Introduction to the atmosphere practice questions.

Radiation arriving from the Sun is mainly shortwave, while radiation leaving the Earth for space is mainly longwave. The reason is that

• a cooler body radiates at longer wavelengths than a hotter one
• the atmosphere lengthens the wavelength of any radiation crossing it
• shortwave radiation cannot travel back out through the atmosphere
• the Earth emits only the radiation that clouds have already reflected

Answer: a cooler body radiates at longer wavelengths than a hotter one

The wavelength at which a body radiates most strongly depends on its temperature, so the Sun at around 5 500 degrees Celsius emits in the visible and near infrared while the Earth at around 15 degrees emits in the far infrared. That difference is what makes the greenhouse effect possible: the atmosphere is largely transparent to the incoming wavelengths and largely opaque to the outgoing ones.

Thick low cloud has two opposing effects on the temperature of the surface beneath it. These are that it

• absorbs incoming sunlight and reflects the outgoing longwave radiation
• reflects incoming sunlight and absorbs outgoing longwave radiation
• reflects both incoming sunlight and outgoing longwave radiation
• absorbs both incoming sunlight and outgoing longwave radiation

Answer: reflects incoming sunlight and absorbs outgoing longwave radiation

A bright cloud top reflects incoming sunlight, raising the albedo and cooling the surface by day, while the cloud absorbs outgoing longwave radiation and re-emits part of it downward, keeping the surface warmer at night. Clouds absorb rather than reflect longwave radiation, and for thick low cloud the reflective cooling usually dominates; how cloud cover responds to warming remains one of the largest uncertainties in climate models.

Atmospheric pressure falls steeply in the first few kilometres above the surface and then far more gradually. The reason is that

• the temperature of the air falls steadily with increasing height
• gravity acts only on the air within the lowest atmospheric layer
• most of the mass of the atmosphere is compressed into its lowest layer
• the composition of the air changes markedly with increasing height above ground

Answer: most of the mass of the atmosphere is compressed into its lowest layer

Air is compressible, so it is densest where the weight of everything above it presses hardest, and around three quarters of the atmosphere's mass lies within the troposphere. Pressure at any height is simply the weight of the air above, which is why it falls fastest where the air is densest.

Water vapour is the most abundant greenhouse gas in the atmosphere, yet climate policy targets carbon dioxide instead. The reason is that

• water vapour does not absorb longwave radiation effectively
• water vapour is present only in the stratosphere, far above the weather
• human activity emits no water vapour anywhere in the system
• the amount of water vapour held is set by temperature, not emissions

Answer: the amount of water vapour held is set by temperature, not emissions

Vapour added to the air condenses out within days, so its concentration is controlled by how warm the air is rather than by how much is released. That makes water vapour a feedback that amplifies whatever warming another gas causes, rather than a driver that policy can act on directly.

A student measures 800 W m-2 of shortwave radiation arriving at a snowfield and 640 W m-2 reflected from it, and reports the albedo of the snow as 1.25. Identify the error in the student's work.

• The two values were divided the wrong way round; albedo is reflected radiation divided by incoming radiation.
• The reflected value should have been subtracted from the incoming value, because albedo is a difference and not a ratio.
• Albedo is defined for outgoing longwave radiation, so the shortwave measurements should not have been used at all.
• The two values should have been added together, because albedo describes the total radiation a surface handles.

Answer: The two values were divided the wrong way round; albedo is reflected radiation divided by incoming radiation.

Albedo is the fraction of incoming shortwave radiation that a surface reflects, so it is 640 / 800 = 0.80 and can never exceed 1. A value above 1 is the signal to check which way round the ratio was taken, and 0.80 also sits sensibly inside the 0.8 to 0.9 range expected for fresh snow.

Which of these processes in the atmospheric system is a transformation rather than a transfer?

• Dust raised over the Sahara and carried west across the Atlantic by the trade winds
• Water vapour condensing into cloud droplets as an air parcel rises and cools
• Warm air rising from a heated land surface into the free troposphere above it
• Sea spray lifted from the ocean surface and mixed upward through the boundary layer

Answer: Water vapour condensing into cloud droplets as an air parcel rises and cools

A transfer moves matter or energy from one place to another without changing its form; a transformation changes the form, and condensation changes water from vapour to liquid. The three distractors are all transfers, because the material arrives elsewhere in exactly the state in which it left.