Weather

The constantly changing condition of Earth's atmosphere creates our weather - clear skies, wind, cloud, rain, and snow. The amount of sunshine we get and how strong it is determines the temperature and pressure in the atmosphere.

The amount of moisture it contains determines how high up clouds form and whether they produce mist, rain, or snow, as well as when storms occur. When we study the weather, we can see predictable seasonal patterns around the world, known as climates.

Weather Systems

Patterns of weather depend on the nature of the local air mass and pressure system, which can change over the course of a year. For example, in the summer, continental land surfaces heat up, making warm, dry air rise. This produces a low pressure weather system, which draws in more warm air from the surroundings and can cause storms. In winter, continental land surfaces cool, and colder, dense air sinks down from the atmosphere above.

Pressure fronts

A pressure front divides two different air masses. Air masses with differing moisture content, density, temperature, and pressure do not mix easily, and the front between them is often marked by rising banks of clouds. For example, a low pressure air mass with warm air will rise up above a high pressure air mass with cold air. Any moisture carried by the warm air will condense as the air cools, forming clouds and possibly rain.

Monsoons

Monsoon winds are massive seasonal winds that bring heavy summer rain to subtropical regions, such as Southeast Asia and India. In winter, they bring dry, cooler weather. Monsoon winds are strongest in Asia, but they also occur in West Africa, northern Australia, and parts of North and South America. Monsoon winds change direction between summer and winter.

Summer

In summer, the South Asian monsoon blows from the Indian Ocean across India, bringing the torrential rains of the wet season, which are essential for the growth of the continent's staple food crops.

Winter

In winter, the South Asian monsoon reverses, bringing the warm dry winds and fine weather of the dry season across the Indian continent and out into the Indian Ocean.

Hurricanes

A hurricane is a huge, rotating tropical storm with high winds and very heavy rain. These storms start from a cluster of thunderstorms, which develop over warm tropical seawater in late summer, then merge together into a larger, spiral hurricane. Their intense low pressure draws in warm, moist winds, which spiral upward as they spin faster.

The rapidly rising air then cools, forming towering storm clouds and torrential rains. When they reach land, hurricanes cause flooding and are highly destructive.

Precipitation

Any kind of falling moisture is called precipitation. Precipitation varies widely, from the tiny droplets of clouds or fog to larger drops of rain, hail, or snow. Whether water falls from a cloud as rain or snow depends on how cold the air temperature is.

Rain: Clouds are made up of tiny droplets of water. Rain falls when these droplets become too heavy to float in the air.

Snow: Snowflakes form when water droplets freeze into crystals, which then stick together as they fall through very cold air.

Sleet: Sleet is a mixture of snow and rain. It forms when rain begins to freeze or when snow begins to melt in air that is above freezing.

Hail: Hailstones are ice pellets that grow from ice crystals in freezing storm clouds. The taller the cloud, the bigger the hailstones.

Fog: Fog forms near ground level, when warm, moisture-laden air is cooled by contact with a cold ground or sea surface.

Cloud formation

When the Sun shines on ponds and lakes, some of the water they hold evaporates into the warm air. This warm water vapor then rises up and away from Earth's surface. As the air rises, it cools. Because cold air cannot hold as much moisture as warm air, the water vapor condenses, and forms clouds.

Clouds that form very high in the atmosphere, above 16,500 ft (5,000 m), are made of ice crystals rather than water vapor.

  1. Moisture Rises: Hot sunlight makes moisture from the ground or sea surface rise into the air as water vapor.
  2. Vapor Condenses: As the vapor rises and cools, it condenses to form visible clouds of tiny water droplets.
  3. Clouds Rise: As water droplets form clouds, they release heat into the surrounding air, lifting the cloud up.

Wind

Winds are common in the Earth’s atmosphere. They vary in scale and intensity from gentle breezes to violent storms like tornadoes, and can be daily or seasonal.

What is wind?

Wind is the movement of air from an area of high pressure to an area of low pressure. The greater the difference between the areas of pressure, the faster the wind moves. Wind speed is measured using the Beaufort scale, which ranges from 0 to 12. At 12 on the scale, hurricane wind speeds can reach 300 mph (480 kph).

Tornadoes

Tornadoes are rotating columns of air that can be violently destructive. Most have wind speeds of less than 120 mph (200 kph), but they can reach 300 mph (480 kph). Tornadoes are characterized by a central spinning, funnel-shaped column of air, which extends from the clouds to the ground. They have the power to destroy crops and buildings. Tornado formation is associated with summer storms, especially in the US.

Thunderstorms

Thunderstorms form in large cumulonimbus clouds. They carry water vapor high into the atmosphere, where it condenses into hail and ice.