Weather and Acid Rain
Name: Rebecca L.
Do different weather conditions like snow,frost, dew,
humidity,rain affect the amount of acid in air moisture?
The form of precipitation and the amount
of water vapor in the air do affect how
acid precipitation and water droplets in
the air and in clouds are.
Generally, the more water drops or snow
flakes, the less acidic the water is,
since the acid particles are dissolved and
The more precipitation and the faster it
falls, the less acidic the rainwater or snow
is, again, because of the dilution factor.
A rapid rain, such as from a thunderstorm,
"cleans" the air, washing out much of the
pollution and making the air temporarily cleaner.
How acid the precipitation is, is also
affected by how much sulfur and nitrogen bearing
pollution and soil is in the air. Soil
generally helps to neutralize the acidic
pollution in rain. Areas of the country with
more alkaline soils, such as west
of the Rocky Mountains, and where it is windy
(so that soil becomes airborne), generally have
less acidic precipitation as a result. The western
part of the country also has fewer pollution sources
and therefore has less acidic precipitation
for this reason as well.
Snow is usually less acidic than rain, as it is
not very good at "washing" pollution from the air.
Snowflakes often form on aerosol particles, which
can be made of pollution, but being solid, they
cannot capture or absorb other pollution effectively.
Rain in Chicago has an average pH of 4.4, whereas
the pH for snow is more commonly around 4.8.
A study of the acidity of dew was performed at
Argonne Nat. Lab. about ten years ago. It showed, as
expected, that the more dew there is, the less acid the
dew is. Again, this resulted from the acidic material
on the leaves of the grass being diluted by the dew water.
Most of the acid material had fallen onto the grass as
dry material. Pollution is removed from the air by three
natural modes: it is deposited on everything in
a dry form of particle, it is absorbed by plants, water
bodies, and the soil in a gaseous form, and it is
removed from the air by precipitation.
A good web site to look at to see the effects of pollution
on precipitation is nadp.sws.uiuc.edu, the site of the
National Atmospheric Deposition Program, which I
participate in. This Program monitors and tracks
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012