The impact of a small lake on heat stress in a Mediterranean urban park: the case of Tel Aviv, Israel

Abstract

Field observations of air and surface temperatures, relative humidity, solar radiation and wind were performed in the daytime hours of the warm season around a pond of 4 ha, located in Begin Park, in the city of Tel Aviv, Israel. Observations were carried out at screened meteorological stations on four randomly selected days, all associated with moderate heat stress. Two of them, one representing a warm and dry day, and other, representing a sultry day, are analyzed in detail. At the downwind side of the pond, lower temperatures, a higher relative humidity and a lower heat stress index were observed consistently when compared with stations located upwind of the pond. This effect is regarded here as the "lake effect". The fact that no significant change was noted in the water vapor pressure during most of the daytime hours indicates that the lake effect was related mainly to cooling rather than to moisture transport from the pond. A positive relationship was found between the lake effect and wind speed in both types of weather. The maximum effect of the wind's speed on the lake effect was observed at midday, at which time the temperature drop reached 1.6 °C, while the relative humidity rose by 6%. As a result, the heat stress index dropped by 0.8–1.1 °C. It is suggested that the temperature drop induced by the pond during the warmest hours of the day was mainly the result of a truncation of the sensible heat flux from the underlying surface when the air, which had previously passed over hot surfaces, swept over the relatively cool water. During the late afternoon and evening hours, when the water became warmer than the surrounding surfaces, latent heat cooling resulting from evaporation became the dominant source of the lake effect, and the lake effect resulted in increasing heat stress. It is concluded that even small bodies of water have a relieving effect on humans in the daytime hours, within the range of 40 m, under both dry and humid hot weather conditions.

Appendix

Dependence of heat stress index on dry temperature

Following Tanner (1968) the wet bulb temperature (T _w) is given by:

$$ T_{\rm w} = T + (e - e^\ast)/(s + \gamma) $$

(3)

where e and e* are water vapour pressure and saturation water vapor pressure, respectively, s is ∂e*/∂T and γ is the psychrometer constant (positive). Since the saturation water vapor pressure can be written as

$$ e^\ast = A\exp (BT), $$

(4)

where both A and B are positive constants, then

$$ S = Be^\ast $$

(5)

Inserting Eqs. 3 and 5 in Eq. 1 yields

$$ {\rm{HSI}} = T + 1/2\left[ {(e - e^* )/(Be^* + \gamma )} \right] $$

(6)

Assuming that e is constant, i.e., there is no addition or subtraction of moisture to the air, the dependence of HSI on T is obtained by differentiating with respect to T, yielding, after some algebraic manipulations:

$$ \partial (\hbox{HSI})/\partial T = 1 - 1/2[Be^\ast /(Be^\ast + \gamma )] $$

(7)

Since both B, e* and γ are positive, the numerical value of the second term in the right-hand side of Eq. 7 is smaller than unity, so the derivative is positive, implying that when the water vapour pressure of an air mass remains unchanged and its temperature decreases, the heat stress decreases as well.