## 1 Introduction  
In the last decades, a continuation of the long-term global warming trend was observed, and regional and local impacts have already become apparent [1]. These impacts are expected to become worse with ongoing climate change [2, 3]. For Europe, temperature increase is projected in all seasons [4]. Seasonal shifts and changes in frequency and intensity of weather extremes will amplify the impacts in many economic sectors such as agriculture [5]. It is expected that approximately 26% of all damages and losses associated with medium to large scale climate-related disasters are attributed to agriculture with its sectors crops, livestock, fisheries, aquaculture, and forestry [6]. The situation of optimizing animal husbandry conditions is also addressed by European legislation in the European Convention for the Protection of Animals kept for Farming Purposes, where the obligations to ensure suitable microclimatic conditions of livestock breeding are also formulated. Increasing air temperature and humidity reduce the ability of cows to cool themselves. Increases in these climate variables lead to, or increase existing, heat stress in cattle [7, 8, 9, 10]. Heatstress is defined as the sum of external forces acting on an animal that causes an increase in body temperature and evokes a physiological response [11]. Heat stress occurrence can be a phenomenon of one or a few days, but it may also be a phenomenon extending over a certain period. It is the joint result of multiple factors, of which ambient temperature and relative humidity are the two most important ones [12; 13; 14]. Other risk factors contributing to heat stress are breed, parity, lactation stage, milk production level, level of feed intake, ration composition, body condition score, feedstuff quality, pasturing condition, housing conditions, and cattle behavior [15; 16]. The diagnosis of heat stress can, in general, be set by observing clinical signs in affected cattle and an inventory of risk factors, together with high ambient temperatures and a certain level of relative humidity [17; 18]. However, sometimes the negative effects of heat stress show up later and indirectly; there is a carry-over effect (summer heat stress can have effects up into autumn). The parameter of choice to confirm a probability diagnosis based on clinical signs is the Temperature--Humidity--Index, THI [19; 20; 21]. It can be calculated with Equation (1). \[THI=0.8T_{a}+RH(T_{a}-14.4)+46.4 \tag{1}\]  
where \(T_{a}\) is the ambient temperature, and \(RH\) is the relative air humidity in percentage [22; 23]. At a THI \(>\) 64, the heat stress is \"mild\"; at a THI \(>\) 72, the heat stress is \"moderate\", and at a THI \(>\) 77, the heat stress is \"severe\"; at THI \(>\) 84, death occurs [24; 25; 26]. Cows are well able to adapt to changeable temperature and humidity conditions throughout the year [16]. This can be confirmed by a relatively wide range of neutral temperatures established for dairy cattle. Fluctuations of temperature within a range of \(-\) 0.5 \({}^{\circ}\)C to 20.0 \({}^{\circ}\)C and 60-80% relative humidity [15] is generally accepted as a thermoneutral zone that does not significantly induce physiological or behavioral changes among cows. The level of air temperature generally accepted as 25.0 \({}^{\circ}\)C-26.0 \({}^{\circ}\)C or 24.0 \({}^{\circ}\)C-27.0 \({}^{\circ}\)C [15; 27; 28] is the upper critical temperature, above which the dairy cow welfare is disturbed. Although air temperature and relative humidity may be most important in determining the exchange of heat between the animal and its surroundings, other relevant microclimate factors, such as air movement and sunlight, also play a significant role in levels of heat stress [29; 30; 31]. Changes in air velocity influence the convection cooling of cattle which, in combination with solar radiation, has a very significant impact on the regulation of the thermal balance of cows [32; 33].