Glossary of Common Weather Terms


Departure from normal, difference from the long-term average.


An area of high pressure around which the winds circulate anti–clockwise in the Southern Hemisphere. Also called a high.

Barometric Pressure

The pressure exerted by the atmosphere at a given point. It can be thought of as the weight of air in the column above you. Pressure is measured by a barometer and normally expressed in inches of mercury or hectopascals (previously millibars).


The historical record and description of average weather in a region or place. Statistics are normally derived from several decades' worth of data. The word is derived from the Greek word klima, meaning inclination, in reference to the position and angle of sun.

Climate Change

The Earth's climate has exhibited marked natural climate changes, with time scales varying from many millions of years down to a few years. The larger climate changes, such as the onset and recession of the Great Ice Ages, has been due to changes in the earth's orbit and the tilt of its axis, which caused systematic variations in the amount and distribution of solar radiation, and changes in circulation and overturning in the oceans and atmosphere. Over periods of a few years, fluctuations in global surface temperatures of a few tenths of a degree are common. These smaller changes are influenced by factors such as volcanic eruptions, and the El Niño–Southern Oscillation pattern.


The study of climate.


An area of low pressure around which the winds circulate clockwise in the Southern Hemisphere. Also called a low.

Dew Point

The temperature to which air must be cooled for condensation to take place.


Liquid precipitation in the form of water droplets. The droplets are less than 0.5 mm in diameter and therefore much smaller than rain. Drizzle normally falls from low stratiform clouds and stratocumulus clouds.


A drought is a prolonged period of low rainfall so that soil moisture is insufficient for sustained plant growth. It can take from two weeks to three months with insufficient rainfall before this criterion is met, depending on the time of the year.

El Niño

The cyclical warming of sea surface temperatures off the western coast of South America that can result in significant changes in climate in the tropical Pacific Ocean and elsewhere around the earth. Is strongly related to changes in pressure patterns and wind patterns around the tropical Pacific Ocean.

Enhanced Greenhouse Effect

If extra amounts of greenhouse gases (e.g. carbon dioxide, methane, CFCs) are added into the atmosphere, such as that from human activities, the natural greenhouse effect is increased, with extra warming occurring in the lower atmosphere.


The total amount of water that is transferred from the earth's surface to the atmosphere, mainly through evaporation of liquid water, and transpiration by plants.

Global warming

Global warming refers to an overall increase in the Earth’s surface temperature which over time changes global climate patterns. Global warming due to natural influences has occurred in the past, but the term is more often used to refer to the warming that is occurring and is expected to continue as a result of increased emissions of greenhouse gases.

Greenhouse Effect

The overall warming of the Earth’s lower atmosphere mainly due to carbon dioxide and water vapour which permit the sun's rays to heat the earth, but then restrict some heat–energy from escaping back into space.


Solid precipitation in the form of balls or pieces of ice. Hail forms in thundercloud when water droplets are continuously taken up and down though the cloud by updraughts and downdraughts, allowing the water droplets to freeze when they reach the cloud top.

Indian Ocean Dipole (IOD)

The Indian Ocean Dipole refers to the difference in sea surface temperature between the western and eastern regions (poles) of the Indian Ocean. The IOD affects the climate of countries that surround the Indian Ocean Basin, including Australia, and is a significant contributor to rainfall variability in this region.


The line drawn on a weather map connecting points of equal barometric pressure.

La Niña

The cyclical cooling of sea surface temperatures off the western coast of South America that can result in significant changes in climate in the tropical Pacific Ocean and elsewhere around the earth. Is strongly related to changes in pressure patterns and wind patterns around the tropical Pacific Ocean.


Liquid precipitation in the form of water droplets. Rain is generally continuous and falls from dense, sheet-like clouds, called stratiform clouds.

Rain Shadow

The region on the lee side (sheltered side) of a mountain range where rainfall is much less than on the windward side.


An elongated area of high atmospheric pressure that is associated with an anticyclone. Sometimes referred to as a ridge of high pressure.

Sea Surface Temperature (SST)

The temperature of the water's surface. It is usually measured using buoys, ship data, and satellites.


Precipitation from individual clouds, often characterised by the sudden beginning or ending. Showers fall from fluffy looking clouds, called cumuloform clouds. Showers can consist of liquid water, ice (hail), or snow, or a mixture.


Frozen precipitation in the form of translucent ice crystals in a complex branched hexagonal form.

Southern Oscillation

A periodic reversal of the pressure pattern (position of highs and lows) across the tropical Pacific Ocean, associated with the El Niño and La Niña climate pattern.

Southern Oscillation Index (SOI)

An index used to monitor the state of the Southern Oscillation. Normally, the SOI is calculated from the standardised monthly mean pressure at Tahiti and Darwin. These two stations give a good approximation of the pressure pattern across the Pacific Ocean on the longer time frames of a month or more.


The temperature of the ambient air at the surface of the earth. This is usually measured in degrees Celsius in New Zealand, and is measured by a thermometer. The thermometer is normally housed in a wooden louvered screen called a Stevenson screen, at a height of approximately 1.3 metres above ground level. This screen allows air to flow through the thermometer enclosure, so that a representative temperature reading is made.


A tornado is a violently spinning funnel of air descending from a cloud. It is made visible by the dust that is sucked up and also from the condensation of water droplets in the centre of the funnel.

Tropical Air Mass

Air that originates in the tropics or subtropics. When a tropical air mass lies over the country, it is often very warm and humid.

Tropical Cyclone

An intense storm that originates in the tropics, forming in a single, warm airmass. It has a very low central pressure, an 'eye' (the area with the lowest pressure, and relatively calm winds), and an 'eye–wall'(a ring of very active thunderstorms around the eye).


An elongated area of low atmospheric pressure that is associated with a cyclone, or low. Sometimes referred to as a trough of low pressurep>

Water Cycle

Also called the hydrological cycle, it is the vertical transport of water in all its states between the earth, atmosphere and the oceans.


The state of the atmosphere at a specific time. It is the short-term variations in the atmosphere, as opposed to the long-term, climatic changes. It is usually referenced to in terms of sunshine, cloudiness, humidity, rainfall, temperature, wind, and visibility.


Air that flows around pressure systems such as highs and lows, or due to heating and cooling of land and sea (the sea breeze, the land breeze). Measured by its speed, and direction. Winds are called by the direction on the compass they blow from i.e northerly winds blow from the north.

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