Study of Weather
ICSE Class 7 · Geography · Chapter 5
What this chapter covers
- Weather and its elements
- Temperature
- Atmospheric pressure
- Humidity
- Winds
- Clouds
- Precipitation
- Weather and climate
- Isotherms and isohyets
Key definitions
Cirrus clouds are thin, white, feathery clouds found very high in the sky.
Cumulus clouds are thick, puffy clouds with flat bases and rounded tops, like heaps of cotton wool.
Stratus clouds are grey, flat clouds that spread in layers or sheets across the sky.
Nimbus clouds are dark grey or black rain-bearing clouds that hang low in the sky.
Key points
- Weather is the condition of the atmosphere at a particular place and time; it happens mostly in the troposphere.
- The elements of weather are temperature, atmospheric pressure, humidity, winds, clouds and precipitation.
- Temperature depends on latitude, altitude, distance from the sea, ocean currents and winds, and cloud cover.
- Daily range is maximum minus minimum temperature; mean daily temperature is their average.
- Atmospheric pressure is the force exerted by the weight of the air on a unit area of the Earth's surface; it falls with height and is lower where air is warm.
- Low pressure brings rising air, clouds and rain; high pressure brings sinking air and clear, dry weather.
- Humidity is the water vapour in the air; relative humidity gives it as a percentage of what the air could hold.
- Condensation begins when air cools to its dew point.
- Winds blow from high pressure to low pressure and are named after the direction they come from.
- Sea breezes blow by day, land breezes by night, and monsoon winds change direction with the seasons.
- The four basic clouds are cirrus, cumulus, stratus and nimbus; cumulonimbus brings thunderstorms.
- Precipitation falls as rain, snow or hail; dew forms directly on cool surfaces.
- Climate is the average weather of a place over about 30 years or more.
- Isotherms join places of equal temperature; isohyets join places of equal rainfall.
Questions answered
What is weather? Name its six main elements.
Weather is the condition of the atmosphere at a particular place and time. Its main elements are temperature, atmospheric pressure, humidity, winds, clouds and precipitation.
Why is the air near the ground usually warmer than the air higher up?
Sunlight passes through the air without heating it much and warms the land first. The warm ground then heats the air just above it, so the air closest to the ground gets the most heat.
On a winter day, a town recorded a maximum of 22 °C and a minimum of 8 °C. Find the daily range and the mean daily temperature.
Daily range = 22 - 8 = 14 °C. Mean daily temperature = (22 + 8) ÷ 2 = 15 °C.
Why does atmospheric pressure fall as you climb a mountain?
There is less air above you pressing down, so the weight of the air, and therefore the pressure, is smaller.
What is relative humidity? What does a relative humidity of 100 per cent mean?
Relative humidity is the water vapour present in the air, shown as a percentage of the most the air could hold at that temperature. At 100 per cent the air is saturated and cannot hold any more vapour.
Explain why a sea breeze blows during the day.
By day the land heats faster than the sea. Air over the land rises and leaves low pressure, so cooler air from the sea, at higher pressure, blows in towards the land.
Name the four basic types of clouds. Which combined cloud, named from two of these types, brings thunderstorms and hail?
Cirrus, cumulus, stratus and nimbus. Cumulonimbus clouds, which combine cumulus and nimbus, bring thunderstorms and hail.
How does hail form?
Strong rising air in a cumulonimbus cloud carries raindrops into its freezing upper part. The frozen drops fall, collect more water and are carried up again, adding layers of ice until they are heavy enough to fall as hailstones.