Air pressure

Air pressure is the force that air molecules exert on a surface due to their weight and movement. 

Normal value at sea level: 1013.25 hPa (hectopascal) = 1 atm = 760 mmHg. 

It decreases with increasing altitude (approximately 1 hPa per 8–10 meters of rise near the ground). 

This is caused by the weight of the air above (gravity). 

Measure: with a barometer (mercury or aneroid barometer). 

Influence on weather: High pressure (H) → often fair, stable weather. 

Low pressure (L) → often cloudy, rain, wind Formula (approximate): P = P₀ × e^(-h/H) (P = pressure at altitude h, P₀ = pressure at sea level, H ≈ 8400 m) 

In short: the more air above you, the higher the pressure. 

That's why you feel your ears popping on an airplane or in the mountains.

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Downdrafts

A downdraft is a strong, downward airflow that develops in severe thunderstorms, where cold air from a cloud plunges downward at high speed and spreads horizontally. 

This results in extreme wind gusts of 150-250 km/h, often with vortices (gustnadoes), but without the rotating trunk of a tornado. 

Downdrafts cause widespread damage over an area of ​​hundreds of meters to kilometers, such as fallen trees, damaged buildings, and injuries, and are often confused with tornadoes. 

Origin and characteristics: The wind is generated by temperature differences (e.g., a >10°C drop after a storm) and occurs in 'bow echo' squall lines. 

They are rare in the Netherlands but are increasing due to climate change, with damage in one direction instead of a narrow track. 

Well-known examples in the Netherlands: Leersum (June 18, 2021): 

Most severe recent event; 9 injured, 100+ houses damaged, 730 hectares of forest destroyed, total damage €13.3 million. 

Winterswijk (July 2, 2025): Major chaos with trees and roofs blown off by a drop of 37→21°C. 

Vethuizen (July 14, 2010): 2 dead at a campsite in Achterhoek. 

Meppel (August 24, 2024): Trees fell like matchsticks; buildings were damaged. 

Historical: Borculo (1925) with possible gustnadoes. 

 Downwinds are difficult to predict and dangerous for traffic and aviation; inspection distinguishes them from tornadoes based on the damage track.

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Storm

A storm is a powerful weather phenomenon with strong winds (from wind force 9, >75 km/h), often accompanied by rain, thunder, snow or hail. 

Storms can cause damage to buildings, trees and infrastructure. 

In the Netherlands, a storm is often associated with autumn and winter weather, such as heavy westerly storms.

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Temperature

The temperature is the degree of heat (or cold) expressed in degrees. 

In the past, people also spoke of temperament. 

The temperature is measured with a thermometer. 

This instrument was invented in 1593 by the Italian Galileo Galilei (1564-1642). 

The best-known scales are Fahrenheit and Celsius. 

Especially in England and the United States, the temperature is still indicated in degrees Fahrenheit, in Europe Celsius has become established. 

Measuring temperature 

At meteorological stations, the temperature of the air is measured according to international agreement. This is done in degrees Celsius at a height of one and a half meters above an open grassy area. 

The thermometer or sensor is in a white box with walls that have the shape of an open blind. 

This gives the wind free rein, but sun and precipitation cannot penetrate the instruments. 

Temperature deviations 

In an urban environment, the temperature can deviate, especially due to the buildings and paving.

It is generally warmer there than in the countryside. 

Inversion 

In general, the temperature decreases as the altitude increases. 

With dry air, the decrease is about 1 degree per 100 meters, with moist air it is about 0.6 degrees. 

After or at the end of a clear night with little wind, the temperature can also increase with altitude up to a certain height. 

This is called an 'inversion'. 

The height to which the temperature decreases is called the 'inversion height'.

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Ice thickness and ice skating

The KNMI has investigated the ice thickness in Central Netherlands using computer calculations. 

An ice layer of more than 10 centimetres occurs in some winters for dozens of days. 

Record ice thickness 

In the winter of 1991, the ice was thicker than ten centimetres on eighteen days. 

In the very severe winter of 1963, it was even thicker on eighty days. 

That winter, the ice must have been more than forty centimetres thick. 

In the winter of 1996, the ice reached a thickness of 25 centimetres. 

That was 3 to 4 centimetres less than in the winter of 1979 and in the winters of 1985, 1986 and 1987. 

On 11 January 1997, at the end of the first frost period, the ice in De Bilt had grown to 32 centimetres.

This is the thickest layer of ice since the winter of 1963. 

Skating ice The carrying capacity of good ice is 4 to 5 centimetres for an adult man. 

A large crowd of people is safe with an ice thickness of 27 centimetres. 

Elfstedentocht 

Most Elfstedentocht (eleven out of fifteen) were held with ice thicknesses of more than 25 centimetres. 

In 1985, the tour was held on 17 centimetres of ice, in 1917 on only 15 centimetres. 

The thickest Elfsteden ice was in 1940, when it was over 41 centimetres.

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