Theme 2: The Natural Environment

Theme 2: The Natural Environment
Unit 2.4: Weather

Syllabus Content Describe how weather data is collected.
You should be able to: Further details Describe how weather data is collected. Make calculations using information from weather instruments. Use and interpret graphs and other diagrams showing weather and climate data Describe and explain the characteristics, siting and use made of a Stevenson screen. Instruments such as: Rain gauge, maximum-minimum thermometer, wet-and-dry bulb thermometer (hygrometer), sunshine recorder, barometer, anemometer and wind vane, along with simple digital instruments which can be used for weather observations; observations of types and amounts of cloud Syllabus Content

Weather vs. Climate Weather describes the condition of the atmosphere of a place over a short period of time. E.g. Temperature, precipitation, wind and sunshine. Climate is the average conditions of the atmosphere of a place over a long period of time (at least 30 years, usually 50). Average temperature and average precipitation patterns are used.

Is this weather or climate data?

Is this weather or climate data?
Climatic maps are used to show information such as distribution of temperature and rainfall of a country, region or the world.

Unit 2.4: Weather Stevenson screen

Stevenson Screen - Characteristics
A white, wooden box on stilts with louvered sides. White wooden box to reflect heat. Louvered sides to allow air to flow freely. Doubled layered roof to prevent direct heating from the sun. Stand on stilts to prevent heat from the ground to be trapped. It houses maximum and minimum thermometers and wet and dry bulb thermometers (hygrometer).

Siting the Stevenson Screen
In an open space, away from trees and buildings, in the middle of a field so readings are not affected by shelter they provide; Above grass not above concrete, as temperatures above concrete will be artificially high; In a fenced / walled compound to avoid tampering.

Elements of the Weather
Temperature Rainfall Relative Humidity Air Pressure Wind Cloud Cover Sunshine

Unit 2.4: Weather – Elements of the Weather
temperature

Elements of Weather - Temperature
This refers to the degree of heat or cold in the air. It tells us how hot or cold the air is. > 20°C – high temperatures < 10°C – low temperatures A thermometer is used to measure temperature in degree Celsius (◦C) or degree Fahrenheit(◦F). Isotherm: a line drawn on a map joining places of equal temperature. A Six’s Thermometer or the maximum and minimum thermometer is used to record the highest and lowest temperatures of a day.

Maximum and minimum thermometers
Used to measure the maximum and minimum temperatures of a day. Metal index in the maximum thermometer will stay at the maximum temperature recorded. Metal index in the minimum thermometer will stay at the minimum temperature recorded.

Maximum and Minimum Thermometers
Maximum Thermometers Minimum Thermometers When temperature rises, the mercury expands and pushes the index along the tube. When temperature falls, the mercury contracts but the index stays where it was pushed to by mercury. The maximum temperature is at the point where the index is. The index is then drawn back to the mercury by magnet for next reading. When temperature falls, the alcohol contracts and its meniscus pulls the index along the tube. When the temp rises, the alcohol expands but the index stays where it is. It is read the same way as the maximum thermometer. Maximum and Minimum Thermometers

Six’s Thermometer

Made up of Mercury and Alcohol
Mercury – Very high boiling point Alcohol – Very low freezing point Read off the base of the Indices (Indicators)

Activity - Reading the Six’s
Max = 30oC Min = 5oC Max = 20oC Min = 10oC Max = 7.5oC Min = -15oC

Describing Temperature
Temperature calculation Definition Formula Mean daily temperature Sum of hourly temperatures divided by 24 hours Sum of hourly temperatures 24 Diurnal temperature Range Difference between the max. and min. temperatures recorded in a day Max. daily temperature – min. daily temperature Mean monthly temperature Average daily temperatures recorded in a month Sum of mean daily temperatures in the month/Number of days in the month Mean annual temperature Average temperature recorded in a year Sum of mean monthly temperatures in the year/12 Annual temperature range Difference between the max. and min. mean monthly temperatures recorded in a year Max. mean monthly temperatures – min. mean month temperature

Calculating Temperatures (1/5)
Daily temperature Range = Maximum Temp - Minimum Temp For example Max temp = 30°C Min Temp = 10°C Daily temp range = 20°C

Mean (average) daily temperature = Maximum Temp + Minimum Temp 2 For example Max temp = 30°C Min Temp = 10°C Average daily temp= (30+10)/2 = 20°C

Mean (average) monthly temperature = Total average daily temp for the month Number of days in the month Mean (average) monthly temperature = Average max temp + Average min temp 2 Only use this when average maximum and average minimum temperature are given.

Mean (average) Annual temperature = Total average monthly temp in a year 12 Annual temperature range = Highest mean Lowest mean monthly temp monthly temp

Location X Mean Annual Temperature = Total of mean monthly temperature
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Temp 23 22 20.5 16.5 14 10.5 10 11 12.5 17.5 21 Mean Annual Temperature = Total of mean monthly temperature 12 = ( ) = (192.5) = 16.04°C Annual Temperature Range = Highest Temperature – Lowest Temperature = 23 – 10 = 13°C

Location Y Mean Annual Temperature = Total of mean monthly temperature
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Temp 25.5 26 26.5 27 27.5 28 Mean Annual Temperature = Total of mean monthly temperature 12 = ( ) = (321) = 26.75°C Annual Temperature Range = Highest Temperature – Lowest Temperature = 28 – 25.5 = 2.5°C

Temperature Summary of Temperature Data Calculations:
Average daily or mean daily temperature: Average daily or mean daily temperature Maximum temperature + Minimum temperature 2 = Daily temperature range Daily temperature range = Maximum temperature - Minimum temperature Average monthly temperature Average monthly temperature Sum of average daily temperatures for the month Number of days in the month = Average annual temperature Average annual temperature Sum of average monthly temperatures in a year 12 =

precipitation

Elements of Weather - Precipitation
This refers to any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of include rain, sleet, snow, and hail. The most common form measured is rainfall. Isohyet: a line on map that joins areas of equal rainfall Rainfall (and snowfall) is measured using an instrument called a rain gauge. Snow (and other solid forms) are measured using a snow gauge. It consists of a cylinder containing a collecting can which in turn contains a glass/plastic jar and funnel.

Rain Gauge Rain falls through a funnel and is collected in the bottle
General Location Rain falls through a funnel and is collected in the bottle Excess rain that overflows is collected in the copper cylinder Amount of rain collected is poured into a measuring cylinder, calibrated in millimeters The gauge is placed in open space so that only raindrops enter the funnel. No runoff water should enter. It is sunk into the ground, so that the top of funnel is 30 cm above the surface. This is to prevent the heat from the ground causing evaporation and to prevent rain splashing up from ground into the funnel. Rainwater falling into the funnel collects in the jar, which is emptied every 24 hrs and measured in a tapered glass cylinder , graduated in mm. Check rain gauge everyday, at same time, to avoid false reading because of dew effect. Rain Gauge

A rain gauge and its parts
copper cylinder funnel outer cylinder glass bottle

Calculating Rainfall (1/4)
Daily rainfall = Sum of all rainfall readings in a day

Monthly rainfall = Sum of all daily rainfall readings in a month

Total annual rainfall = Sum of the total monthly rainfall received for the year.

Mean (average) monthly rainfall = Total annual rainfall 12

humidity

Elements of Weather - Humidity
Humidity is the amount of moisture (water vapour) in the air. There are two types of humidity measurements: Absolute Humidity – measured by mass (g / m³) Relative Humidity – measured as percentage or ratio. The maximum amount of humidity the air can hold depends on temperature. A hygrometer measures the humidity of the air. A hygrometer has two thermometers, a dry one and a wet. There is a difference between the dry and wet bulb thermometers because of latent heat created during the process of evaporation Relative Humidity is calculated by using a table that looks at the difference between the wet bulb and the dry bulb. A hygrometer can also be used to find dew point.

Humidity Question: Answer: Question:
Why do we feel sticky or clammy on hot days? Answer: Too much water vapour in the air slows down the evaporation of our perspiration. Question: How then, does water vapour enter the air? What affects the humidity of the air? How do we measure the humidity of the air?

The maximum quantity of moisture that can be held in the air
Specific Humidity The maximum quantity of moisture that can be held in the air This depends on air temperature

It is highest in low latitudes and lowest in high latitudes.
Absolute Humidity The actual quantity or amount of water vapour (moisture) held by a parcel of air, at a given temperature, expressed as grams per cubic metre (g/m³). It is highest in low latitudes and lowest in high latitudes.

Absolute Humidity The maximum absolute humidity of warm air at 30°C is approximately 30g of water vapour - 30g/m³. The maximum absolute humidity of cold air at 0°C is approximately 5g of water vapour - 5g/m³. From the examples, it is important to note that warm air can hold more water vapour (moisture) than cold air. Put another way, as temperature increases, the ability of a parcel of air to hold moisture also increases.

Relative Humidity Relative humidity also measures water vapour but relative to the temperature of the air. It is expressed as the amount of water vapour in the air as a percentage of the total amount that could be held at its current temperature. Relative humidity is the proportion of the actual amount of water vapour in a mass of air compared to the maximum amount of water vapour the air can hold at a given temperature.

Relative humidity As an example, consider two containers:
Container one has a maximum volume of 30g of water and is half full- it contains 50% of its capacity. Container two has a maximum volume of 5g of water and is three quarters full- it contains 75% of its capacity. Container one contains four times as much water as container two, yet actually contains a lower percentage.

Relative Humidity Actual amount of water vapour in the air (g/m3)
x 100% Maximum amount of water vapour the air can hold (g/m3)

Relative Humidity Relative Humidity changes when:
Atmosphere gains or loses water vapour Evaporation Temperature changes Lower temperature relative humidity rises Raise temperature relative humidity decreases

Relative humidity is affected by
The amount of water vapour in the air Temperature When the amount of water vapour in the air increases without any change in temperature, relative humidity increases. When the amount of water vapour in the air decreases without any change in temperature, relative humidity decreases. Warm air can hold more water vapour than cool air. When temperature increases without any change to the amount of water vapour in the air, the rise in temperature makes air more able to hold water vapour, hence relative humidity decreases as temperature increases. Relative humidity is affected by

Relative Humidity Saturation of the air occurs when relative humidity is 100%. When saturation occurs, the air can hold no more water vapour at that temperature. The temperature at which saturation occurs is known as the dew point temperature. Dew-point temperature: temperature at which air with a given humidity will reach saturation when cooled without changing its pressure.

Measuring Relative Humidity

Measuring Relative Humidity
Relative humidity is measured using a wet and dry bulb thermometer This is also known as hygrometer. The ‘dry-bulb’ is a glass thermometer, which records the actual air temperature. The ‘wet-bulb’ is similar thermometer, but with the bulb enclosed in a muslin bag which has one end in a container of water.

A Hygrometer Dry Bulb Thermometer Wet Bulb Thermometer Container of distilled water with muslin wick

How does the hygrometer work?
The dry bulb thermometer records the actual temperature of the surrounding (ambient) air. In the case of the wet- bulb thermometer, temperatures cause the water on the wet cotton to evaporate. The evaporation cools the bulb of the wet-bulb thermometer. The drier the ambient (surrounding) air, the more evaporation and associated cooling can take place, the lower the reading of the wet-bulb thermometer.

How does the hygrometer work?
Find the difference between the wet-bulb and the dry-bulb temperature to derive the wet bulb depression. Derive the relative humidity using the relative humidity table. Relative humidity is the intersect between the dry-bulb temperature and the difference between the wet-bulb and dry-bulb temperature. Gnerally, the smaller the difference between the dry and wet bulb raedings, the more humid the air.

A Sling Psychrometer This a variation on the normal hygrometer. It has all the elements of a hygrometer. However, it is not housed in a Stevenson Screen but can be used to determine relative humidity at any point in time, with the help of Relative Humidity Tables.

Relative Humidity Table

What is the relative humidity?
Dry Bulb – 12ºC Wet Bulb – 7ºC Dry Bulb – 22ºC Wet Bulb – 19ºC

Air pressure

Elements of Weather – Air Pressure
What is air pressure? Refers to the force exerted on an unit area of the earth’s surface by the weight of a column of air above it. In other words, air has weight and exerts a force on the Earth’s surface. What influences air pressure? Temperature Altitude

What is the relationship between pressure and temperature?
As temperature increases, pressure decreases. This is because as the temperature rises, the atmosphere expands, becoming less dense and rises. This creates areas of low pressure on the Earth’s surface. As temperature decreases, pressure increases. This is because as the temperature falls, the atmosphere contracts, becoming more dense and descends. This creates areas of high pressure on the Earth’s surface. Rising air cools, becomes dense and eventually sinks back down to Earth.

What is the relationship between pressure and altitude?
Low-pressure areas have less atmospheric mass above their location. High-pressure areas have more atmospheric mass above their location. As altitude increases there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing altitude.

As the altitude increases pressure decreases.
As the altitude decreases pressure increases.

Measuring Air Pressure

Measuring Air Pressure
Air pressure is measured using a barometer. There are two types of barometer: a mercury barometer an aneroid barometer A barograph records the pressure.

Mercury Barometer This consists of a glass tube, closed at one end, with an open mercury- filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir.

Aneroid Barometer This is an instrument for measuring pressure that does not involve liquid. It uses a small, flexible metal box called an aneroid cell (capsule). The evacuated capsule (a vacuum) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer.

Mercury Barometers Aneroid Barometers

Air Pressure Also called barometric pressure.
At sea level, the weight of the air on the surface of the Earth is 1.03kg/cm³. Air pressure is measured in millibars (mb). Average air pressure at sea level is 1013 mb. High air pressure > 1013 mb. Low air pressure < 1013 mb. Isobars are lines drawn on maps connecting places of equal pressure.

An Isobaric Map

Importance of Atmospheric Pressure
The variation in temperature and altitude across the Earth’s surface causes atmospheric pressure at sea level to also vary. This creates global belts or zones of high and low pressure, which in turn affects global wind patterns, wind speed and direction and humidity.

GLOBAL PRESSURE BELTS Rising air at the equator causes the equatorial belt of low pressure Descending air at about 30ºN and 30ºS causes the sub-tropical belt of high pressure Polar high pressure Mid latitude low pressure Sub-tropical high pressure Rising air at about 60ºN and 60ºS causes a mid-latitude belt of low pressure Equatorial low pressure Descending air at the poles causes the polar high pressure areas Sub-tropical high pressure Mid latitude low pressure Polar high pressure

ASSOCIATED SURFACE WIND PATTERNS
Winds always blow from high pressure to low pressure. They are deflected because of the Coriolis Force which come about because of the rotation of the earth. Polar high pressure Mid latitude low pressure Sub-tropical high pressure Winds in Northern Hemisphere are deflected to the right. Equatorial low pressure Winds in the southern hemisphere are deflected to the left. Sub-tropical high pressure These wind belts shift seasonally. Mid latitude low pressure Polar high pressure

wind

Elements of Weather - Wind
Air moves from an area of high pressure to an area of low pressure. This movement of air is known as wind. Wind is described in terms of speed, direction and frequency.

Wind Speed Wind Speed This refers to the rate at which air is moving. It is measured in several units including meters per second (m/sec); knots per hour (k/hr); kilometres per hour (km/h). Wind speed is affected by pressure gradients. A pressure gradient refers to the difference in atmospheric pressure which exists between the area a wind blows from and the region it is blowing to. The steeper the gradient (i.e. the larger the difference), the greater is the wind speed. Wind speed is measured by an instrument called an anemometer. It consists of 3 or 4 metal cups fixed to metal arms that rotate freely on a vertical shaft. When the wind blows, the cups rotate. The rate of the rotations is recorded on a meter to give speed in km/hr. Wind speed can also be determined by its effects using a Beaufort Wind Force Scale. This relates wind speed to observed conditions at sea or on land. Measuring Wind

Anemometers  Beaufort Wind Force Scale

Wind Direction Wind Direction This refers to the direction from which the wind is blowing. Winds blow from regions of higher to regions of lower pressure. Wind direction is determined using a wind vane. This consists of a horizontal rotating arm which turns on a vertical shaft. The rotating arm has a tail and pointer. As the wind blows, the arm swings around and the pointer faces into the wind. The cardinal points (NEWS) are marked on arms which are rigidly fixed to shaft. Measuring Wind

MeasuringWind This is used to record wind frequency. This looks at:
Wind Rose Wind Rose This is used to record wind frequency. This looks at: Percentage of time wind blows from a particular direction. Winds that blow most frequently from a specific direction are called prevailing wind. The wind direction for a specific place can be shown on a wind rose. This is made up of a circle from which rectangles of equal size radiate. The directions of rectangles represent points of the compass. The number of rectangles show the number of days/times the wind blows from that direction. In the centre of the rose the number of days with no wind is recorded. MeasuringWind

Land and Sea Breezes

Land Breeze During the night, the land cools down faster than the sea.
As cool air over the land sinks, a high pressure area is formed. Air over the sea is warmer as the sea loses heat slower. The warm air over the sea rises and forms a low pressure area. As air moves from a high pressure area to a low pressure area, this causes air to move from the land towards the sea, forming a land breeze.

Land Breeze

Sea breeze During the day, the land heats up faster than the sea.
As the warm air over the land rises, a low pressure area is formed. Air over the sea is cooler as the sea heats up slower. The cool air over the sea sinks and forms a high pressure area. As air moves from a high pressure area to a low pressure area, this causes air to move from the sea towards the land, forming a sea breeze.

Sea breeze

clouds

How are clouds formed? Water is converted to water vapour through evaporation. As water vapour rises, it starts to cool. When the water vapour cools to dew point temperature, and there are particles for the water vapour to condense on, condensation takes place to form water / cloud droplets. These particles are called condensation nuclei. Examples include dust, soot or sea salt As these water droplets bump against each other and become larger in a process called coalescence, clouds form.

Measuring Clouds The amount of clouds in the sky is visually assessed by observing how much of the sky is covered or obscured by cloud. The unit of measurement used is oktas (eights). The altitude, direction and velocity of clouds is measured using a nephoscope. A ray of light of known velocity is emitted from nephoscope, which will strike with the base of the clouds. The travel time of the receiving signal is used to get the height of the clouds.

Oktas Mirror Nephoscope Measuring Clouds

Types of Clouds Clouds are grouped into 4 categories based on height and composition: High Level Clouds: Composed of ice crystals Prefix ‘cirro’ Medium Level Clouds: Composed of water droplets or a mixture of ice crystals and water droplets Prefix ‘alto’ Low Level Clouds: Composed of water droplets Contains a subset of vertically developed clouds Prefix ‘cumulo / nimbo Extensive Vertical Development: Clouds whose bases are at low level but their tops are much higher than other low level clouds

Types of Clouds The names given to clouds are important and can help to identify different types based on shape or characteristics. The names are used as suffixes or prefixes. Stratus (strato -): layered / flat clouds that appear as sheets across the sky. Cumulus (cumulo -): puffy clouds with a flat base. Look like puffs of wool. Nimbus (nimbo -): rain-bearing clouds

Types of clouds These clouds have bases between 2000m and 7000m.
High Level Clouds Medium Level Clouds These form at altitudes above 6000m; usually 5500 – 14000m. Because the air is cold and dry at such altitudes, high clouds are composed almost exclusively of ice crystals. These clouds also tend to be very thin. Cirrus- white, long, wispy filaments / streamers. Little vertical development. Cirrocumulus- infrequent. Occur individually or in long rows. Have a rippled appearance looking like fish scales. Cirrostratus- cold, dry clouds with a sheet-like appearance. Often with a halo and varying vertical development. Thick Cs clouds show impending bad weather. These clouds have bases between 2000m and 7000m. They are usually composed of water droplets or mixture of water droplets and ice crystals (if temperatures are low enough). Altocumulus- puffy / rippled generally in shades of grey. Altostratus- combination of water droplets and ice crystals. A thin layer, grey, to blue-grey. Block out the majority of sunlight and give grey, dreary days. Types of clouds

Clouds with Extensive Vertical Development
Low Level Clouds Clouds with Extensive Vertical Development These have their bases lying below 2000m. They are usually composed of water droplets. Stratus- typically a uniform grayish color covering over the entire sky. These clouds are often mistaken for a fog, but they do not touch the ground. They may be associated with small amounts of precipitation, such as drizzle, but do not produce larger precipitation. Stratocumulus- low, rounded, and "lumpy" with small patches of blue sky in between. These clouds are much larger elements than altocumulus but are not associated with precipitation. Nimbostratus- thick dark gray layers, associated with continuously falling rain or snow. Nimbostratus clouds can occupy the entire sky and have moderate vertical development. Although composed of water droplets, cumulonimbus clouds can include ice crystals. Cumulus- puffy, "floating cotton" appearance of the cumulus (Cu) cloud is a common sight. These clouds can be distinguished from a stratocumulus cloud by the large amounts of sky visible between each cloud, compared to the relatively small space between the Sc clouds. Cumulonimbus- large cauliflower-shaped towers, often with ‘anvil tops’. A great deal of vertical development, also known as thunderstorm clouds. Types of clouds

Cirrus

Stratus

Cumulonimbus

Cumulus with vertical extent
Cirrus Stratus

sunshine

Sunshine The amount of sunshine we have depends on latitude and how much cloud there is in the sky. For example, in the Eastern Sahara desert, the sun is covered by clouds for less than 100 hours a year. This, the number of sunshine hours is very high - more than 3,600 hours each year. In Britain sunshine duration ranges from 1,850 hours in Southern England to 1,200 hours in North Scotland. There is usually more sunshine where atmospheric pressure is higher since higher pressure generally means less cloud cover.

Annual sunshine hours map of the world

Measuring Sunshine The intensity of sunshine cam be measured by a solar radiometer / pyrheliometer / pyranometer. The duration / amount of sunshine i.e. the number of hours and minutes of sunshine received in one place is measured using a Campbell-Stokes Sunshine Recorder. It works by using a glass sphere (mounted on a metal frame) to focus the sun’s rays onto a strip of photo-sensitive card gradated in hours and minutes. As the sun moves round during the day, a trace is scorched onto the card. At the end of the day, card is removed and replaced. The length of the trace represents the amount of sunshine that the location received.

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