Precipitation – the variety of phenomena in Poland

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Poland lies in a temperate warm transitional climate with a predominantly maritime type in the west and a continental type in the east. Incoming from over the Atlantic Ocean, moist polar-marine air masses have a major impact on the amount and intensity of precipitation. The Baltic Sea has the most humid air, so there is increased precipitation. In the rest of the area, it is possible to see the dependence of their number on the relief of the surface. The highest totals are recorded in undulating areas – in the mountains and lake districts. There, air can stop at hills that are natural obstacles.

In contrast, the lowest precipitation totals are found in the flat, lowland areas of central Poland, especially in Kujawy and Greater Poland. These regions are located in the so-called. “rain shadow” of the lake district and at the same time away from the marine reservoir. The highest precipitation totals are observed in the summer, but the cool semester brings more variation in terms of the type of precipitation.

Rain or drizzle – how to distinguish precipitation?

Rain and drizzle are the only liquid precipitation distinguished in meteorology. To recognize them, the criterion of droplet size is used. We will call rainfall with a droplet diameter of more than 0.5 mm, and drizzle below this limit. In practice, however, it is difficult to run with a yardstick, and to tell the difference between the two it is worth using a more empirical approach. When it rains, because of its larger diameter, we can see individual drops, and in very heavy rainfall also streams of water. It will also rumble against the windowsill and collect in puddles.

Individual drops will form circles on the surface of the water. Instead, in the case of small drizzle droplets, we have the impression that they are floating in the air, forming a suspension. We can even see areas of floating droplets and faint air movements. The drizzle will not be heard or seen on the surface of the water. The only cloud that can bring drizzle precipitation is the low, stratus Stratus. Rain, on the other hand, occurs with higher-lying and thicker stratus clouds (Stratocumulus or Nimbostratus) or clumps (extended Cumulus or Cumulonimbus clouds) (Zaleski, 2015). In the case of simultaneous Stratus clouds and above them Stratocumulus or Nimbostratus clouds, we can observe simultaneous drizzle and rain.

Snow

When the temperature drops below 0ᵒC the most common precipitation that can be expected will be snow. It is formed by resublimation of water vapor in the cloud to form ice crystals. The crystals, clumping together, form larger structures and, under the influence of gravity, fall out of the cloud. Their shape, size and concentration depend on the temperature, humidity and atmospheric pressure in which they were formed. Although they form fancy patterns and shapes, their base is always a hexagon. At temperatures down to -5ᵒC, ice crystals form flakes in the form of plates and small stars. As the temperature drops and with low humidity, snow is formed in the form of needles or spatial columns and prisms.

The largest and most extensive structures can be observed when the temperature of their formation ranges from -10ᵒC to -20ᵒC and high humidity. Snowfall in such conditions will be in the form of large, clumped plates or huge aggregated crystals – dendrites (Zaleski, 2015). Snowflakes will resemble shreds of cotton wool majestically floating in the air. Snow composed of ice crystals is a type of precipitation occurring from all types of precipitation clouds. In addition, as in the case of drizzle, when low stratus clouds are deposited or dense fog is present, we may have to deal with granular snowfall. These will be fine (up to 1 mm), opaque ice particles that will not bounce off the hard surface. It occurs at temperatures from 0ᵒC to -10ᵒC.

Is it hail yet?

The snowfall is quiet. Individual flakes coat the surface rather than bounce off it. It often falls quietly and unnoticed. What if we have to deal with precipitation that rumbles against the windowsill or hums outside the window? Bouncing up, bouncing off hard surfaces or hitting us in the face? Will it always be hail? Although it is in the form of spherical ice particles, it does not always look like typical hail. It is smaller, white rather than glassy, and falls slowly rather than with tremendous force or carried by strong winds. It can be snow crumb, ice crumb or ice grains (Zawiślak, 2010). This precipitation is typical, unlike hail, for the cold season, and we can distinguish between the two quite easily.

However, starting with hail, it is the precipitation of ice particles larger than 5 mm in diameter. They can be spheroidal, conical or irregular in shape, and transparent or milky. Their structure can be varied, not always forming layers around the nucleus, as in an onion. We also encounter hailstones with a homogeneous structure. Such precipitation is always fleeting and is produced in extended Cumulonimbus storm clouds. They can occur throughout the year but most often occur from May to October, when the temperature at ground level is significantly above 0ᵒC.

Other forms of precipitation of spherical ice particles will be between 1 and 5 mm in size and are more likely to occur in the cool half of the year, when the temperature at ground level is near or below 0ᵒC. Snow and ice crumbles are transient precipitation associated with extended clump clouds (Cumulus and Cumulonimbus). During precipitation, both forms of crumbles will be heard and will bounce back (ice crumbles with greater intensity). Snow crumbles will be in the form of white balls, which can splash when reflected. They are also easy to squish between your fingers. Ice crumbles, on the other hand, are in the form of hard balls of ice, not susceptible to disintegration or crushing. They are formed as a result of the merger of an aqueous phase with snow croup in the cloud and are a transition stage to hail (Zaleski, 2015).

A different genesis than hail or crumble will be ice grains, formerly known as ice rain. It is continuous precipitation from stratus clouds (Nimbostratus or Altostratus) in the form of transparent grains 1-3 mm in diameter. It is formed when a snowflake melts and refreezes as it falls, when there are alternating layers of air with positive and negative temperatures in the atmosphere. It is a rather short-lived phenomenon that does not generate losses, such as hailfall.

Precipitation without a cloud

Can we observe precipitation without a cloud over our heads? When visibility is good, there is no fog, instead we can see the sun, but there are tiny ice crystals floating and falling in the air? The answer is yes. It will be a fallout of diamond dust – very fine ice crystals formed in frigid (below -10ᵒC), cloudless and safe weather. These crystals will be especially visible when shining in the sunlight, creating a diamond veil. They can also cause halo phenomena – white or rainbow concentric rings around the sun or moon.

Gov’t

It is a well-known fact that snow cover is a dangerous phenomenon that can cause property damage (downed power lines, broken trees or slippery road accidents). However, not every frozen puddle can be called a golem. Namely, it is a phenomenon that occurs as a result of freezing rain or drizzle. It will be in the form of a compact, homogeneous and generally transparent ice layer, covering all surfaces exposed to precipitation. It is formed when rain or drizzle falls as overcooled water droplets on a surface with a temperature below 0ᵒC.

A prerequisite for the formation of snow cover is a layer of positive temperatures at an altitude of 100-600 meters and a negative temperature of objects or the ground surface. Water droplets formed in a layer of positive temperatures do not have time to freeze and fall as ice grains and as overcooled water hit the frozen surface, momentarily freezing. In the airspace, snowmelt can occur on aircraft exposed to overcooled precipitation or overcooled water droplets in the cloud during flight (Szewczak, 2014). Colloquially, the ice may be referred to as black ice or glass.

Blizzard and blizzard

Atmospheric phenomena characteristic of the winter period, and often mistaken for each other, are snowstorms and blizzards. They are by no means precipitation, but they are worth mentioning just in the context of this article. The distinction between the two is simple, and they do not always have to occur together. A snow blizzard is what we call the phenomenon of the wind lifting the snow already on the ground. We distinguish between two types of snow blizzard: high and low. A low snow blizzard will occur at lower wind speeds, when the snow is lifted to insignificant heights and does not restrict visibility at the observer’s eye level.

A high snow blizzard occurs at higher wind speeds (no hard limit has been set), and the lifted snow limits the observer’s visibility and often obscures the sky or sun. During a blizzard, the sky may be cloudless, and there is no snowfall (Zaleski, 2015). A blizzard, on the other hand, is the occurrence of strong winds accompanied by snowfall (Bear, 2003). Snow is carried in different directions, significantly reducing visibility. Blizzard may occur in areas without snow cover. It happens that both phenomena occur together when there is already lingering but fresh snow.

As you can see, precipitation is not just rain and snow. And although we do not have dozens of terms for types of snow, as in the languages of peoples living in the northern areas of the Earth, Polish winter is also characterized by various forms of precipitation. And this is only a small part of the meteorological phenomena and picturesque ice formations that we can observe during cold weather.

You can read about how much benefit rainwater can bring in the article: Rainwater – how much can it benefit a city?


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