10 FACTORS INFLUENCING RAINFALL TYPE AND AMOUNT

relief or topography

relief features such as mountains and hills result in the rising and cooling of moist wind to for relief rainfall

aspect

windward slopes which are on the path of rain bearing winds receive heavier rainfall than leeward slopes which face away




forest and water bodies

areas near forest and large water bodies experience high rainfall and more often due to high rate of evaporation

10 FACTORS INFLUENCING RAINFALL TYPE AND AMOUNT
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air pressure

high-pressure areas receive low rainfall than low-pressure areas due to pushing of airmasses from high pressure to low pressure. the high-pressure areas have descending dry air

airmasses

when warm and cold airmasses meet frontal rainfall is formed

ocean currents

it influence rainfall whereby coasts washed by warm currents experience heavy rainfall when moist onshore winds are warmed by the current and made to hold on to the moisture which they release on reaching the land.




the coast washed by cold ocean currents on the other hand experience low rainfall as the result of moist wind being cooled and moisture in them condensed resulting in rain falling over the ocean thereby bringing little or no rainfall to the coastal areas.

this is the cause of western margins deserts for example Kalahari and Namib desert

Apparent movement of the sun (I.T.C.Z)

In June – July when the sun is overhead in the north, intense heating occurs creating a low-pressure belt and the convergence of moist winds which result into heavy rainfall in the north.




The same condition is experienced in the south in December – January. The north and south, therefore, experience a single rainfall maximum due to the movement of the sun.

Because the sun is overhead at the equator twice a year, a double rainfall maxima is experienced in the equatorial areas such as around the Lake Victoria basin

Prevailing winds

They have a rainfall effect on the areas over which they blow because they transfer weather characteristics to the areas where they move.




The Southeast trade winds emerge from the Indian ocean when they are moist so they are responsible for the heavy rainfall experienced along the E.African coast as well as the northern shores of lake Victoria while the Northeast trade winds from the Arabian desert are responsible for the low and unreliable rainfall in Northeastern Uganda and Northwestern Kenya

Latitudinal location

Areas located at or near the equator experience heavy rainfall which is evenly distributed throughout the year with a double rainfall maxima in March and September because the sun is overhead at the equator twice a year while areas far away from the equator experience moderate to low rainfall with a single rainfall maximum because the sun is overhead at the tropic of cancer in the north and tropic of Capricorn in the south once in a year




Corriolis force effect

According to Ferrel’s law, the Southeast trade winds are deflected to the right of their path as they cross the equator due to the rotation of the earth.

This is responsible for the heavy rainfall received around the northern and north eastern shores of Lake Victoria while low and unreliable rainfall experienced in the Ankole Masaka corridor

Perturbation

It refers to the development of low-pressure belts over the Indian ocean due to intense insolation.

These forces winds from the interior of E.Africa to blow offshore (seaward) resulting in heavy rainfall over the Indian ocean and dry conditions in Northeastern Kenya

Coastal configuration




The Northeast and Southwest alignment of the coast forces winds to blow parallel to the coast instead of blowing onshore.

This is responsible for the low rainfall received in Northeastern Kenya

human activities

Human activities such as deforestation, overgrazing, the sinking of boreholes, and swamp reclamation among others reduce the rate of evaporation and evapotranspiration resulting in low rainfall e.g in the Karamoja region and Turkana land.




On the other hand, afforestation and re-afforestation result in an increase in the rate of evaporation and evapotranspiration hence increasing the amount of rainfall in the areas where the trees are planted

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7 FACTORS INFLUENCING ATMOSPHERIC PRESSURE




Atmospheric pressure it is the weight (force) of the air exerted per unit area on the earth’s surface.

Atmospheric pressure it is the weight (force) of the air exerted per unit area on the earth’s surface

It is mathematically expressed as:
Pressure = Force/Area

The weight of air is the vertical column exerted from the upper limit of the atmosphere to the earth’s surface which is approximately 1.034kg/cm3 over the sea level.




It is measured and recorded in units known as Millibars. However, the average pressure or weight of the air on the earth’s surface is measured in Millibars per unit area.

The weight of air is the vertical column exerted from the upper limit of the atmosphere to the earth’s surface which is approximately 1.034kg/cm3 over the sea level

Factors influencing atmospheric pressure:

Temperature

There’s an inverse relationship between the temperature and atmospheric pressure of a place.




Hot temperatures lead to low pressure while cold temperatures lead to high atmospheric pressure.

When air is heated, the air molecules expand and spread over a wide area resulting into low pressure while low or cold temperatures result into contraction and condensation of air molecules thereby exerting high pressure on the earth’s surface.

For that matter, Polar regions are high pressure zones while equatorial areas are low pressure belts.




Hot temperatures lead to low pressure while cold temperatures lead to high atmospheric pressure. When air is heated, the air molecules expand and spread over a wide area resulting into low pressure while low or cold temperatures result into contraction and condensation of air molecules thereby exerting high pressure on the earth’s surface

Altitude

Atmospheric pressure decreases with an increase in altitude.




This is because air at a high altitude spreads over a wide area which reduces its weight thereby causing low pressure.

So, high altitude areas like highlands / mountain tops have low pressure due to the gravitational force towards the low altitudes.

Low altitude areas such as foot hills and sea level on the other hand experience high pressure because the air near the ground supports the weight of air above it hence, the underlying or bottom air molecules constantly push downwards onto the earth’s surface.




This is also due to the high concentration of air impurities like dust particles and carbon dioxide at low altitudes.

Atmospheric pressure decreases with an increase in altitude. This is because air at a high altitude spreads over a wide area which reduces its weight thereby causing low pressure

Rotation of the earth

As the earth rotates, air at the poles (North and South poles) is blown away towards the equator.

It crosses parallels which are getting longer.

The cold dense air at the poles crosses from the high latitudes towards the equator spreading over a wide area leading to low pressure.




This accounts for the low pressure at the equator.

Air rising at the equator spreads out as it moves towards the poles.

It crosses parallels that are getting shorter and contracts to occupy a small space. Its pressure therefore rises.

This accounts for the high pressure at the horse latitudes.




Latitudinal location

Air pressure tends to increase away from the equator towards the Polar Regions.

The equatorial region experiences low air pressure because of the intense or high insolation (heat) from the overhead sun while Polar Regions that experience a low intensity of insolation experience high pressure

Nature of the earth’s surface




Land and water surfaces experience varying air pressure because of differences in the rate of heating and heat loss.

This however affects atmospheric pressure at a local scale. During the day, land surfaces absorb heat faster than water surfaces leading to low pressure over the land and high pressure over the sea.

Conversely, at night, low pressure develops over the sea and high pressure over the land because the land surface cools faster than the water surface

Amount of water vapour in the atmosphere




Moist air is cold and dense hence it exerts high pressure on the earth’s surface while dry air with little or no moisture is warm and light (less dense) hence exerting low pressure on the earth’s surface

Apparent movement of the sun (Influence of the I.T.C.Z)

Low-pressure belts shift with the apparent movement of the overhead sun. When the sun is overhead in the Northern Hemisphere (Tropic of Cancer) in June – July, high temperatures are experienced in the north leading to low pressure and high pressure over the Southern Hemisphere.




In December – January when the sun is overhead in the Southern Hemisphere (Tropic of Capricorn), temperatures rise and low pressure develops over the south while the north develops high pressure.

FEATURES AND INSTRUMENTS IN THE WEATHER STATIONS

what is Stevenson screen?




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

A Stevenson screen is a prominent feature on most weather stations. It is used to house weather instruments, for example, the six’s thermometer, hygrometer, barometer, thermograph, and hydrometer.

what is a weather station?

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.




Most instruments used in measuring weather elements are found in a weather station.

This is a place where weather events are recorded for example weather station inside Dar es salaam International Airport. It is situated in an open space away from buildings and trees.

Trees can affect weather phenomena for example by acting as windbreaks thus affecting the wind speed and direction readings.

Buildings act as thermal sinks i.e they store heat energy thus distorting heat readings.

factors to be taken into account when setting a weather station

  • the site should be free from flooding
  • the site should be an open place where there is little obstruction of weather elements
  • it should be an accessible place to simplify the recording process
  • the area where the weather station is located should be fairly level or gently sloping ground (5 degrees) so that it is easy to position the weather instrument
  • the place should provide a wide view of the surrounding landscape and the sky
  • the place should have security




Instruments and items found in a weather station are:

1. Stevenson screen.
2. Six’s thermometer.
3. Hygrometer.
4. Barometer.
5. Sunshine recorder.
6. Anemometer.
7. Rain gauge.
8. Evaporation dish.

 Stevenson screen is a prominent feature on most weather stations. It is used to house weather instruments, for example, the six’s thermometer, hygrometer, barometer, thermograph, and hydrometer.

 The following are some of the prominent features of the Stevenson screen.

  • It is painted white in order to reflect excessive heat.




  • It is 1.2 meters above the ground to avoid ground weather conditions from affecting the readings yielded by the housed instruments. 
  • It has louvered sides to allow for the free circulation of air. 
  • It has a double roof to reduce excessive heat from solar radiation.
  • It has metal legs to avoid attacks by termites. Sometimes the legs are made of treated wood for the same reason. 
  • it is made of wood which is bad conductor of heat

importance of Stevenson screen

  • provide shade conditions for accurate temperature recording
  • ensure the safety of thermometers because they are delicate

recording and calculating weather condition

temperature

temperature is the degree of hotness of object or place




it is measured using three types of thermometer namely:

  • maximum thermometer
  • minimum thermometer
  • six’s thermometer

maximum thermometer

it is used to measure the highest temperature reached in a day

it uses mercury




how maximum thermometer works
  • the temperature rises causing the mercury to expand
  • mercury pushes the index up
  • when the temperature falls mercury contracts
  • the maximum temperature is read from the scale at the lower end of the index
  • the thermometer is reset by shaking it to force the mercury back into the bulb

minimum thermometer

this is used to record the lowest temperature reached in the day. it uses alcohol

how minimum thermometer works
  • temperature falls causing the alcohol to contract
  • alcohol pulls the index down
  • when the temperature rises alcohol expands and rises in the tube
  • the index remains where it was pulled
  • the minimum temperature reading is obtained from the scale at the lower end of the index




calculating temperature

diurnal or daily temperature range

this is the difference between the maximum and minimum temperature for any one day

mean daily temperature

this is the average of the maximum and minimum daily temperature




mean monthly temperature

this is the sum of the mean daily temperature in the month divided by the number of days in that month

mean monthly minimum temperature

this is the sum of daily minimum temperatures divided by the number of days in that month

mean monthly maximum temperature

this is the sum of the daily maximum temperature divided by the number of days in that month.

mean annual temperature

sum of mean monthly temperature divided by 12

mean annual temperature range




this is the difference between the highest and lowest mean monthly temperature in the year

rainfall

amount of rainfall in the day is measured by the instrument called a rain gauge

rain gauge should be made of impenetrable material which can not absorb water

how rain gauge works

  • it is taken to an open space to prevent water from dropping into the funnel
  • it is sunk in the ground to prevent evaporation
  • the funnel top is left 30 cm above the ground to prevent splashes of the water and the runoff
  • after 24 hours water is emptied into a measuring cylinder
  • the reading of the amount of rainfall is got from the measuring cylinder in millimeters
  • the figure represents the millimeters of water falling on each square millimeter of the ground
  • a rain gauge can be used to measure snowfall by melting it before the reading is taken.

calculating rainfall




monthly rainfall total

the sum of the rainfall recorded in the month

annual rainfall total

the sum of monthly rainfall totals for 12 month

mean monthly rainfall

the sum of the rainfall totals for the particular month over several years divided by the number of years of observation

mean annual rainfall

the sum of mean monthly rainfall for 12 months of the year

sunshine

duration of sunshine is measured using Campell stokes sunshine recorder

how compell stokes works

  • the spherical lens focuses light on the sensitive paper
  • the paper burns when the sun is shining
  • the total hours of sunshine is got by adding all the burnt sections from the calibration on the side of sensitive paper
  • the sensitive paper is changed every day

humidity




humidity is the condition of the atmosphere concerning its water content

it is measured with the hygrometer or psychrometer which consists of a wet and dry bulb thermometer kept on the Stevenson screen.

wet bulb thermometer is the thermometer covered with muslin bag immersed in water while dry bulb thermometer has no muslin

how hygrometer works

  • when the air is dry there is a lot of evaporation from the muslin
  • evaporation cools the bulb of the thermometer resulting in the low temperature reading
  • when humidity is high there is little evaporation from the muslin
  • the wet bulb thermometer is cooled art the slower rate and both thermometer shows almost the same temperature reading.
  • the difference in the reading between the two thermometer is used to determine relative humidity

interpretation of hygrometer reading

  • when the two reading are the same, relative humidity is 100% that is the air is saturated
  • if the difference is small the humidity is high
  • if the difference is big it means humidity is very low




calculating humidity

absolute humidity

it is the actual amount of water vapor a given volume of air can hold

it is expressed in g/m3

specific humidity

this is the mass of water vapour in the given mass of air. it is expressed in g/km

relative humidity

this is the ratio between the absolute humidity and the maximum amount of water vapour the air can hold expressed in percentage

relative humidity = absolute humidity/maximum amount of water the air can hold at the same temperature




For example if the 22 centigrade contain s 10g/m3 and the given air can hold a maximum of 20g/m3 what is its relative humidity

solution

10*(100/20)=50%

wind direction

wind direction is determined using the wind vane

how wind vane works

  • as the wind blows the arrow swings
  • the arrow points in the opposite drection of the wind flow
  • the direction is read from the cardinal compass points
  • the arrow will point in the direction from which the wind is blowing
  • for example if it points to south then the wind is blowing from south towards north




the wind sock

is used to indicate the general direction of the wind flow

windsock is not kept in the weather station because it does not give the accurate direction of wind flow

they are seen near airstrip for the benefit of the pilots

how wind sock works

when the wind blows the sock stretches out in the direction the wind is blown

wind speed or velocity

this is measured by using the anemometer

how anemometer works

  • when the wind blows the hemispherical cups rotates
  • the number of rotation is obtained from the meter on the lower part of the anemometer
  • the units for measuring wind speed is called knot

cloud cover




the amount of cloud cover is observed using eyes

it is given in oktas

okta = is approximately 1/8 of the sky is covered with clouds

atmospheric pressure

this is the force exerted by gases in the atmosphere on some areas or bodies on the earth’s surface

atmospheric pressure is measured using barometers of three types namely: mercury, aneroid, and Fortin barometers




mercury barometer

how mercury barometer works
  • air exerts pressure on the mercury in the beaker
  • the height of the mercury in the tube is proportional to atmospheric pressure
  • the reading are taken in mmHg
  • it is 760mmHg at the sea level
advantages of mercury barometer
  • it is accurate
disadvantages of the mercury barometer
  • it is cumbersome to carry around
  • it can be damaged quite easily while being carried around

aneroid barometer

this is used to measure changes in the atmospheric pressure

how aneroid barometer works
  • has airtght compartment (vacuum)
  • compartment expands when pressure decreases
  • it collapses when pressure increases
  • the movement is transmitted by lever to a pointer on a dial
  • the reading are in kg/cm3




evaporation

the rate and amount of evaporation is measured using piche and tank evaporimeter

piche evaporimeter

how piche evaporimeter works
  • when there is lot of sunshine water evaporates from the blotting paper
  • the level of water in the glass tube reduces
  • the rate and amount of evaporation is got by looking at the scale on the outside of the glass tube
  • the units are in mm




tank evaporimeter

how tank evaporimeter works
  • the tank with water is put in the open
  • water evaporate when there is a lot of sunshine
  • water in the tank reduces
  • the rate and amount of evaporation is obtained from calibration in the inner side of the tank in mm

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4 TYPES OF TEMPERATURE INVERSION




Temperature inversion is a condition in which the temperature of the atmosphere increases with altitude in contrast to the normal decrease with altitude.

When temperature inversion occurs, cold air underlies warmer air at higher altitudes.

There are four kinds of temperature inversions: ground, turbulence, subsidence, and frontal.




A ground inversion

develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation.

If the temperature of surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions.




If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.




A turbulence inversion

often forms when quiescent air overlies turbulent air.

Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer.

The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.




A subsidence inversion

develops when a widespread layer of air descends.

The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result, the lapse rate of temperature is reduced.

If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion.




Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.

A frontal inversion

occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below.

This kind of inversion has considerable slope, whereas other inversions are nearly horizontal.




In addition, humidity may be high, and clouds may be present immediately above it.

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7 CAUSES OF TEMPERATURE INVERSION




Temperature inversion is a condition in which the temperature of the atmosphere increases with altitude in contrast to the normal decrease with altitude.

When temperature inversion occurs, cold air underlies warmer air at higher altitudes.




Image result for TEMPERATURE INVERSION

    Normally, temperature decreases with an increase in elevation. It is called the normal lapse rate.

At times, the situation is reversed and the normal lapse rate is inverted. It is called Inversion of temperature.

Inversion is usually of short duration but quite common nonetheless. A long winter night with clear skies and still air is an ideal situation for inversion.

The heat of the day is radiated off during the night, and by early morning hours, the earth is cooler than the air above.

Over polar areas, a temperature inversion is normal throughout the year. Surface inversion promotes stability in the lower layers of the atmosphere.




Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere.

Dense fogs in the mornings are common occurrences, especially during winter season. This inversion commonly lasts for few hours until the sun comes up and beings to warm the earth.

The inversion takes place in hills and mountains due to air drainage. Cold air at the hills and mountains, produced during night, flows under the influence of gravity.




Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above.

This is called air drainage. It protects plants from frost damages.

Causes of Temperature Inversions

Normally, air temperature decreases at a rate of 3.5°F for every 1000 feet (or roughly 6.4°C for every kilometer) you climb into the atmosphere.




When this normal cycle is present, it is considered an unstable air mass and air constantly flows between the warm and cool areas.

As such the air is better able to mix and spread around pollutants.During an inversion episode, temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing.




This is why inversion layers are called stable air masses.
Temperature inversions are a result of other weather conditions in an area.

They occur most often when a warm, less dense air mass moves over a dense, cold air mass.

This can happen for example, when the air near the ground rapidly loses its heat on a clear night. In this situation, the ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day.




Additionally, temperature inversions occur in some coastal areas because upwelling of cold water can decrease surface air temperature and the cold air mass stays under warmer ones.

Topography can also play a role in creating a temperature inversion since it can sometimes cause cold air to flow from mountain peaks down into valleys.




This cold air then pushes under the warmer air rising from the valley, creating the inversion.

In addition, inversions can also form in areas with significant snow cover because the snow at ground level is cold and its white color reflects almost all heat coming in.

Thus, the air above the snow is often warmer because it holds the reflected energy.




IMPORTANCE OF WEATHER TO HUMAN




Weather can be defined as the physical condition or state of the atmosphere at a particular time and place.

Most weather happens in the part of the earth’s surface called the troposphere

temperature, pressure, wind, humidity, and precipitation interact with each other.




they influence the atmospheric conditions like the direction and velocity of the wind, amount of insolation, cloud cover, and the amount of precipitation.

Weather can be defined as the physical condition or state of the atmosphere at a particular time and place. Most weather happen the part of the earth's surface called the troposphere

these are known as elements of both weather and climate. the influence of these elements differs from place to place and time to time.

it may be restricted to a small area and for a small duration of time,




we very often describe the influence of weather elements in name of weather such as sunny, hot, warm, cold, fine depending upon the dominant element of weather at a place and point in time.

therefore, the weather is the atmospheric condition of a place for a short duration with respect to its one or more elements.

two places even a short distance apart may have different kinds of weather at one and the same time




 Importance of weather:

  • Good weather improves people’s lives
  • Weather determines the kind of clothing to be worn by people in an area.
  • Knowledge of the weather of a place enables people to carry out economic activities which can be sustained by the weather in that place. e.g. dairy cattle do well in a cool and wet place.
  • By studying the weather of a place over a long time, we can establish its climate.




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