The term “weather” refers to the atmospheric conditions which exist in an area at a particular time. The weather in a particular location can change from day to day or even within a few minutes. If you speak of how rainy it was this morning or how hot it was yesterday, you are speaking about the weather. Climate refers to the general atmospheric characteristics expected to prevail in a particular place. We learn about the climate of an area by studying weather patterns in that area over a 30-35 year period. It is important for Geography students to know the difference between weather and climate.
The Elements of the Weather
There are a number of elements which make up the weather. They are:
Temperature: This refers to how hot or cold the atmosphere is. It is measured in degrees Celsius or degrees Fahrenheit. Temperature is measured using a thermometer. 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.
Precipitation: This term refers to the various ways in which water falls from the atmosphere to the ground. This includes rain, snow and hail. Rainfall is measured in millimeters using a rain gauge. The amount of precipitation that an area receives has a major impact on the vegetation and soil in that area. Some areas receive very little precipitation while others receive very much. In some areas rainfall is seasonal, while in others it rains throughout the year.
Pressure: This refers to the weight of the atmosphere pressing down on the Earth’s surface. It is measured in millibars (mb). The instrument used to measure atmospheric pressure is the barometer. Low pressure is associated with rising air, which often results in cloud formation and rainfall. High pressure is associated with descending air and fair weather.
Winds: Winds blow from areas of high pressure to areas of low pressure. We are concerned with two aspects of wind; its direction and its speed.
Wind direction is measured using a wind vane or wind sock. As the wind blows, these instruments turn to indicate wind directon. Winds are named according to the direction that they blow from. For instance, an easterly wind is a wind which blows from the east.
Wind speed is measured using an anemometer. A commonly used type is the cup anemometer. It is composed of three cups mounted on horizontal arms which are attached to a vertical shaft. When the wind blows, it causes the cups to spin around which causes the shaft to turn. The faster the wind blows, the faster the cups spin and the faster the shaft turns. A device connected to the shaft gives the wind speed in miles or kilometers per hour.
Humidity: This term refers to the moisture in the atmosphere. Absolute humidity is a measure of the total amount of moisture in the air. However, the amount of moisture that air can hold depends on its temperature. Relative humidity is the amount of water in the atmosphere given as a percentage of the total amount of water that the air can hold at that temperature. Therefore, relative humidity of 75% indicates that the air is currently holding 75% of the total amount of water that it can hold at that particular temperature. Relative humidity is measured using a hygrometer.
Clouds: Clouds form when water vapour condenses in the atmosphere. When air rises, it cools. A rising current of air is called an updraught (also spelled updraft). As the air cools, its ability to hold moisture is reduced. The water vapour in the rising air condenses, forming clouds. When discussing clouds, we are concerned with cloud cover and cloud type.
Cloud cover: The amount of the sky that is obscured by clouds is known as cloud cover. Cloud cover is measured in oktas (eighths). One okta of cloud cover means that one eighth of the sky is obscured by clouds. Four oktas of cloud cover means that half the sky is obscured by clouds. Eight oktas of cloud cover means the entire visible sky is obscured by clouds. The human eye is used to judge cloud cover.
Cloud type: There are many types of clouds. Clouds are named according to their appearance. The three basic cloud types are cirrus, cumulus and stratus.
- Cirrus clouds: These clouds form high in the atmosphere where they are blown into thin, feathery formations by high level winds.
- Cumulus clouds: The word cumulus means “heap” or “pile”. Cumulus clouds are puffy looking clouds formed when water vapour condenses as a result of strong updraughts. They look like floating heaps of cotton.
- Stratus clouds: These clouds are formed in layers. A stratus cloud looks like a sheet of cloud.
Many other types of cloud display features of more than one of the basic cloud types. For instance a cirrostratus cloud displays features of cirrus as well as stratus clouds. They are high level,thin, feathery clouds formed in layers. It is important to note that the prefix “alto” is used to describe mid level clouds and the terms “nimbo” or “nimbus” are used to describe clouds that produce rain. Therefore, altocumulus clouds are mid-level cumulus clouds and nimbostratus clouds are stratus clouds that produce rain.
A distinct type of cloud is the cumulonimbus cloud. This type of cloud forms when very strong updraughts rise high into the atmosphere forming very tall clouds. The top of this type of cloud may spread out, giving the cloud an anvil shape. Cumulonimbus clouds produce heavy rain and thunderstorms. The diagram below shows various cloud types as well as the altitudes at which they form. Click to enlarge the image.
The CXC/CSEC Geography syllabus requires students to study some of the major weather systems which affect the Caribbean. Students must also understand how various factors affect weather and climate.