Weather is defined as conditions of the atmosphere which occur at a place at specific time periods, that is, from hour to hour or day to day. It changes from time to time and from place to place. For example, it may be raining in the morning and Sunny in the afternoon.

Weather may also be defined as the day-to-day state of the atmosphere, and its short-term variation ranges from minutes to several weeks. Weather is an important aspect of our lives. The weather often determines how and where we live, what we do, what we wear and what we eat. The scientific study of weather is called meteorology, and a person who studies weather is called meteorologist.

Weather is an important because it affects human activities. The following are some of the reasons why weather is important;

Weather condition is a great determinant of timing of doing various human activities. For example, during the morning people can actively work in agricultural fields but as the weather gets hotter, people cannot engage in production so actively due to an excessive atmospheric temperature which could lead to sweating and discomfort.

Weather change can also influence other human activities as well. For example, on a rainy day, very few people can participate in outdoor activities. Farmers may not go to the farms. Likewise, people may fear to travel because sometimes the rain may cause floods, demolish bridges or erode the roads.

Therefore, there is a need to adjust human activities according to change in the weather so as to avoid adverse weather conditions. For example, working during rainy or very Sunny days may affect our health. Therefore, weather affects the efficiency of our daily activities.

Weather elements refer to a combination of natural phenomena that make up the weather. There are several elements that make the weather. The weather elements are temperature, pressure, precipitation, wind, humidity, clouds and Sunshine

The study of these elements can provide the basis for forecasting weather and defining the climate. The following sub sections present detailed information about each element of weather

The temperature is the state of hotness or coldness of the atmosphere. It is usually measured by an instrument called thermometer and expressed in degrees on a Celsius or Fahrenheit scale.

This refers to the deposition of moisture on the Earth’s surface from the atmosphere. This moisture includes rain, snow, ice, hail, mist and sleet.

Example: Demonstrating the rainfall formation

Boil some water in a pot. Wait for some minutes until the water starts boiling, and then hold a container filled with cold water over the pot. As the steam comes in contact with the container, it condenses to form droplets which will then fall down. This is a simple demonstration on how rain is formed.

Rainfall formation

The Sun’s heat causes water to evaporate from the surface of the oceans, lakes, rivers and other water bodies. This vapour rises into the atmosphere where it condenses to form clouds. Because the air is cooler at higher altitudes, the vapour is cooled to form small droplets that join together to form larger drops which are then too heavy to remain in the air, so it falls as rain. The figure 4.5 shows the water cycle, also called the hydrologic cycle.

Types of rainfall

(i). Convectional rainfall This is formed through the rising of the moist air currents, which condenses at higher altitudes to form clouds that result in rainfall

(ii). Orographic rainfall Sometimes moist winds are forced by a high mountain to rise and the moisture in it condenses to form rainfall. The rain formed in this manner is called orographic rainfall. The side of the mountain facing away from the direction of wind gets little or no rainfall. This phenomenon is called the rain shadow effect

An example of the rain shadow effect in Tanzania is found on the western side of Mount Kilimanjaro. Winds blow from the Indian Ocean in the east and are forced by this mountain to rise up and drop moisture on the eastern and south western slopes. When these winds blow over to the western side of the mountain, they are already relatively dry. As a result, they bring very little rainfall to the Masai steppe. Other examples are the Rocky Mountains which affect the rain-bearing winds from the Pacific; and the Andes in Chile which affect the rain-bearing winds from the Pacific on the Patagonia plateau

iii. Cyclonic rainfall This occurs when large masses of air with different characteristics of temperature and moisture meet. As the warmer and moist air is forced up over the cooler and dry air, it expands, cools and water vapour condenses to form clouds and rainfall

On the other hand, tropical cyclones are formed over oceans in the tropics between latitude 8°N and 8°S. They usually bring very heavy rainfall and are associated with thunderstorms and very fast-moving winds which often cause destructions along coastal settlements. In the Caribbean and USA, tropical cyclones are called hurricanes. In Africa they are known as cyclones, while in China and Japan they are called typhoons but in North Australia, they are known as Willy–Willies.

Humidity is the state of the atmosphere in relation to the amount of water vapour it contains. Humidity indicates the degree of dampness of the air and it is one of the main influences on weather. It is expressed in either absolute or relative terms. Absolute humidity is the actual amount of water vapour present in a certain volume of air at a given temperature, expressed in grams per cubic meter. Relative humidity is the amount of water vapour present in a mass of air, expressed as a percentage of the total amount of water vapour that would be present when that air is saturated at that temperature. Air is saturated when the atmosphere cannot hold any more water vapour. This condition depends on the temperature and pressure of the air.

The air around us has weight. Atmospheric pressure (or air pressure) is the weight of the air resting on the Earth’s surface. It is the weight exerted by air on the Earth’s surface

The force with which air presses down on a unit area is called atmospheric pressure. But this pressure is exerted equally in all directions. Atmospheric pressure can be demonstrated by the following experiment

Take a glass full of water, cover the top of the glass with a piece of thin paper, and then hold the glass upside down. The water in the glass will not spill out because pressure of the air is pressing the paper so that it does not fall out.

Wind is air in motion, from high pressure areas to low pressure area. It is a moving air mass, especially on the Earth’s surface. Usually, there is a movement of air from high pressure to low pressure areas, which is caused by differences in heating of air over different parts of the Earth’s surface. The air that moves from a region of high pressure to that of low pressure is called wind. Heated air expands, becomes less dense and rises up. Cooled air contracts, becomes denser and sinks down. When air sinks, its pressure increases because it is compressed, but when air rises, its pressure decreases because its molecules are spread over a large area. Areas from where heated air is rising are called areas of low pressure, while areas in which cool air is sinking are called areas of high pressure.

During the day the land is usually warmer than the sea, and the air pressure on the land is lower than that over the sea. Therefore, air blows from sea to land. This kind of air movement (wind) is known as sea breeze. But during the night the land is cooler than the sea and there is low pressure on the sea. Therefore, winds blow from the land to the sea. This air movement is called land breeze.

On the Earth’s surface, the regions of the north and south poles are very cold and have high pressure while the belt along the equator is very hot and has low pressure. This makes air move from the poles towards the equator. In the equatorial belt, rising air is replaced by air moving in from the north and south of the equator. We should then expect two belts of wind blowing towards the equator. But this is not exactly so because the earth rotates from west to east. According to Ferrel’s law air or water moving freely in any direction over the Earth’s surface is turned (deflected) to the right of its course in the northern hemisphere and to the left in the southern hemisphere. Therefore, wind blowing from the north towards the equator in the northern hemisphere will blow from the northeast and not from the north, and any winds blowing from the south towards the equator in the southern hemisphere will blow from the southeast and not from due south.

In the equatorial belt of low pressure, between 5^oN and 5^oS latitudes, intense solar heating causes the moist air to raise in great convection columns. This belt is called the doldrums or low pressure belt. The rising air spreads out and moves towards the poles. In so doing, it cools and thus contracts, and develops high pressure. This occurs around 30°N and 30°S. Thus, the air sinks and builds up high pressure at these latitudes. These latitudes are called horse latitudes or subtropical high-pressure cells

In latitudes 30°N and 30°S some of the high pressure air moves over the surface towards the equator as the north east and south east trade winds. Some moves over the surface towards the poles as westerlies

In each hemisphere, there are three wind systems which operate between the indicated latitudes namely

Occasionally in the westerly wind system, depressions and anticyclones develop. A depression is an area of low pressure in which winds blow inwards in a circular motion. This motion is anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. A depression develops when cold heavy air comes in contact with warm most air. Depressions are usually associated with cyclonic rains. Anti-cyclones are areas of high pressure in which winds blow in a clockwise, circular motion in the northern hemisphere. They are associated with cool fine weather with no rain and they normally follow a depression.

Cloud cover refer to the fraction of the sky obscured by clouds when observed from a particular location. It is also known as cloudiness, cloud age or cloud amount. The cloud cover is observed by using eyes. However, Ceilometer is an instrument used to measure cloud height and cover. Ceilometer works day or night by shining an intense beam of light which is modulated at the audio frequency, at overhead cloud.

The amount of Sunshine depends on latitude and amount of cloud in the sky. In some of the world's deserts, the number of Sunshine hours is very high, more than 3,600 hours each year. In the Eastern Sahara Desert, the Sun is covered by clouds for less than 100 hours a year. Hours of Sunshine are usually recorded on a simple machine called Campbell-Stokes recorder.

A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and precipitation amounts

Wind measurements are taken with minimal or without obstructions, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated measurements are taken at least once an hour. Weather conditions t at sea are taken by ships and buoys, which measure slightly different meteorological quantities such as sea surface temperature, wave height, and wave period

Weather station data can be used to gauge current weather conditions and to predict the future weather forecast, like temperature high/lows, cloud cover and probability of precipitation. Weather stations are used by meteorologists, weather buffs, gardeners, farmers, outdoor enthusiasts, students, pilots and anyone who enjoys weather data or relies on the weather to make decisions

Selecting an appropriate site for the weather station is critical for obtaining accurate meteorological data. Typically, the site should represent the general area of interest, and be away from obstructions such as buildings and trees

When establishing a weather station the following guidelines must be considered

A Stevenson screen or instrument shelter is an enclosure intended to shield meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard weather station

The Stevenson screen contains instruments like thermometers (ordinary, maximum/minimum), hygrometer, psychrometer, dew cell, barometer and a thermograph. Its purpose is to provide a standardized environment in which to measure temperature, humidity, dew point and atmospheric pressure.

Sometimes, traditional Stevenson Screen can be used. This is a box shape, constructed of wood, in a double louvered design. However, it is possible to construct a screen using other materials and shapes, such as a pyramid. The World Meteorological Organization (WMO) agreed standard for the height of the Stevenson Screen is between 1.25 m and 2 m above the ground.

The interior size of the screen will depend on the number of instruments that are to be kept in it. A single screen may measure 76.5 by 61 by 59.3 centimeters and a double screen 76.5 by 105 by 59.3 centimeters. The unit is either supported by four metal or wooden legs or a wooden post. The whole screen is painted with several coats of white to reflect Sunlight radiation and will usually require repainting every two years. Figure 4.30 shows the interior of Stevenson screen

The location of the screen is very important to avoid data degradation by the effects of ground cover, buildings and trees. It is recommended that the screen be placed at least twice the distance of the height of the object; at least 20 m from any tree that is 10 m high. In the northern hemisphere, the door of the screen should always face north so as to prevent direct Sunlight on the thermometers. In Polar Regions, with 24-hour Sunlight, the observer must take care to shield the thermometers from the Sun and at the same time avoiding a rise in temperature being caused by the observer's body heat

The general purposes of the Stevenson Screen are

Temperature is usually measured by an instrument called thermometer and expressed in degrees on a Celsius or Fahrenheit scale. There are two types of thermometers used to measure temperature namely maximum and minimum thermometers. The maximum thermometer shows the highest temperature reached during a given period, for example, a day; while the minimum thermometer shows the lowest temperature recorded (Figure 4.1 shows maximum and minimum thermometers)

The maximum thermometer is made of glass and contains mercury in the bulb. The minimum thermometer is also made of glass but contains alcohol instead of mercury. The thermometer is marked in degrees of Celsius or Fahrenheit. When the temperature rises, the mercury expands and extends along a glass tube. Changes in temperature are shown by the length of mercury. For example, if the lowest temperature reads 12.5°C and the maximum temperature reads 24.0 °C, then the change in temperature is calculated as 24.0−12.5=11.5⁰ c

The Six’ thermometer can also be used for measuring maximum and minimum temperature. The thermometer consists of a U-shaped glass tube. The metal index nearest to the bulb indicates the minimum temperature and the other metal index records the maximum temperature

Temperature is a very important factor in determining weather. It influences or controls other elements of weather, such as precipitation, humidity, clouds and Sunshine. The factors affecting (modifying) temperature include latitude, altitude, distance to the ocean and/or sea, orientation of mountain ranges toward prevailing winds (aspect) and ocean currents

Rainfall is measured by an instrument called rain gauge. Normally, the reading is done once in every 24 hours. Figure 4.9 show rain gauge

Humidity is measured by an instrument called hygrometer (Figure 4.12). This instrument consists of wet and dry bulb thermometers. The wet bulb thermometer is kept moist (wet) by wrapping it in a muslin bag which is dipped in a container of distilled water. When the air is not saturated, water evaporates from the muslin and this cools the wet bulb causing mercury to contract. The dry bulb is not affected in the same way. So the two thermometers show different readings. But when the air is saturated the two thermometers show the same readings. Therefore, when there is a big difference in readings between the two thermometers, humidity is low and when there is a small difference, humidity is high

The hygrometer (figure 4.12) consists of dry (left) and wet (right) bulb thermometers. Absolute humidity is calculated after finding the dew point. Dew point is the critical temperature at which air becomes saturated with water vapour. Further condensation causes the formation of tiny drops of water called dew

Atmospheric pressure is measured by an instrument called a barometer. There are two types of barometers, namely mercury barometer and aneroid barometer. Mercury barometer measures pressure in millimeters usually expressed symbolically as mmHg and read as millimeters of mercury. The pressure at sea level is 76 mmHg. This is called standard pressure. Figure 4.13 shows simple mercury barometer while 4.14 shows aneroid barometer

Wind direction is measured by a wind vane or wind sock. The wind vane consists of a freely rotating arm, fitted over a central rod. The arrow of the wind vane always points in the direction from which the wind blows, and the wind is named after this direction. Four arms marking the direction of the cardinal points are fixed to the stationary central rod

Wind sock consists of a sock-like sheet of cloth fitted to the top of a tall wooden or metal bar, just like the flag is fitted to the flag post. The tail of the sock points away from the direction of wind, and the direction of wind is named after the head of the sock. Windsocks are mainly used to show wind directions at airports and airstrips in order to direct pilots when landing or taking off

Wind speed is measured by an instrument called an anemometer (figure 4.23). This instrument consists of three or four horizontal arms pivoted on a vertical shaft. Metal caps are fixed to the end of the arms so that when there is a wind the arms rotate. This movement operates a meter which records the speed of wind in kilometers per hour

The cloud cover is observed by using eyes. However, Ceilometer is an instrument used to measure cloud height and cover. Ceilometer works day or night by shining an intense beam of light which is modulated at the audio frequency, at overhead cloud

Sunshine are usually recorded on a simple machine called Campbell-Stokes recorder (figure 4.25)

It works by using a glass ball focusing the Sunlight and rays onto a strip of card. As the Sun moves around during the day, the card is scorched, creating a record of how many Sunshine hours there were

The maximum and minimum temperatures which are recorded for the day are used to calculate

The monthly range of temperature is the difference between the highest or maximum recorded temperature and the lowest or minimum recorded temperature in a particular month.

The annual range of temperature in a particular year is the difference between the highest and the lowest monthly temperature

When reading and recording of data done over a period of time, the obtained data can be shown on maps. These maps are called temperature maps. When comparing the temperature in different parts of the world, we use temperature maps. Different places on a map with the same temperature can be joined by lines called isotherms as shown in Fig. 4.3

Usually temperature decreases at the rate of 0.6°C for every 100 meters increase in altitude. Therefore, temperatures in highlands are lower than temperatures in lowlands

Apart from isotherms, another way of presenting the temperature data is using a graph. In this case, temperature figures are plotted on the graph and points are joined by a smooth line

Rainfall figures entered in the record book for the month or several months can be represented in the form of graphs known as histograms (Figure 4.10). Mean monthly rainfall records are usually obtained by adding up the amount of rainfall of a particular month for example January), then divide the total records by the number of days in that month

Another way of presenting rainfall figures is by drawing lines on a map to link all places that receive the same amount of rainfall. These lines are called isohyets. They are usually drawn at uniform intervals

Pressure is expressed in millimeters with reference to the height of mercury column. When using an aneroid barometer, we express pressure in millibars of force per unit area. In physics, a unit of force known as a dyne per square centimeters is called a bar and is now the standard unit of pressure measurement. A bar is then divided into one thousand units called millibars. At sea level, pressure is normally 760 mmHg or 1.034 kilograms of force per square centimeter. This is equivalent to 1015.9 millibars or approximately one bar

Figure 4.15 below shows the height of mercury column at high and low pressures. When the atmospheric pressure is high, mercury level is pushed up the glass tube. At low pressures, mercury column drops down

Pressure is shown on a weather map, usually called synoptic map. Lines drawn on a weather map joining places with the same pressure are called isobars

The pressure is greatest at sea level where the whole thickness of the atmosphere exerts its weight. But atmospheric pressure decreased at the rate of 10 millibars for every 100 meters increase in height. This is because the thickness of the atmosphere decreases, thus it exerts less pressure

Okta is the usual unit of measurement of the cloud cover. One okta represents approximately 1/8 of the sky with cloud cover. If approximately 3 segments out of 8 are covered in clouds, then the cloud cover is written as 3/8 cloud cover. These are 3 oktas. 8/8 means the cloud is completely covered by clouds. The figures below represent the symbols used to represent cloud cover in oktas

Simple observation can be made such as: Clear - no cloud cover. Partly cloudy or less than half cloud cover. Mainly cloudy - more than half cloud cover but with some breaks in the cloud. Overcast - complete cloud cover

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location and time

The nature of modern weather forecasting is highly complex and quantitative. There are different methods that can be used to determine a weather forecasting. The method a weather forecaster to choose depends upon his/her experience, the amount of information available to the weather forecaster, and the level of difficulty that the forecast situation presents. The following are various methods used in weather forecasting;

i. Numerical method The numerical method involves use of numbers (mathematics). This method is based on the fact that gases of the atmosphere follow a number of physical principles. If the current conditions of the atmosphere are known, then the physical laws may be used to forecast future weather

Numerical weather forecasting is made possible by making observations of the atmosphere by means of radiosonde stations all over the world. A radiosonde is an instrument carried by balloon or other means to various levels of the atmosphere and transmitting measurements by radio transmitter. The radiosonde is attached to helium or hydrogen-filled balloon, generally called a weather balloon, and the balloon lifts the radiosonde to altitudes exceeding 30 km

Radiosondes measure atmospheric pressure, air temperature, water vapour (humidity) and winds (speed and direction). Modern radiosondes contain a GPS receiver to calculate wind speed and direction, and a radio transmitter to send the data back to the ground

Usually, the same time each day (0000 and 2400 UTC), weather personnel across the planet release radiosondes to the sky. The data obtained are processed, correlated with data from other radiosondes, and used to create an instantaneous picture of weather. Figure 4.31 shows radiosonde

The data are used not only to understand current weather patterns but also as inputs for longer-range computer-based forecasting models.

ii. Satellites, Radiosonde data are supplemented by means of radiometric observations from satellites which also provide data on humidity and cloud cover. For viewing large weather systems on a worldwide scale, weather satellites are invaluable. Satellites show cloud formations, large weather events such as hurricanes, and other global weather systems. With satellites, forecasters can see weather across the globe including the oceans, continents, and poles. Recent satellite data is very detailed, even to the point of showing states and countries. The satellite image in figure 4.32 below shows the satellite image

iii. Persistence method, This method assumes that the conditions at the time of the forecast will not change. For example, if it is Sunny with 30°C readings today, the persistence method predicts that it will be Sunny with 30°C next day (tomorrow). If it reads 10 mm of rainfall today, the persistence method would predict the same amount of rainfall for the next day (tomorrow). Generally, this is the simplest way among the forecasting methods

iv.Trends method,This method involves determining the speed and direction of the movement for fronts, high and low pressure centers and areas of clouds and precipitation. Using this information, the weather forecaster can predict where he or she expects those features to happen in future. For example, if a storm system is 100 kilometers west of your location and moving to the east at 20 kilometers per day, using the trends method you would predict it to arrive in your area in 5 days

v. Climatology method, This method involves averaging weather statistics accumulated over many years to make the weather forecast. For example, if using the climatology method to predict the weather for Dar es Salaam on 4^th, July, would go through all the weather data that has been recorded for every 4^th, July and take the average. If making a forecast for temperature and precipitation, then the similar procedure of using the recorded weather data to compute the averages for temperature and precipitation will apply, For example, if these averages were 33°C with 0.18 inches of rain, then the weather forecast for Dar es Salaam on 4^th, July, using the climatology method, would call for a high temperature of 33°C with 0.18 inches of rain. The climatology method only works well when the weather pattern is similar to that expected for the chosen time of year. If the pattern is quite unusual for the given time of a year, the climatology method will not work