Weather and Climate | ICSE Class 7 Geography Notes

This note covers weather and climate, temperature, atmospheric pressure, wind, humidity, condensation, precipitation, cloud types, weather instruments, temperature records, weather stations, isotherms and isohyets.

What is the difference between weather and climate?

Weather is the state of the atmosphere at a particular place and time. The atmosphere is the layer of gases surrounding Earth. Descriptions such as hot, cold, rainy, cloudy and windy tell us about different aspects of the weather.

Climate is the pattern of weather experienced by an area or region over a long period, several decades at least. A decade is ten years. Climate describes a long-term pattern rather than the conditions experienced during a single afternoon.

Why does the time period matter?

Weather is what we experience from hour to hour or day to day, and it keeps changing. Climate brings together the patterns found across many years. A short record of temperature or rainfall therefore describes weather during that period; it cannot by itself establish a region's climate.

How do seasons connect the two?

Seasons are recurring parts of the year, each lasting a few months. Weather changes with the season, while the recurring pattern of seasons is connected with climate. Thus, weather, seasons and climate are related, but they describe different time scales.

Clothing, food and crops are connected with seasonal conditions. Recognising this connection helps explain why climate matters in daily life. It also helps us avoid confusing a change in today's weather with a change in the long-term climate.

Which elements describe the weather of a place?

An element of weather is an aspect of atmospheric conditions that we observe or measure. Temperature, atmospheric pressure, humidity, precipitation, wind and clouds together help us describe the weather. Knowing just one of these does not provide the whole description.

Water vapour is water in its gaseous state. It differs from the liquid droplets and solid ice crystals that form clouds. Humidity concerns water vapour in the air, while cloud observations concern visible collections of droplets or crystals.

Why do we need instruments?

Words such as warm and chilly describe how conditions feel, but people may experience the same temperature differently. Instruments provide measurements that people can compare. A shared scale makes an observation more useful than a description based entirely on personal sensation.

Keep the element separate from the instrument's name. Temperature is the condition being measured; a thermometer is the measuring instrument. In the same way, wind speed is a measurement of moving air, whereas an anemometer is the device used to obtain it.

How does a thermometer measure air temperature?

A thermometer measures temperature. Ambient temperature means the temperature of the immediate surroundings. Some thermometers show this temperature, while others record the highest and lowest temperatures reached during a day.

Maximum temperature means the highest temperature recorded during the stated period. Minimum temperature means the lowest. Both terms need a time period: the maximum for a day and the maximum found in a week's records describe different selections of observations.

What do temperature units mean?

The Celsius scale and the Fahrenheit scale are temperature scales. The symbols °C and °F mean degrees Celsius and degrees Fahrenheit respectively. A temperature of 15°C is the same as 59°F, so the number must be read together with its scale.

Thermometers often contain a coloured liquid that expands when temperature increases. The level of this liquid is read against a scale. Digital thermometers are increasingly preferred because they are more precise and can record more data.

Draw a liquid-filled bulb connected to a narrow tube. Add a temperature scale beside the tube. Label the bulb, liquid column and Celsius scale. The drawing should make clear which level is read against the scale.

How can daily readings be summarised?

The range of temperature is the maximum temperature minus the minimum during a particular period, usually 24 hours. The mean daily temperature is obtained by adding the day's maximum and minimum and dividing the sum by two.

Daily range = daily maximum temperature − daily minimum temperature. Here, the equals sign means “is equal to”, and the minus sign means subtraction. The result expresses the difference between the two readings in degrees.

Mean daily temperature = (daily maximum temperature + daily minimum temperature) ÷ 2. The plus sign means addition; the brackets group the readings to add first; the division sign means divide. The divisor 2 represents the two readings used.

How can we interpret a week of temperature records?

Case study: A city in Madhya Pradesh

The following record gives daily maximum and minimum temperatures for a city in Madhya Pradesh. The dates use the day.month.year format. Each row pairs readings from the same day, and both temperature columns use degrees Celsius.

On 03.03.2025, the maximum was 32°C and the minimum was 18°C. The daily range was therefore 32 − 18 = 14°C. The mean daily temperature was (32 + 18) ÷ 2 = 25°C.

How does a weekly comparison differ?

The highest maximum shown anywhere in the record is 32°C, on 03.03.2025. The lowest minimum is 14°C, on 05.03.2025. The difference between those extremes is 32 − 14 = 18°C. Those extremes occurred on different dates.

That 18°C difference describes the extremes across the displayed week. It is not the daily range for 03.03.2025. To calculate a daily range, use the maximum and minimum in the same row rather than taking readings from different days.

The maximum temperatures rise from 29°C on 28.02.2025 to 32°C on 03.03.2025. They then fall to 28°C on 05.03.2025 and rise to 29°C on 06.03.2025. The sequence therefore does not show a continuous rise throughout the week.

Note: Read the date, the column heading and the unit before calculating. Keep the printed observations separate from the range or mean calculated from them.

This record supports statements about the weather during the dates shown. Establishing the climate would require a much longer record. Even careful calculations from a short table cannot turn a week's observations into a description of several decades.

What is atmospheric pressure, and how is it measured?

Atmospheric pressure is the pressure exerted by the weight of the air above and around us. A barometer measures this pressure. Air pressure is an element of weather even though we cannot describe it simply by looking at the sky.

Atmospheric pressure is higher near the sea coast and lower as we go higher into the mountains. At greater heights, the air is thinner. This helps explain why people sometimes feel breathless or tired at high altitudes. Altitude means height above sea level.

Does pressure remain fixed at the coast?

Pressure is not always high in the plains or on the coast. It sometimes drops dramatically, producing a depression, or low-pressure system. Such a system can sometimes develop into a storm or cyclone. A cyclone is a weather system with winds rotating around a low-pressure centre.

A pressure reading therefore needs its location and time. A general comparison between the coast and a mountain does not mean that coastal pressure remains unchanged. Weather observations track changes as well as differences between places.

How is a barometer represented?

There are several types of barometer. Their readings are generally displayed in millibars, a unit of pressure abbreviated as mb. Normal atmospheric pressure at the sea coast is around 1013 mb. Keep “around” in this description: it is not an unchanging reading.

Draw a circular dial with a pointer and a marked pressure scale. Label the pointer and the scale, using “millibars” for the unit. This is an external recognition sketch, not a drawing of the internal mechanism.

For identification, link the instrument directly to its element: a barometer measures atmospheric pressure. Do not confuse it with a thermometer, which measures temperature, or with a hygrometer, which measures humidity. These measurements describe different properties of the air.

How are wind direction and wind speed measured?

Wind is the movement of air from areas of high pressure towards areas of low pressure. Its direction tells us where it comes from, while its speed tells us how quickly the air moves. A complete description distinguishes these two properties.

What does a wind vane show?

A wind vane, also called a weather vane, indicates wind direction. It has a rotating arm, with a pointer at one end and a tail at the other. Wind pushes the tail and turns the arm. The pointer shows the direction from which the wind comes.

Draw a horizontal arrow with a tail, supported on a vertical pivot so that it can turn. Add fixed direction labels: north, south, east and west. Label the pointer and tail, and state that the pointer indicates where the wind comes from.

A windsock is another indicator used to show wind direction. At an airfield it helps pilots during take-off and landing. Its appearance differs from the arrow-and-tail wind vane, so distinguish the instruments when identifying a picture.

What does an anemometer show?

An anemometer measures wind speed. A cup anemometer has three or four cups attached to arms that rotate around a vertical shaft. Stronger wind produces faster rotation. A meter uses the rotations over a period of time to calculate speed.

Wind speed can be expressed in kilometres per hour, abbreviated as km/h. This unit describes distance travelled in an hour. Remember the instrument pair: the wind vane indicates direction, while the cup anemometer provides speed.

Draw cups at the ends of arms around a central vertical shaft. Label a cup, an arm and the shaft. Add curved arrows to show rotation, and write that faster rotation corresponds to stronger wind.

Wind information is useful to pilots, sailors and farmers. Its speed and direction help in weather forecasting. A greater wind speed also causes soil to dry faster, connecting wind observations with the conditions affecting agriculture.

What is humidity, and how does a hygrometer measure it?

Humidity is the amount of water vapour present in the air. A hygrometer is an instrument used to measure humidity. There are several types of hygrometer, based on different principles, so its appearance can vary.

Relative humidity compares the moisture present in the air with its full capacity at a given temperature. It is expressed as a percentage. The symbol % means per cent, or out of a hundred. Temperature must be considered when interpreting this comparison.

Saturated air contains moisture to its full capacity at the given temperature. It has a relative humidity of 100%. This does not mean that the air consists entirely of water; the percentage describes its moisture content relative to saturation.

Case study: Drying clothes in Delhi and Kochi

Consider the comparison in which Delhi has relative humidity of 52% and Kochi has 84%, with the same temperature in both places. Evaporation is the change of liquid water into water vapour. Water evaporates more slowly when the air already contains more moisture.

On the humidity information given, wet clothes are likely to dry faster in Delhi. Delhi's relative humidity is lower. Keep the temperature condition in the explanation and retain “likely”: this is a comparison using the stated conditions, not a permanent claim about either city.

Draw a simple instrument face with a pointer and a scale labelled “relative humidity (%)”. Label the pointer, scale and instrument name. Use the sketch to identify what is measured, without adding an unstudied internal mechanism.

Humidity measurement also matters in food processing and museums. Museums monitor humidity to help preserve their exhibits. These uses show why measuring water vapour can be important even when no rain is falling.

How do condensation, dew and precipitation differ?

Condensation is the change of water vapour into liquid water, caused by loss of heat. Cooling moist air can lead to condensation. This process differs from evaporation, which changes liquid water into its gaseous form.

The dew point is the temperature at which a given sample of air becomes saturated. Water vapour can condense on cooler objects or around tiny particles in the air. These particles include dust, smoke and salt.

How does cooling lead towards rainfall?

1. Moist air loses heat and cools towards the temperature at which it becomes saturated.
2. Water vapour condenses around tiny particles in the air, producing small water droplets.
3. Continued condensation helps the condensed particles grow in size.
4. When the particles can no longer remain suspended against gravity, they fall towards Earth's surface as precipitation.

Gravity is the pull that draws objects towards Earth. Precipitation is water released from the atmosphere that falls to the surface in liquid or solid form. It includes rain, snow and hail; these forms differ in their physical state and appearance.

Why is dew different from rain?

Dew forms when moisture is deposited as liquid droplets on cooler solid surfaces. It forms where those surfaces are cooled rather than falling from clouds. Clear skies, calm air, high relative humidity, and cold, long nights are ideal conditions for dew formation.

The dew point must be above the freezing point for dew to form. The freezing point is the temperature at which liquid water freezes, 0°C. Frost consists of minute ice crystals deposited on cold surfaces at or below freezing conditions.

Note: Study rain, dew, hail and snow together, but distinguish their formation. Dew develops on a cool surface; rain, snow and hail fall through the air as precipitation.

Keep three questions separate when comparing these forms: is the water liquid or solid, does it fall through the air, and does it form on a surface? This approach distinguishes snow from dew without confusing either with invisible water vapour.

How can the main cloud types be recognised?

A cloud is a mass of tiny water droplets, ice crystals, or both, suspended in the atmosphere. Clouds form when water vapour condenses in the air. Their shapes and appearance vary, so recognising cloud types involves more than describing the sky as cloudy.

What are the four main groups?

Cirrus clouds occur high in the atmosphere and are white. Their thin, feathery form contrasts with cumulus clouds, which look like cotton wool. Cumulus clouds occur in scattered patches and have a flat base.

Stratus clouds spread in layers across large portions of the sky. They generally form through loss of heat or the mixing of air masses with different temperatures. An air mass is a large body of air with broadly similar temperature and moisture conditions.

Nimbus clouds are dense and dark. They form at middle levels or very near the surface. Sometimes they are so low that they seem to touch the ground. Retain “sometimes”: this description does not apply to every nimbus cloud.

Draw separate labelled sketches: feathery cirrus, cotton-like cumulus with a flat base, a broad layer of stratus, and a dense dark mass of nimbus. Use the shapes as recognition sketches rather than a measured height chart.

The useful comparison is between feathery, cotton-like, layered and dense dark forms. Cloud names describe different appearances; they are not names for instruments. A cloud also differs from water vapour: its droplets or crystals form a visible mass.

How does a rain gauge measure rainfall?

A rain gauge collects rainwater so that the amount of rainfall can be measured. Its collecting funnel directs water into a measuring tube or cylinder. A scale allows the collected water to be read as a rainfall depth.

Rainfall is expressed in millimetres, written as mm. A millimetre is a unit of length. The reading expresses a depth of rainfall, so the unit is different from the degrees used for temperature or the percentage used for relative humidity.

The drawing shows a collecting funnel above a narrow measuring tube within a cylindrical outer container. Labels identify the collecting funnel, measuring scale and measuring tube. The scale runs alongside the tube.

See Fig. 2.6 in your NCERT textbook

Where should the instrument be placed?

Place a rain gauge in an open area away from objects that obstruct the rain. It should stand on a flat surface and remain stable so that wind does not tilt or topple it. The purpose is to allow rain to enter the collector.

Take readings at the same time each day when keeping a daily record. Record the date, the measured rainfall and the unit together. A number without its date and unit gives an incomplete account of the observation.

How is a rainfall record interpreted?

Daily measurements can be compared over a week or a month to study variation in rainfall. Such a record concerns the specified observation period. It must not be treated as an annual total or a description of the climate unless the required longer record is available.

A rain gauge and a hygrometer therefore answer different questions. The rain gauge measures rain received; the hygrometer measures humidity in the air. Moist air does not itself provide a rainfall measurement, and the two instrument readings should not be interchanged.

What do isotherms and isohyets show on maps?

An isotherm is a line on a map joining places with equal temperature. An isohyet is a line joining places receiving equal amounts of rainfall over the same period. These lines help show patterns across an area instead of presenting observations only as a list.

How should an isotherm be drawn and read?

For an isotherm, first identify the temperature information represented. Join positions with the same temperature and label the line with that temperature and its unit. A temperature line is not a boundary between countries or an indication of wind direction.

Schematic means simplified to explain a relationship.

Make a schematic map sketch with several observation points marked “same temperature”. Join those points with a smooth line and label it “isotherm”. This demonstrates equality of temperature without claiming observations for an actual region.

In this sketch, the important relationship is equality of temperature along the line. The positions illustrate the meaning of the term rather than the location of any particular climate region.

How does an isohyet differ?

An isohyet represents equal rainfall, not equal temperature. Its rainfall amounts must refer to the same period. Otherwise, a daily amount at one place could be confused with a monthly or annual amount at another place.

Make a schematic map sketch with observation points marked “same rainfall for the same period”. Join those points with a smooth line labelled “isohyet”. State the common observation period when real rainfall data are supplied.

Both lines connect places with equal values, but the measured element determines the name. Begin with the map title and the unit before interpreting a line. A temperature measurement and a rainfall measurement cannot be combined along the same equal-value line.

How do weather stations turn observations into useful information?

A weather station brings weather instruments together so that conditions at a place can be measured and tracked. Readings taken at regular intervals help describe change over time. Together, the readings provide more information than an isolated temperature or rainfall observation.

How are measurements recorded?

An Automated Weather Station, abbreviated as AWS, is a self-operating system that uses sensors to measure and record weather data. A sensor is a device that detects a condition so it can be measured. Such stations record temperature, humidity, wind, precipitation and pressure.

Weather stations support agriculture, aviation, navigation and environmental monitoring. Aviation concerns aircraft and flying; navigation concerns finding and following a route. These activities benefit from timely information about weather conditions.

How do observations support forecasts?

Meteorology is the systematic study of weather and its changes. Meteorologists are the people who study it. A weather forecast is a prediction of future weather. Meteorologists collect measurements, study the data and use scientific methods to try to predict coming conditions.

Forecasts can help people prepare for heavy rain and storms. Fishermen can be warned when stormy weather is expected at sea. If a cyclone is expected, a coastal area might need evacuation, meaning the movement of people away from danger.

A useful weather report identifies the place and observation period, names the elements measured and gives readings with units. It distinguishes recorded conditions from predictions. This keeps the report clear about what has been observed and what is expected to happen.

Glossary

• Weather — The state of the atmosphere at a particular place and time.
• Climate — The pattern of weather experienced in a region over several decades at least.
• Atmosphere — The layer of gases surrounding Earth, within which weather conditions develop.
• Temperature — A measure of how hot or cold the air or another substance is.
• Atmospheric pressure — The pressure exerted by the weight of air above and around us.
• Humidity — The amount of water vapour present in the air at a place.
• Relative humidity — Moisture present compared with the air's full capacity at a given temperature, expressed as a percentage.
• Evaporation — The process through which liquid water changes into water in its gaseous state.
• Condensation — The change of water vapour into liquid water through loss of heat.
• Dew point — The temperature at which a given sample of air becomes saturated.
• Precipitation — Water falling from the atmosphere to Earth's surface in liquid or solid form.
• Dew — Water droplets deposited by condensation on cool surfaces such as grass and leaves.
• Isotherm — A line on a map joining places that have equal temperatures.
• Isohyet — A line joining places receiving equal rainfall amounts over the same period.
• Weather station — A place where instruments are brought together to measure and track weather conditions.

Common errors and misconceptions

• Misconception: A week of hot weather establishes a region's climate. Correct: Climate describes patterns over several decades at least; a week's record describes weather over that week.
• Misconception: Temperature range and mean daily temperature are the same. Correct: Range is maximum minus minimum. Mean daily temperature is their sum divided by two.
• Misconception: A barometer measures rainfall. Correct: A barometer measures atmospheric pressure; a rain gauge measures rainfall.
• Misconception: A wind vane measures wind speed. Correct: It indicates direction. A cup anemometer measures speed using its rotating cups.
• Misconception: Relative humidity of 100% means the air is entirely water. Correct: It means the air is saturated with water vapour at the given temperature.
• Misconception: Dew falls from clouds like rain. Correct: Dew forms by condensation on cool surfaces; rain falls through the air.
• Misconception: Clouds are simply invisible water vapour. Correct: Clouds are visible masses of tiny water droplets, ice crystals, or both.
• Misconception: Isotherms and isohyets represent the same element. Correct: Isotherms join equal temperatures; isohyets join equal rainfall amounts for the same period.

Exam-style questions with model answers

Q1. Distinguish between weather and climate by defining each and stating its time scale. [2 marks]

1. Weather is the state of the atmosphere at a particular place and time, experienced from hour to hour or day to day.
2. Climate is the pattern of weather in an area or region over a long period, several decades at least.

Q2. A day's maximum temperature is 32°C and its minimum is 18°C. Calculate the daily range and mean daily temperature, showing the calculation for each. [2 marks]

1. The daily range is maximum minus minimum: 32 − 18 = 14°C.
2. The mean daily temperature is the sum of maximum and minimum divided by two: (32 + 18) ÷ 2 = 25°C.

Q3. Identify the instruments used for atmospheric pressure, humidity and rainfall. For each, state what it measures and a unit or form in which the reading is expressed. [3 marks]

1. A barometer measures atmospheric pressure, the pressure exerted by the weight of air. Its readings are generally displayed in millibars, abbreviated as mb.
2. A hygrometer measures humidity. Relative humidity expresses the moisture present compared with saturation at a given temperature, as a percentage.
3. A rain gauge measures rainfall collected during an observation period. The rainfall reading is expressed as a depth, using a unit such as millimetres.

Q4. Give one recognisable description of each of these cloud types: cirrus, cumulus, stratus and nimbus. [4 marks]

1. Cirrus clouds are thin, detached white clouds with a feathery appearance. They form high in the atmosphere.
2. Cumulus clouds look like cotton wool, occur in scattered patches and have a flat base beneath their upper part.
3. Stratus clouds form layers that cover large portions of the sky. Their spread-out form distinguishes them from separate cotton-like patches.
4. Nimbus clouds are black or dark grey, extremely dense masses that block the Sun's rays. Sometimes they seem to touch the ground.

Q5. Delhi has relative humidity of 52% and Kochi has 84%, at the same temperature. Using this humidity comparison, explain in three points where wet clothes are likely to dry faster. [3 marks]

1. Delhi has the lower relative humidity, 52% compared with Kochi's 84%, so its air is less close to saturation at the given temperature.
2. Water evaporates more slowly when the air already contains more moisture. Kochi's higher relative humidity therefore makes evaporation slower in this comparison.
3. Wet clothes are consequently likely to dry faster in Delhi on the information given. The conclusion retains the question's condition that both places have the same temperature.

Q6. Explain how rainfall is measured with a rain gauge. Give five points covering its purpose, collector, measuring part, placement and timing of observations. [5 marks]

1. A rain gauge measures the amount of rainfall received during an observation period. The result is expressed as a rainfall depth, commonly in millimetres.
2. The collecting funnel receives rain and directs the collected water into the measuring part of the instrument.
3. The water is collected in a measuring tube or cylinder. A scale is used to read the collected rainfall.
4. The gauge should stand in an open area away from objects that obstruct rain, on a flat surface where it will not tilt or topple.
5. For daily observations, readings should be taken at the same time each day. Record the date and rainfall unit with each measurement.

Q7. Give four distinctions that explain isotherms and isohyets: their meanings and the measurement information required when labelling each. [4 marks]

1. An isotherm is a line joining places that have equal temperatures. It represents the temperature element of weather across an area.
2. An isohyet is a line joining places receiving equal rainfall amounts. The compared amounts must cover the same period.
3. An isotherm label should identify temperature and its unit, such as degrees Celsius, so the line's value is clear.
4. An isohyet label should identify rainfall depth and its unit, such as millimetres, with the common observation period stated for the map.

Q8. Explain wind measurement in five points: the two properties recorded, the wind vane's construction, its reading, the cup anemometer's operation, and the wind-speed unit km/h. [5 marks]

1. Wind is described by its direction and speed. These are separate properties, so knowing direction alone does not provide a speed measurement.
2. A wind vane has a rotating arm with a pointer at one end and a tail at the other. Wind pushes the tail and turns the arm.
3. The wind vane's pointer indicates the direction from which the wind comes. Fixed direction labels help identify that direction.
4. A cup anemometer has cups that rotate around a vertical shaft. Stronger wind makes them rotate faster, and a meter calculates speed from the rotations.
5. The abbreviation km/h means kilometres per hour. It is a speed unit expressing distance travelled in an hour, rather than a direction.

Key takeaways

• Weather describes atmospheric conditions at a particular place and time; climate describes patterns over several decades at least.
• Temperature, pressure, humidity, precipitation, wind and clouds together provide a fuller description of the weather.
• Daily temperature range is maximum minus minimum; mean daily temperature is their sum divided by two.
• A thermometer measures temperature, a barometer measures pressure, a hygrometer measures humidity, and a rain gauge measures rainfall.
• A wind vane indicates where wind comes from, while a cup anemometer measures wind speed.
• Condensation forms liquid droplets; dew develops on cool surfaces, while rain, snow and hail fall as precipitation.
• Cirrus, cumulus, stratus and nimbus can be distinguished by their feathery, cotton-like, layered and dense dark appearances.
• Isotherms connect equal temperatures; isohyets connect equal rainfall amounts for the same observation period.

Test yourself

Why is a single rainy day insufficient to describe a region's climate?

Climate describes weather patterns over several decades at least, whereas one rainy day is a short-term weather observation.

What does ambient temperature mean?

Ambient temperature is the temperature of the immediate surroundings of the place being observed.

What does relative humidity of 100% indicate?

It indicates air saturated with water vapour at the given temperature, rather than air made entirely of water.

Which two instruments distinguish wind direction from wind speed?

A wind vane indicates wind direction, while a cup anemometer measures wind speed.

How does dew differ from rain?

Dew forms by condensation on cool surfaces; rain falls from clouds as liquid water.

Which cloud type has a cotton-like appearance and a flat base?

Cumulus clouds have a cotton-like appearance, occur in patches and have a flat base.

What is the difference between an isotherm and an isohyet?

An isotherm joins places with equal temperatures; an isohyet joins equal rainfall amounts for the same period.

Why are readings taken regularly at a weather station?

Regular readings track changes in weather and provide information useful for mapping and forecasting conditions.