What causes April showers?

Is April Really The Month of Showers?

Spring weather can be rather unpredictable. Clear mornings can turn to stormy afternoons at the blink of an eye. But what causes April showers?

April mornings are often bright and fair but the afternoons that follow can bring damp deluges of rain. Showers can happen at any time of year but intemperate, maritime climates are most common during spring. In some areas, April weather can seem positively schizophrenic – but what causes April showers?

Cumuliform Cloud Showers

A shower is any type of precipitation that comes from cumuliform clouds. Cumuliform clouds are a family of clouds that include cumulus and cumulonimbus, which tend to have significant vertical development but which are often rather isolated from other clouds. Near the equator, cumuliform clouds can be up to 17km tall but may have a relatively small surface area when viewed from below.

Cumuliform cloud formation occurs when hot air rises from the land or sea. Solar radiation heats the land and the layer of air closest to it, creating a thermal. As the air gains in altitude, it cools at the dry adiabatic lapse rate of around 1°C per 100m. As the temperature drops the relative humidity increases as air at lower temperatures has a lower carrying capacity for moisture and other particulates. When it reaches 100% humidity – known as the dewpoint – moisture in the air begins to condense, resulting in cumuliform clouds.

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Solar Seasons and Temperature Lag

The conditions for the formation of cumuliform are influenced by the seasons. To understand how we need to understand the difference between solar seasons and seasons as they are experienced on Earth.

Traditional conceptions of winter define it as beginning at the winter solstice (around the 22nd of December in the northern hemisphere) and lasting until the vernal equinox. Solar winter is a little different – it is considered to be the one and a half months each side of the winter solstice and spans the quarter of the year when the hemisphere receives the least solar radiation.

The coldest temperatures in winter are usually felt towards the end of January or the beginning of February and solar winter is often over by this time. This is because both the land and the sea take a while to heat up and cool down. When the winter solstice comes, the land is still benefiting from residual warmth from the late summer and autumn; by February it has had three months with relatively little incoming solar radiation.

Why April Showers Occur

What does this have to do with April showers? Well, while the land takes a while to heat up in the spring, the ocean takes even longer. In April, there are high amounts of incoming solar radiation but the land remains relatively cool and the ocean cooler still. This discrepancy is perfect for the regular daily formation of cumulus clouds.

Clear mornings allow the sun to heat the land, creating large columns of warm air. Cool but humid air from the ocean rushes in to fill the void, creating a refreshing sea breeze. This air in turn is heated. The system acts as a vertical conveyer belt, transporting large amounts of moisture into the atmosphere. Eventually, this moisture cools to form clouds, and when the saturation point is reached the clouds burst and it starts to rain. This usually occurs during the early afternoon.

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Azores Effect Influence on Cumulus Formation

All this is also augmented by the Azores effect. High altitude winds oscillate with the seasons and as the Azores high pressure moves north during the spring so does the jet stream. The presence of this cold north-easterly wind increases the condensation rate of rising air and thus aids cumuliform formation. Showers can occur at any time of year, but the combination of solar radiation, cool land temperatures, and even cooler ocean temperatures create ideal conditions for the formation of diurnal cumuliform clouds. These clouds, when saturated, deliver short and often intense bursts of rain that are referred to as April showers.

References:

  • Angwin, R (15 April 2004) “If it’s April – it must be showers”, Bbc.co.uk
  • Linacre, E & Geerts, B (1997) Climates and weather explained, Routledge, New York

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