent, air is said to be saturated, meaning the air cannot hold any more water vapor. Excess water vapor will fall as precipitation. Clouds and precipitation occur when air cools below its saturation point. This usually happens when warm, humid air cools as it rises.

The most humid places on Earth are islands near the Equator. Singapore, for instance, is humid year-round. The warm air is continually saturated with water from the Indian Ocean.

Clouds come in a variety of forms. Not all of them produce precipitation. Cloud cover is measured using a mirror or mirror nephoscope and the unit is (oktas).

Wispy cirrus clouds, for example, usually signal mild weather. Other kinds of clouds can bring rain or snow. A blanketlike cover of nimbostratus clouds produces steady, extended precipitation. Enormous cumulonimbus clouds, or thunderheads, release heavy downpours. Cumulonimbus clouds can produce thunderstorms and tornadoes as well.

Clouds can affect the amount of sunlight reaching the Earth’s surface. Cloudy days are cooler than clear ones because clouds prevent more of the sun’s radiation from reaching the Earth’s surface. The opposite is true at night—then, clouds act as a blanket, keeping the Earth warm.

Weather Systems

Cloud patterns indicate the presence of weather systems, which produce most of the weather we are familiar with: rain, heat waves, cold snaps, humidity, and cloudiness. Weather systems are simply the movement of warm and cold air across the globe. These movements are known as low-pressure systems and high-pressure systems.

High-pressure systems are rotating masses of cool, dry air. High-pressure systems keep moisture from rising into the atmosphere and forming clouds. Therefore, they are usually associated with clear skies. On the other hand, low-pressure systems are rotating masses of warm, moist air. They usually bring storms and high winds.

High-pressure and low-pressure systems continually pass through the mid-latitudes, or areas of the Earth about halfway between the Equator and the poles, so weather there is constantly changing.

A weather map is filled with symbols indicating different types of weather systems. Spirals, for instance, are cyclones or hurricanes, and thick lines are fronts. Cyclones have a spiral shape because they are composed of air that swirls in a circular pattern.

A front is a narrow zone across which temperature, humidity, and wind change abruptly. A front exists at the boundary between two air masses. An air mass is a large volume of air that is mostly the same temperature and has mostly the same humidity.

When a warm air mass moves into the place of a cold air mass, the boundary between them is called a warm front. On a weather map, a warm front is shown as a red band with half-circles pointing in the direction the air is moving.

When a cold air mass takes the place of a warm air mass, the boundary between them is called a cold front. On a weather map, a cold front is shown as a blue band with triangles pointing in the direction the air is moving.

A stationary front develops when warm air and cold air meet and the boundary between the two does not move. On a weather map, a stationary front is shown as alternating red half-circles and blue triangles, pointing in opposite directions.

When a cold front overtakes a warm front, the new front is called an occluded front. On a weather map, an occluded front is shown as a purple band with half-circles and triangles pointing in the direction the air is moving. Cold fronts are able to overtake warm fronts because they move faster.