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Name: Priya
Status: Student
Grade: 9-12
Location: AZ
Country: United States
Date: February 2009


Question:
Imagine that its summer and that the sun is shining on the ocean and on a stretch of land. Which will heat up more during the day? Which will cool more slowly at night? Does it matter if the land is wet?



Replies:
In general the land surface, being darker and less reflective, will absorb a great deal of solar radiation, and will have a significant increase in temperature on a bright sunny day. The ocean, at the same time, will reflect a greater portion of the radiation, and in addition, since water has such a high thermal coefficient, will have a much smaller increase in temperature. Thirdly, the water will spread the radiation to a depth that is not possible on the land. For all these reasons the water will remain cooler than the land on sunny day.

At night, the darker colours of the land will cause it to radiate its heat quickly, and its temperature will drop rapidly. The water will radiate more slowly, has a greater heat 'reserve' due to its high thermal coefficient, and as the surface cools, it will be displaced by warmer water from below due to the density changes that occur. As a result, an hour or so after sunset, the ocean will be warmer than the land, and will remain so for a large part of the night.

Anyone who lives close to the sea will tell you that on sunny days there is a consistent 'on shore' breeze during the day, and an 'off shore' breeze during the night. These 'sea breezes' are a direct result of the differential warming and cooling of the land and sea surfaces, as convection currents are set up by warm air rising above whichever is the warmer surface.

Nigel Skelton
Tennant Creek High School
AUSTRALIA


Priya,

The land will heat up more. There are several reasons for this.

Much of the solar radiation that reaches the ocean is reflected back to the atmosphere. Whatever is not reflected can penetrate into the water since it is somewhat transparent, thereby distributing any heating of the water to a certain depth, and therefore mass, of water.

In general, soil and vegetation reflect less solar radiation than water, particularly if the vegetation is healthy. The solar radiation heats up the very top of the land surface the most, as solar radiation cannot penetrate very deeply into the soil. Vegetation also absorbs solar radiation and stores and uses some of that energy in it. Therefore, a land surface will heat up more quickly and to a much higher temperature than the open ocean will.

Shallow ocean or lake waters will warm more rapidly and to higher temperatures than deep waters, because shallow water has less mass to heat up.

At night, the rate of loss of energy from either the ocean or the land is determined primarily by the temperature of the surface of either, as energy loss at night occurs only in the infrared wavelengths. If the land surface temperature is higher than the ocean surface temperature (which is normally true), the land surface will cool more rapidly and will lose more energy at night than the ocean will.

Until about late August, the land surface does not lose as much energy at night as it gained during the day (there are more daylight hours than nighttime hours); this is probably also true for the ocean. So the average temperature of the surface of the ocean and land both tend to continue to increase until September (in mid latitudes at least). If the ocean surface has a lower temperature than the land surface, it will cool more slowly than the land surface at night. The difference in temperature between the surface and the air above it determine the rate of cooling. Therefore, the land surface will cool more at night than the ocean surface (although not to a lower temperature than the ocean surface).

The wetness of the surface is only important for the land surface (since the ocean is always wet). If soil is wet, more energy will be removed from the land surface through evaporation from the soil and through the transpiration of plants during the day and night than would be removed if the soil were dry (since less evaporation and transpiration can occur for a dry soil). However, it is surprising that this effect does not always result in a greatly increased loss of energy for a wet soil. I will explain why below.

For a dry soil, the loss of energy occurs primarily through "sensible heating" (energy lost directly to the air above if the air is cooler than the temperature of the land surface); this can almost make up for the lack of energy lost through evaporation and transpiration. This almost results in an "energy balance", where outgoing energy equals incoming energy from the Sun over a 24 hour period. If this were not the case, the Earth would heat up incredibly quickly in Summer and all living things would fry by July.

David R. Cook
Meteorologist
Argonne National Laboratory


Priya,

Have you ever been to the beach on a sunny day? The sand gets really hot during the day. If you went back at night the sand would be much cooler. The temperature of the water would change little. The reason is that water has a high heat capacity relative to the sandy beach, meaning, that it takes much energy to heat up the water (or cool it off). Sand with water will not heat as rapidly as dry sand. The water will not exhibit much of a change in temperature throughout the day, but the temperature of the sand will change dramatically (assuming, of course, that it is sunny outside).

Indeed, the relative differences in temperatures between the sand and water during the day are commonly reversed during the evening. The difference in temperatures drives the wind. Day time winds generally blow onto the shore and night time winds generally blow off shore (assuming the body of water is large enough – like an ocean). Recalling that hot air rises, how can you explain the changing direction of the wind between the day and evening?

Leslie Kanat


For several reasons, the land will heat up more quickly and cool off more rapidly as well. I see from your location that you probably have not spent much time at the beach so you have not directly experienced this fact.

Water reflects much of the sunlight falling on it, the waves tend to increase evaporation which cools the water and it takes more heat energy (as measure by a property called specific heat) to warm up the same volume of water than land. Not only that, but heat travels much faster through water so heat flows away from the water surface more rapidly. It is only the first few inches of the land that heat up from the sun .

In the evening, the ground also gives up its heat more easily so it cools more quickly than the ocean. Also it is just the first few centimeters of land that needs to cool, but you need to cool far more of the ocean to get the same temperature drop.

Sailors are well aware of the temperature differences as the wind goes from the sea to the land in the daytime as heated air over the land rises. Unfortunately, in the evening the wind flows in the opposite direction making it impossible for sailboats to come into port after dark. Many a sailor has arrived at their destination as the sun is going down and has had to wait until the next morning to sail into port. These are called land and sea breezes.

Robert Avakian



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