Weather Glossary & Terminology

Relative Humidity

Humidity is the amount of water vapor in the atmosphere.

A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.

Humidity is the amount of moisture in the air. Commonly described as 'relative' humidity which is expressed as a percentage.

A relative humidity of 100% means the moisture content of the air is the maximum possible.

i.e. totally saturated with water vapor and can not hold any more, creating the possibility of rain.

Note:
The hotter the air, the more moisture it can hold.

The effect of Humidity on Humans

Humans are sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature.

If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. Conversely, if the relative humidity is low, we will feel cooler than the actual temperature because our sweat evaporates easily, cooling us off.

Dew Point Temperature

The temperature to which the "humid" air must be cooled at a constant barometric pressure, for water vapor to condense and form fog or clouds. The dew point is a saturation temperature.

Air Pressure

Air pressure varies over time, and these temporal differences are usually caused by the temperature of the air. Cool air is denser (heavier) than warm air. Warm air is less dense (lighter) than cool air and will therefore rise above it. Areas of high pressure can be caused when cool air is sinking and pressing on the ground. At this time, the weather is usually dry and clear. In contrast, when warm air rises, it causes a region of low pressure. With low pressure, the weather is often wet and cloudy.

Therefore:

Low pressure

Rotation: counterclockwise.
Warm air (with moisture) moving up, creating clouds / Conditions: Wet & Cloudy.

High pressure

Rotation: clockwise.
Cold air moving down / Conditions: Dry and clear.

Note:
On a weather map, lines called isobars join up areas where the pressure is the same.
The closer together the isobars are, the more windy it will be.

Warm front symbol

When a warm air mass pushes into a cooler air mass,

Cold front symbol

When cold air is advancing and pushing underneath warmer air.

Stationary Front symbol

When a cold front or warm front stops moving.

Occluded Front symbol

When a warm air mass gets caught between two cold air masses.

Air Temperature

Air temperature (Also termed surface temperature in meteorology) is the ambient temperature indicated by a thermometer exposed to the air but sheltered from direct solar radiation typically by the use of a Stevenson Screen.

Location of Stevenson Screen / Thermometer

The screen/thermometer should be set at a height of approximately 1.25m above ground level, typically over a grassed area, and should be situated at least three times the height of any nearby object (such as a tree or a house) away from the object, so there are no obstructions to the passage of air around the screen.

Ideally, the screen should not be placed on or near a wall or a fence but on a stand of its own. Walls and fences are heated by direct solar radiation, and they heat the air in contact with them. Therefore, if the screen is sited on the fence, the air warmed by the fence will rise into the screen, giving you a higher temperature.

A typical Stevenson screen is shown below:

Wind Chill

Wind chill (often popularly called the wind chill factor) is the felt air temperature on exposed skin due to wind. It measures the effect of wind on air temperature. The wind chill temperature is usually lower than the air temperature since the air temperature is usually lower than the human body temperature.

Beaufort Scale (wind speeds)

The Beaufort scale is an empirical measure that relates wind speed to observed conditions. Its full name is the Beaufort wind force scale. It was created by Sir Francis Beaufort in 1805 and is widely used to estimate wind strengths.

How it works

The scale starts at 0 and goes up to 12 (or sometimes even higher), with each number corresponding to a range of wind speeds and specific observable effects.

At 0 (Calm), the wind speed is less than 1 mph (or less than 1 kph), and smoke rises vertically.

As the scale increases, the wind speed and its effects become more pronounced. For example, at 3 (Gentle Breeze), the wind speed is 8-12 mph (12-19 kph), and leaves and small twigs are in constant motion.

At the higher end, 12 (Hurricane), the wind speed exceeds 73 mph (118 kph), causing devastation.

Below is a table illustrating the Beaufort Scale:

Clouds and Rain

Clouds come in 3 basic shapes

Cirrus (wispy)

Description: Thin, white feathery or wispy clouds, usually separated or detached.




Height: Highest of all clouds, forming at 30,000 feet (9144 meters) or more in the sky.




Composition: Primarily made of ice crystals.




Appearance: Often give a delicate, streaky look to the sky.

Cumulus (heaped & puffy)

Description: Flat-based, billowing clouds with vertical doming. Often have a "cauliflower-like" appearance.




Height: Typically found at lower altitudes but can grow very tall.




Composition: Mostly water droplets, though ice crystals can form at higher altitudes.




Appearance: Prominent during the summer months and give a puffy, cotton-like look to the sky.

Stratus (layered)

Description: Thin, layered clouds, often occurring as continuous or rippled sheets covering large portions of the sky.




Height: Usually found at lower altitudes.




Composition: Mostly water droplets.




Appearance: Frequently gray and thick, giving the sky a uniform, overcast look.

Cloud altitude ranges

High clouds (between 5500m and 14000m)comprise of ice crystals:

Cirrus: white filaments.




Cirrocumulus: small rippled elements.




Cirrostratus: transparent sheet, often with halo.

Medium clouds (between 2000m and 7000m) comprise of water droplets or a mixture of ice crystals & water droplets:

Altocumulus: generally white, layered, rippled elements.




Altostratus: grey thin layer.




Nimbostratus: thick layer, dark with possibility of rain or snow.

Low clouds (below 2000m) comprise of water droplets:

Stratocumulus: Thin, layered clouds, often occurring as continuous or rippled sheets covering large portions of the sky.




Stratus: layered uniform grey.




Cumulus: 'white cauliflower' clouds with flat base.




Cumulonimbus: large dark towers, often with 'anvil' tops, associated with heavy rain & thunderstorms.

What are clouds ?

Clouds are made of tiny water droplets, ice crystals, or both. Storms and clouds are concentrated in the lower seven miles or so of the atmosphere, known as the troposphere. Clouds can serve as excellent indicators of changing weather. In general, flat clouds indicate stable conditions, while piled-up clouds mean unstable air.

How do clouds form ?

Clouds are formed by the gathering of condensing water vapor. Moist air near the Earth's surface rises from the ground into the atmosphere either by the sun's heat or by a colder invading air mass (called a cold front) that pushes the warm, moist air upward. This lifting of the air drops both the air pressure and temperature. When the air temperature reaches the air's dew point, the water vapor condenses and forms clouds.

The darker clouds we see when a storm is brewing are thicker clouds filled with lots of water molecules, which restrict the amount of sunlight that can pass through, making the clouds appear dark.

What is rain ?

To get rain, the condensing water vapor in the clouds needs to become heavy enough to fall out of suspension. In the beginning, the tiny droplets aren't heavy enough to fall, so they just hang there, suspended in the air.

To become heavier, the droplets need to grow into drops. This happens as they acquire more water and become larger. Some will collide with other droplets and become larger, or they will grow as water condenses out of the air directly into the droplet.

After a while, the droplets will reach a size/mass when they won't be able to stay floating in the cloud because they are too heavy and therefore will start to fall. Some may get blown back up into the clouds until they are heavy enough to overcome the force of the wind. Then they will fall to earth as rain and will continue to keep falling as long as the conditions are right to make the clouds and let the water droplets grow heavy enough to fall.